# VERY technical question about depth of field (you've been warned)



## Senor Hound (May 17, 2008)

Okay, so I was messing around with a depth-of-field calculator which I have linked here:

http://www.dofmaster.com/dofjs.html

It says that a digital camera with a 1.5 or 1.6 sensor will have less of a depth of field at any given aperture and focal length than the film camera (i.e. 50mm at f/1.8).  I told someone else here this, and they disagreed with me, and said the same lens should have the same depth of field no matter what the camera is, cause they have the same lens and aperture, so there shouldn't be any possible way for it to have a different depth of field.

I thought it was because a digital sensor might be less capable of reproducing depth of field, but I found out this isn't the reason.  So now I think its because the lens has to enlarge the image more than film does, so the tolerance for what looks in focus is higher (circle of confusion).  Therefore, depth of field decreases.  And if this is true, that means that when you blow up a photo, your depth of field actually decreases the larger you make it, until you get to a point where you have no focused depth of field and the whole image looks blurry.

Can someone figure this out for me?  I've looked on three websites with depth of field calculators and they all tell me the same thing.  And I'm not the best at science, so I'm in the dark.

Can anyone who has a PhD in optics help me out? 

Edit:  BTW, I found this:

http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size

It tells me what I thought was true, but it doesn't tell me why that's the case.  And it is open source, so maybe its wrong too...


----------



## RyanLilly (May 17, 2008)

Maybe I'm not understanding the DOF calculators terminology, But If they Mean that Between the "Near limit" and the "far limit" is what is in focus. Then the thing seems to be completely wrong, or just backwards.

I can't think of the physics behind it right now, I'm sure I could figure it out if I drew some picture to help myself visualize it, but its late, But  the smaller the sensor or film, the larger the DOF, or in other words more distance is in focus.


----------



## RyanLilly (May 17, 2008)

OK, Now I had to think about it. The Wider the lens the the larger the DOF, The longer the lens, the smaller the DOF.

There is nothing wrong with that DOF Calculator. I was thinking that the focal length was 35mm equivilent, but it uses the actual physical size. 

Smaller sensors or film only have larger DOF, because they require very short lenses to get a reasonably wide viewing angle. So to obtain the same angle as a 38mm on 35mm film, might require a lens of only 5 or 6mm, so optically you have extremely long DOF


Choose a canon 5D or even a crop body with a 38mm,
Then choose a random P&S, like a powershot 5xx and 6mm lens, This should be about 38mm equiv. same f-stop for both, and look at the total DOF for each.


----------



## Alex_B (May 17, 2008)

In order to get the same field of view, you will have to use a longer lens with the film camera.

Hence you should compare 50mm at f/1.8 on a 1.6 crop sensor with 80mm f/1.8 on film or on a full frame digitial sensor.

Then the 1.6 sensor will have more DOF.


----------



## Alex_B (May 17, 2008)

oh, it has all been said now  .. I was too slow


----------



## Senor Hound (May 17, 2008)

Alex_B said:


> In order to get the same field of view, you will have to use a longer lens with the film camera.
> 
> Hence you should compare 50mm at f/1.8 on a 1.6 crop sensor with 80mm f/1.8 on film or on a full frame digitial sensor.
> 
> Then the 1.6 sensor will have more DOF.



I'm not talking about field of view, I'm talking about depth of field.  Why does a digital camera have a shorter depth of field when using the EXACT same lens and aperture as a film camera?


----------



## Alex_B (May 17, 2008)

Senor Hound said:


> I'm not talking about field of view, I'm talking about depth of field.  Why does a digital camera have a shorter depth of field when using the EXACT same lens and aperture as a film camera?



Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).

Hence you enlarge also the "unsharpness" more, to put it in such handwaving words.

To understand why it is usually said that smaller sensors produce larger DOF, you need to talk about field of view and different focal lengths though.


----------



## RyanLilly (May 17, 2008)

RyanLilly said:


> Choose a canon 5D or even a crop body with a 38mm,
> Then choose a random P&S, like a powershot 5xx and 6mm lens, This should be about 38mm equiv. same f-stop for both, and look at the total DOF for each.



If I want to take the exact same picture, same framing proportion and field of view with a Full Frame camera and a P&S, Take a 5D, 38mm lens@f/3.2, and to take the same picture with a Powershot a560(about a  6x crop factor) you need a  6mm lens @ 3.2. 

A Canon 5D DOF= 4.13 feet 
Powershot 560= infinate DOF, even though the feild of view is exactly the same.

Now If you could stick a real 38mm lens on that powershot, the field of view would look like a 228mm on the 5D, and the DOF would be a mere .66 ft.

Basically there is a ratio between sensor size and focal length, the wider the ratio is, the smaller the DOF, The closer the ratio is the larger the DOF


----------



## Senor Hound (May 17, 2008)

Alex_B said:


> Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).
> 
> Hence you enlarge also the "unsharpness" more, to put it in such handwaving words.
> 
> To understand why it is usually said that smaller sensors produce larger DOF, you need to talk about field of view and different focal lengths though.



Okay, that's what I thought.  With that being said, does that mean the depth of field on an 8x10 photo is less than the depth of field on a 6x4 if the film negative is the same size?  If so, then that would explain why you can only blow up a photo so far, until you get to a point where the depth of field is so insanely small it all looks out of focus?

Thank you for being so smart and so patient with me.  I find stuff like this interesting, especially when its (somewhat) applicable to my hobby.


----------



## Alex_B (May 17, 2008)

Senor Hound said:


> Okay, that's what I thought.  With that being said, does that mean the depth of field on an 8x10 photo is less than the depth of field on a 6x4 if the film negative is the same size?  If so, then that would explain why you can only blow up a photo so far, until you get to a point where the depth of field is so insanely small it all looks out of focus?
> 
> Thank you for being so smart and so patient with me.  I find stuff like this interesting, especially when its (somewhat) applicable to my hobby.



no image is tack sharp the more you enlarge the more you will realise grain and general unsharpness, *if you view it from the same distance.* But an 8x10 you will usually automatically view from a larger distance than a 6x4, hence the effect appears less pronounced or even becomes irrelevant. You usually fill your field of attention with the whole image, instinctively.

only if you go to very large print sizes, you will often not step that far back, and hence you realise the limited resolution.


----------



## Sark (May 17, 2008)

Two identical focal length lenses when set to the same f-stop produce exactly the same DOF. I'm not sure why Alex-B is introducing print size into the equation, because it's not relevent to the DOF, only the image sharpness.

The confusion with DOF when discussing sensor size is due to the fact that a smaller sensor requires a shorter lens to match the field of view of a larger lens used with a larger sensor.

Sensor size alone has no effect on DOF.

Sark


----------



## Sark (May 17, 2008)

Alex_B said:


> Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).


A smaller 10MP APS sensor needs to be enlarged no more than a 10MP full frame sensor. If by small and  larger your refering to MP, then yes, that effects image size, but not DOF. 
Sensor size, or MP alone has no effect on DOF.

Sark


----------



## Alex_B (May 17, 2008)

Sark said:


> Two identical focal length lenses when set to the same f-stop produce exactly the same DOF. I'm not sure why Alex-B is introducing print size into the equation, because it's not relevent to the DOF, only the image sharpness.



define image sharpness without print size.  what appears sharp subjectively depends on print size.
you can define sharpness of course with respect to the recording medium, but this in practice is of little use.


----------



## Alex_B (May 17, 2008)

Sark said:


> A smaller 10MP APS sensor needs to be enlarged no more than a 10MP full frame sensor. If by small and  larger your refering to MP, then yes, that effects image size, but not DOF.
> Sensor size, or MP alone has no effect on DOF.
> 
> Sark



we do not need to include MP in this discussion since they do not tell much about resolution or sharpness. Of course MP define a certain limit, just as grain does. but we can assume in what we are talking about here sharpness is not limited by MPs.

from a smaller recording sensor to a certain size on screen or in print means more enlarging than from a larger sensor to the same size. And by large and small i only refer to physical size of the recording medium. nothing else.


----------



## Sark (May 17, 2008)

Alex_B said:


> define image sharpness without print size. what appears sharp subjectively depends on print size.
> you can define sharpness of course with respect to the recording medium, but this in practice is of little use.


Yes, I said that ..."*it's not relevent to the DOF, only the image sharpness.*" ..but image sharpness is not the same as DOF.

It's also true that image resolution is not a factor of the pixel count, but again, image resolution ( optical, or otherwise) is not relevent to DOF, only image sharpness. 

To clarify. Are you suggesting image sharpness effects DOF.

Sark


----------



## Alex_B (May 17, 2008)

ok, maybe I made sloppy use of the word sharpness here. my apologies.

_DOF in a sense refers to what appears sharp, not blurry, it refers to the detail on objects being resolved or not._ I agree that way of phrasing it is not very precise.

So my basic point is, in order to define DOF via the circle of confusion on the recording medium (be it film or a sensor), the size of the recording medium (format) has to be taken into account. For a smaller format, the circle of confusion will be smaller since it will be enlarged more than a larger format.

On 35mm film this circle might be .03mm, then it will be .019mm on a 1.6 crop sensor. This means it gives the same sized circle in the final image on screen or print.

At least this is my understanding of this.

You never view the image at its original size, you always enlarge.


----------



## Sark (May 17, 2008)

Originally Posted by *Senor Hound* 
_I'm not talking about field of view, I'm talking about depth of field. *Why does a digital camera have a shorter depth of field when using the EXACT same lens and aperture as a film camera?*_



Alex_B said:


> Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).


The above is why I had some confusion (and disagreement) with your reply. You seem to have suggested print size and image sharpness effect Depth of Field. I dispute this. DOF relates to sharpness I agree, but image sharpness is not the same as DOF. 
The original post was discussing the relevence of sensor size on DOF. I would still argue that sensor size has no effect on DOF. 

Sark


----------



## Alex_B (May 17, 2008)

Sark said:


> The original post was discussing the relevence of sensor size on DOF. I would still argue that sensor size has no effect on DOF.



And I would say it has. 

And that effect of sensor size alone points just the opposite way than the effect of focal length:
So when you want the same FOV, you will need a longer lens for the larger sensor. The longer lens will reduce your DOF. 

And this effect is larger than the sensor size effect. So overall, if you try to take the same image, same composition, you need the same FOV, meaning a longer lens for the larger sensor, meaning a net reduction in DOF.

All assuming the same f-stops being used of course.


----------



## Alex_B (May 17, 2008)

He, I like this discussion, but I have the feeling we are confusing those watching us. Where is _Helen _when you need her??


----------



## Sark (May 17, 2008)

Quote:
Originally Posted by *Sark* 


_The original post was discussing the relevence of sensor size on DOF. I would still argue that sensor size has no effect on DOF. _




Alex_B said:


> And I would say it has.
> 
> And that effect of sensor size alone points just the opposite way than the effect of focal length:
> So when you want the same FOV, you will need a longer lens for the larger sensor. The longer lens will reduce your DOF.
> ...


I guess it depends on whever you are discussing DOF from an optical point of view, or from an image point of view. I think it can get confusing if you are refering to DOF in relation to the final image. There are so many factors that determine the quality of the detail in both the intended out of focus part of the image and the intended in focus part of the image.

I do agree it's relevent and your argument may be right from a practical point of view, but technically speaking, DOF is still a factor of the focal length rather than the sensor size. The fact that sensor size can effect focal length choice doesn't directly infer sensor size effects DOF. Two identical lenses at the same f-stop will always produce the same DOF.

Sark


----------



## Alex_B (May 17, 2008)

OK, I just did some googleing and want to cite some things:

"An acceptably sharp circle of confusion is loosely defined as one      which would go unnoticed when enlarged to a      standard 8x10 inch print, and observed from a standard viewing      distance of about 1 foot."
[http://www.cambridgeincolour.com/tutorials/depth-of-field.htm]

And if you use their DOF calculator and just vary the format size while all else is fixed, and you will see the effect of format size.


----------



## Sark (May 17, 2008)

Will check this out, but must go for now, but will hopefully get back to this. Would be nice to read some input from others. I guess they're all on the Nikon v's Canon thread. 

Sark


----------



## Alex_B (May 17, 2008)

Sark said:


> I guess they're all on the Nikon v's Canon thread.



:lmao:


----------



## RyanLilly (May 17, 2008)

I guess all that has been said above makes pretty good sense with the clarification. Good topic, I enjoyed it.

Holy crap Alex, I replied to this topic yesterday, went to sleep, woke up today, and your still online talking about it! I guess thats all in the same day for you, but man, you a trooper, and its not even time for diner yet! :mrgreen:


I'm sure Helen and dispel all of our deep thinking and ramblings with a swift easy to understand reply. 
Its still early in New York, I guess we can giver her some time to finish her coffee.


----------



## Alex_B (May 17, 2008)

RyanLilly said:


> I guess all that has been said above makes pretty good sense with the clarification. Good topic, I enjoyed it.
> 
> Holy crap Alex, I replied to this topic yesterday, went to sleep, woke up today, and your still online talking about it! I guess thats all in the same day for you, but man, you a trooper, and its not even time for diner yet! :mrgreen:



Right you are 



> * I'm sure Helen and dispel all of our deep thinking and ramblings with a swift easy to understand reply. *
> Its still early in New York, I guess we can giver her some time to finish her coffee.



That is my fear .. errm, hope I mean!


----------



## Senor Hound (May 18, 2008)

Alex_B said:


> OK, I just did some googleing and want to cite some things:
> 
> "An acceptably sharp circle of confusion is loosely defined as one      which would go unnoticed when enlarged to a      standard 8x10 inch print, and observed from a standard viewing      distance of about 1 foot."
> [http://www.cambridgeincolour.com/tutorials/depth-of-field.htm]
> ...



Maybe you are arguing the same point.  Perhaps he's saying the depth of field would be the same if there was the same percentage of enlargement for each medium.  For example, if you made the film photo a 12x9 and the digital 8x6, they would have the same depth of field.

In other words, what I think he's saying is its not the film or sensor that's changing the depth of field, its the resulting extra enlargement necessary to compensate that's the difference.

Then again, maybe he's not.  I will say as the OP of this thread I am very thankful to Alex, because he taught me something (that isn't easy to teach, also), and because he's just a cool cat from what I know of him 

BTW, I don't know if Sark is wrong, but I do know Alex is right, now that he explained it to me and I've looked at other sources to help clarify (and consequently confirm).


----------



## Sark (May 18, 2008)

Last night I read the Depth of Field Tutorial Alex linked and began to doubt some of my own preconceptions. This article suggests Focal length has no effect on DOF. There is a link to further reading matter that claims to prove this fact. However, the DOF calculator clearly contradicts this by changing the DOF when altering the focal length. DOF is clearly shown to decrease with an increase in focal length. I then re-read the article and it appears that this statement is based on the following: _If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length. _I dont disagree with this, but you cant make a statement like _focal length has no effect on DOF_ and then base that statement on more than one variable.
Photographer dont, or rarely at least, change focal length to retain the same FOV.

If you accept *Depth of Field* relates to how much of the foreground and background of an image appears in focus (or out of focus), then how much of the image circle your sensor uses doesnt change the foreground and background DOF. The DOF is the same across the entire image circle. It is therefore the same across the entire sensor area, regardless of sensor size. Only the *Field of View* changes with changes in sensor size. The sensor does no more than crop the image circle.

If you trim a piece of 35mm film so that it is the same size as an APS sized sensor.it wont change the DOF of the remaining smaller piece of film, and trimming/cropping the film area is all youre basically doing to the image circle when using a smaller sensor.

If you want to introduce image sharpness into the equation, then print size becomes relevant, but this is just another variable. Ultimately, sensor size cannot change the information on the image circle, and therefore has no effect on DOF within that image circle. Unless I'm wrong of course?:hail:

Sark


----------



## Alex_B (May 18, 2008)

Sark said:


> If you accept *Depth of Field* relates to how much of the foreground and background of an image appears in focus (or out of focus),



But "appear in focus" always relates to print size and viewing distance.


----------



## Alex_B (May 18, 2008)

Sark said:


> _If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length. _I dont disagree with this, but you cant make a statement like _focal length has no effect on DOF_



That statement has to be seen in the context it was given. And it in fact has little to no effect if you make the subject occupy the same fraction of the viewfinder and given the same format of the recording medium. Because then, if you change the focal length, you will also change the distance to the subject, and the effect of using a shorter lens is compensated by the effect of getting closer to the subject by the same fraction.

If, however, you chose a different focal length in order to maintain the same FOV on a smaller or larger format of the recording medium, then this implies that you remain at the same distance to the subject. Hence DOF will vary.


----------



## Alex_B (May 18, 2008)

Sark said:


> Photographer dont, or rarely at least, change focal length to retain the same FOV.



This is exactly what you do. This is why with your medium format camera you have a totally different set of lenses than with your 35mm small format. And this is also why zoom lenses made in particular for crop sensors, are often shorter than their 35mm counterparts.

Assuming that you as a photographer compose the shot, then you will control two things composition wise. And those are perspective and framing.
Now, the perspective depends on the distance to the subject and on the direction your camera points at, and the framing depends on FOV and again the direction.

So in order to get the same composition on two different formats, you will need two different focal lengths respectively so you can keep your FOV identical.

Of course I am assuming that the photographer controls his composition, and does not just let it happen randomly


----------



## Alex_B (May 18, 2008)

Senor Hound said:


> and because he's just a *cool cat* from what I know of him



Thanks, but *stray dog* describes me better I would guess


----------



## Sark (May 18, 2008)

Alex_B said:


> But "appear in focus" always relates to print size and viewing distance.


 
I agree. I guess the discussion depends on how many variable you bring into the equation. My opinion is based on the image at capture. After this there are various factors that determine image sharpness. These other variables are trickier to quantify. A full frame sensor can produce near flawless detail up to, say A4. But so can an APS sized sensor. Of course it is also true that once print size effects detail, it will effect it to a greater extent with images from the smaller sensor. And I guess this is what you are saying.

Now I think I'll go look for that Nikon v's Canon thread.

Sark


----------



## jamesino (May 18, 2008)

_&#8220;If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length&#8221;.

_But in order for a shorter lens to have the same subject magnification as a longer lens, the shorter lens has to be closer to the subject. Therefore, the focal distance is decreased and since a decreased focal distance decreases the DOF, it's not really a far comparison between a shorter and longer lens.


----------



## Helen B (May 18, 2008)

Depth of field is such a slippery thing.

Here are some simplified relationships (they are not totally accurate in all situations):

Depth of field is

1 - proportional to the square of the subject distance

2 - inversely proportional to the square of the focal length

3 - proportional to the f-number

4 - proportional to the diameter of the acceptable circle of confusion

It's easiest to make comparisons by varying two things at once. For ease of calculation consider a 4/3 sensor and a full frame sensor, and hold 1 and 3 constant (ie f-number and subject distance). The 4/3 sensor has a 'crop factor' of 2, so to get the same size print the image needs to be enlarged twice as much as an image from the full frame sensor. This means that the circle of confusion must be half the diameter (to end up with the same size blur circle in the print).

Therefore the DoF would be halved by switching from a full frame sensor to a 4/3 sensor. To keep the same angle of view from the same camera position, the focal length of the lens needs to be halved for the 4/3 sensor. As the DoF is inversely proportional to the focal length, halving the focal length quadruples the DoF. Therefore the DoF is halved, then quadrupled - which means that the overall effect is to double it by switching to the smaller sensor and keeping the same camera position and field of view.

(It might be worth noting that the diffraction-limited depth of field is unchanged by sensor size - the larger the sensor the more you can stop down for the same amount of blur caused by diffraction if you hold the print size constant.)

How does that sound?

Best,
Helen


PS I'm on holiday


----------



## Sark (May 19, 2008)

Alex...It seems we posted at the same time yesterday, so I did not read all your replies.

Originally Posted by *Sark* 

 
_Photographers dont, or rarely at least, change focal length to retain the same FOV._



Alex_B said:


> This is exactly what you do. This is why with your medium format camera you have a totally different set of lenses than with your 35mm small format.
> 
> So in order to get the same composition on two different formats, you will need two different focal lengths respectively so you can keep your FOV identical.


True in that context, but thats not what I was referring to. A photographer using, say a Nikon D3, doesnt change the focal length with the intention of then changing the focal distance to compensate for the change in FOV. In a real life photography situation the point of using a different focal length is to change the FOV.
In the context of a change in focal length compensating for sensor size, or format size, I agree that it is relevant. Maybe thats where we are at crossed purposes. Im not basing my statements on more than one variable. The original post stated... 

_It says that a digital camera with a 1.5 or 1.6 sensor will have less of a depth of field *at any given aperture and focal length* than the film camera (*i.e. 50mm at f/1.8*). I told someone else here this, and they disagreed with me, and said *the* *same lens* should have the same depth of field no matter what the camera is, cause they have* the same lens*, so there shouldn't be any possible way for it to have a different depth of field._

If by *same *lens, and *any given *focal length, it meant *equivalent* lens/focal length, then yes, sensor size effects DOF because sensor size is affecting a change in the focal length. As I read it, the original post did not infer this, but assumed sensor size alone could affect DOF. 
DOF on the image circle doesnt change with a change in sensor size. Not unless you change the focal length to compensate for the smaller sensor. And this was not suggested.

Of course print size, among other factors, has a perceived effect, but that, as I have already stated, is a difficult one to accurately quantify. That is why I have tried to avoid the relevance of print size etc, in this discussion. However, I now accept it is an error on my part to separate optical DOF and image resolution determined by other factors. Reading more technical info on DOF, it is clear that these two factors are not generally discussed separately.

Personally, I would still argue that for the sake of clarity, DOF as a result of focal length and aperture setting, and image resolution from other factors, are separate issues. In a practical sense at least. 

Consider the following situation. Two identical cameras with the same sensor size are focused on the same subject. Camera one has a high quality 50mm optic set to f 2. Camera two has an average quality 50mm optic set to f 16. Camera one would produce the better image resolution, but I believe most photographers viewing both resulting images displayed at the same size would consider camera twos image as having the greatest depth of field. This may not be scientifically correct, but in my experience, most photographers relate DOF to the effect of the aperture setting.
Even at identical aperture settings, focal length effects DOF, but not necessarily optical resolution. Therefore the perceived image resolution at the important field of focus can remain high, whilst the background can appear more, or less, out of focus, due solely to the focal length affecting DOF.
This, at least, is how I have always percieved it.

Sark


----------



## Helen B (May 19, 2008)

Sark said:


> DOF on the image circle doesnt change with a change in sensor size. Not unless you change the focal length to compensate for the smaller sensor.



DoF also changes if you wish to end up with the same print size from different sensor sizes, if everything else is held constant. DoF is not solely a property of the lens, it is also a property of the degree of magnification of the image.

Best,
Helen


----------



## Dao (May 19, 2008)

This is a very interesting subject.

From what I read, you mean the DoF is related to the circle of confusion.  The smaller the Circle of confusion, the sharper the image (in focus).  Since the smaller sensor increase the size of the circle of confusion (at the finial print).  So that is why it affect DoF (assuming other are constant)

Is that right?


----------



## Senor Hound (May 19, 2008)

Dao said:


> This is a very interesting subject.
> 
> From what I read, you mean the DoF is related to the circle of confusion.  The smaller the Circle of confusion, the sharper the image (in focus).  Since the smaller sensor increase the size of the *circle of confusion* (at the finial print).  So that is why it affect DoF (assuming other are constant)
> 
> Is that right?



Perhaps we should call this thread, the thread of confusion...


----------



## Ben-71 (May 20, 2008)

The original question was about DOF, as influenced by the lens' focal length and the sensor 
or film format.​ 
I see that DOF is regarded, by some here, as the perceived DOF in the 
end product &#8211; the enlargement. ​ 
When the DOF button in my camera is pressed, the aperture closes to the preset value and 
the DOF is visible in the viewfinder. 
I kept a few 1K Watt lights in my studio just for this purpose &#8211; to check DOF on a View camera 
screen or a viewfinder (it takes lots of light to check DOF at small apertures in 1:1+ Macro shots).​ 
So, it was the DOF-chicken that came before the egg-largement... and -- "_appear in focus_" 
certainly does *not *"_always relates to print size and viewing distance_".
DOF is very much there, before the image is enlarged. ​ 
By changing focal length, we change the angle of view, so we change the DOF.
Changing the format of a sensor (or film) is another way of changing the angle of view, hence a 
change of DOF.​ 
What if, instead of comparing two sensors of different sizes, we compare two sensors of 
the same size, but with different pixel number? &#8211; Would the 'lesser' sensor give less DOF? 
No. Only the resolution of the final image would change.​ 
When the day will come and a yet smaller, more advanced sensor, with 
10 times more pixels, will replace the larger sensor that we now have, 
would it change the DOF?
It would, because its smaller size will change the angle of view, while its larger pixel number 
will just allow larger & sharper enlargements.
What we see in the viewfinder (through the same lens), when we press 
the DOF button on the camera, will look exactly the same.​ 
If magnification enters the DOF definition, then what about the lens' resolution?
 &#8211; Would a super-sharp lens give more DOF than a bad one of the same focal length & aperture? 
It wouldn't. 
(Now I see that *Sark* already pointed to this)​ 
So, the DOF is there, before other factors such as enlargement size, 
viewing distance, contrast, and quite a few others, which influence the 
end product, enter the game.​ 
Enlargement introduces yet more variables. 
Even the choice of material (type of paper, canvas, or screen resolution, etc',), influences the 
perceived DOF.​ 
True, by enlarging, part of the basic dots grows beyond 0.02mm or so, but this is just one of 
the variables that change the resolution in general &#8211; perceived DOF included &#8211; in the final 
enlargement. 
Such factors are additional to, and not part of, the DOF, because enlarging does not change 
the proportion between sharp and un-sharp according to distance. 
It makes everything equally less sharp. ​ 
What if a sensor (or, say, a 6x4.5 back on a 4"x5" view camera) crops so that just the sharp part 
out of the DOF is in the final image? Did it increase DOF?
Wasn't there DOF to begin with, but we introduced another element, which is not relevant to DOF ? ​ 
The perceived DOF in the final image is created by many variables that have nothing to do with 
the DOF that was created by the lens & the recorded area of the image circle.​ 
It seems to me that this debate is mainly about semantics.
There's *DOF*, and there's *perceived DOF in the final enlargement* which is influenced by both 
the DOF and by other variables which are not part of the DOF.​ 
Everyone who disagrees with this is kindly requested to return my 2 cents ​


----------



## Alex_B (May 20, 2008)

OK, i have to comment on this: There is definitely nothing like absolute DOF. DOF is *always* related to what we perceive with our eyes. DOF is about perceiving with your eyes! If your eyes had infinite resolution, then DOF would always be perceived as being zero. But lucky as we are, our eyes have only a finite resolution and we do not realise how things in front of the plane the lens focussed on and behind that plane are in fact out of focus. But we just cannot resolve it, so it all seems in focus. This is called DOF then.

If you assume no enlargenment, then you would just look at the unenlarged film, and DOF will be larger than in the enlargement. Actually, on unenlarged 35mm film almost every shot is both in focus and has infinite DOF if you look at it with your bare eyes.

If you look through your viewfinder with your DOF preview button, the percieved image you see is much larger than when you look at the original film or sensor size.


----------



## Alex_B (May 20, 2008)

Ben-71 said:


> the *DOF that was created by the lens & the recorded area of the image circle*.​



This is something which in principle is not and cannot be defined in a sensible way.

If you assume no enlargement, then you have an enlargement factor of 1 which is just a special case of enlargement. This is the case for contact prints. But unless you are into stamps and other miniatures, this makes no sense for small format film.


----------



## Dao (May 20, 2008)

Quoted from Wikipedia

"The image format size also will affect the depth of field. The larger the format size, the longer a lens will need to be to capture the same framing as a smaller format. In motion pictures, for example, a frame with a 12 degree horizontal field of view will require a 50 mm lens on 16 mm film, a 100 mm lens on 35 mm film, and a 250 mm lens on 65 mm film. Conversely, using the same focal length lens with each of these formats will yield a progressively wider image as the film format gets larger: a 50 mm lens has a horizontal field of view of 12 degrees on 16 mm film, 23.6 degrees on 35 mm film, and 55.6 degrees on 65 mm film. What this all means is that because the larger formats require longer lenses than the smaller ones, they will accordingly have a smaller depth of field. Therefore, compensations in exposure, framing, or subject distance need to be made in order to make one format look like it was filmed in another format."


----------



## Alex_B (May 20, 2008)

Dao, true, but this is the other effect, the lens related DOF effect. 

There is also a (smaller) effect related purely to the format, if you keep the lens at the same length. And this is the one which causes dispute here


----------



## Sark (May 20, 2008)

Just to clarify this whole discussion.

I believe you can differentiate between image resolution and depth of field. Whilst I accept they are connected, I believe you can also discuss them separately. I also believe that many photographers will think like this. 

Science agrees there is a perceived level of sharpness that is based on the CoC. At the capture point, controlling the desired sharp subject elements is generally achieved by focusing. Controlling those elements you desire to be out of focus is determined by the f-stop. The f-stop will also affect the depth of perceived sharpness of those elements near the focus point. But it is also possible for the f-stop to have negligible effect on the important in-focus elements, whilst affecting the desired out of focus elements significantly. A single flower in a field of flowers for example. The f-stop can therefore affect DOF without affecting the perceived image resolution at the important point of focus. In this example, enlargement will affect the in-focus element to a far greater extent than the out of focus elements (a very blurred object is a very blurred object even at a higher magnification). Now I would consider this loss of sharpness at the important point of focus (due to print size) as an image resolution issue, seperate to depth of field. 

Sark


----------



## Ben-71 (May 20, 2008)

*Alex_B*
<< OK, i have to comment on this: There is definitely nothing like absolute DOF. DOF is *always* related to what we perceive with our eyes. DOF is about perceiving with your eyes! If your eyes had infinite resolution, then DOF would always be perceived as being zero. But lucky as we are, our eyes have only a finite resolution and we do not realise how things in front of the plane the lens focused on and behind that plane are in fact out of focus. But we just cannot resolve it, so it all seems in focus. This is called DOF then.>>​ 

Agreed. Except for what is, exactly, the definition of 'DOF'*...*  ​ 
<< If you assume no enlargenment, then you would just look at the unenlarged film, and DOF will be larger than in the enlargement. Actually, on unenlarged 35mm film almost every shot is both in focus and has infinite DOF if you look at it with your bare eyes. >>​ 
Not the DOF will be larger on film than in the enlargement, but, typically 
the grain will be smaller. But, again, I do think that this is a question of 
semantics: Do we include more variables, such as contrast, type of paper, 
etc', or we do not.​ 
BTW, I once had no choice but use 35mm for 1~1.5m B&W prints. 
I used a special super-high resolution Kodak film (I think it was the TP1524 
or something, a line film that could also be developed [in Rodinal] as full-tone). 
The only grain you could see, was the paper's grain.
This, I think, strengthens my conviction that enlargements do not change the DOF,
they can only introduce OTHER variables, which may change the perceived DOF 
in the end result.​ 

<< If you look through your viewfinder with your DOF preview button, the percieved image you see is much larger than when you look at the original film or sensor size. >>​ 
Why much larger? Nikon's specification for the D300 *:*
Viewfinder Frame Coverage -* Approx. 100%* (vertical and horizontal)
Viewfinder Magnification -Approx. *0.94x* with 50mm lens at infinity​ 
And, on the Mamiya 6x7 and the Sinar 4"x5" that I used, the matt glasses 
were 1:1 with the film format.​ 
<< This is something which in principle is not and cannot be defined in a sensible way.>>​ 

The definition of DOF is, of course, an arbitrary one, so how about this simple, arbitrary definition &#8211; 
"A dot up to *X*mm on the recording media (film or sensor) is sharp. A dot larger 
than *X*mm is not sharp" ? 
(I didn't invent this "wheel") ​ 
<< If you assume no enlargement, then you have an enlargement factor of 1 which is just a special case of enlargement. This is the case for contact prints.
But unless you are into stamps and other miniatures, this makes no sense for small format film.>>​ 

I don't think so. It is a special case of 1:1 enlargement if a 1:1 enlargement 
is made. Even a contact print can introduce more variables such as contrast, 
or the type of paper used - variables which can change the perceived DOF.​ 
I was talking about no enlargement, as every enlargement introduces more 
variables, that aren't part of what makes the DOF.​ 
It stil seems to me that this debate is mainly about semantics.​

There's *DOF*, and there's *perceived DOF in the final enlargement.*
The perceived DOF in the final enlargement is influenced by both the DOF 
and by other variables which are not part of the DOF.​

It's 4 cents by now&#8230;  ​


----------



## JamesD (May 20, 2008)

Try this experiment, and I mean physically do it.  Don't just read through.  Doesn't matter if you use a film camera or a digital one, but use the same lens.

At night, go outside and take a slightly out-of-focus image of two small sources of light, such as two street lamps, one behind the other.  Use your absolute widest aperture.  Focus on the closest street lamp.

Print the image three times at 8X12 (or 6.66X10).  The first image, print with no cropping.  The second image, crop to 60% width/height, _but do not resize._  For the third image, resize your cropped image so that it's the same size as your first image (ie, it's an enlargement).

Now, examine the images.  The difference between 1 and 2 is analogous to film vs small-frame digital (neglecting resolution), using the same exact lens and aperture.  Your slightly-out of focus light (the back one) should appear as a small blob, the same physical size in images #1 and #2 (ie, perhaps 1/4 inch across).  Now, examine image #3.  In this case, your blob will be a little less than half an inch across.  Comparing it to #1 is analogous to comparing a film frame to a digital frame, when both were shot with the exact same lens and aperture, and enlarged to fill the same sheet of paper.

The fact is that DOF is about "appearing in acceptable focus."  That phrasing is in the definition.  If, with a 35mm lens at f/2.8, your slightly out of focus blob is .03 inches across on the film or sensor, then it's .03 inches across.  Period.  If the sensor or film is smaller rather than larger, it takes up more area in the frame.  That makes the blob bigger in the final 8X10.  Because it's physically bigger on the paper, it may no longer be in "acceptable focus."

Now, if you want the same field of view, then you'll have to use a shorter lens if the sensor/film is smaller.  In this case (and this is the important part)....

Realize that depth of field is related to absolute aperture size (in, say, inches, not f-stops) and subject distance.  _If the subject is 10 feet away, depth of field will be the same with 100mm at f/4 and 50mm f/2._  In both cases, the aperture is 25mm wide.  If you don't believe that, try it.

If you've ever worked with pinholes at all, you'll find all this stuff becomes terribly obvious.  Bigger pinhole, less "focus."  Bigger frame, more "focus."  Aperture is aperture, whether glass is involved or not.


----------



## Alex_B (May 20, 2008)

Ben-71 said:


> Agreed. Except for what is, exactly, the definition of 'DOF'*...*  ​



I will not repeat it as it is more or less given in the follow up statement of James (and it appeared already earlier in this thread in different phrasing):



JamesD said:


> The fact is that DOF is about "appearing in acceptable focus."  That phrasing is in the definition.  If, with a 35mm lens at f/2.8, your slightly out of focus blob is .03 inches across on the film or sensor, then it's .03 inches across.  Period.  If the sensor or film is smaller rather than larger, it takes up more area in the frame.  That makes the blob bigger in the final 8X10.  Because it's physically bigger on the paper, it may no longer be in "acceptable focus."



Exactly, DOF is defined by "appearing in acceptable focus". That is the photographic definition (which can of course be refined by talking of circles of confusion and so on). And it is the only definition I am aware of. Give us an alternative definition which makes sense, then we can discuss further.

_However in this whole thread I have not found any other definition._

The whole photographic world is blurry, however whenever the blur is so small that we cannot resolve it with our eyes, then it appears sharp to us. This is what photography is like. There is no absolute sharpness, or absolute DOF. Both terms are only defined with respect to visual perception.


----------



## Alex_B (May 20, 2008)

Ben-71 said:


> Why much larger? Nikon's specification for the D300 *:*
> Viewfinder Frame Coverage -* Approx. 100%* (vertical and horizontal)
> Viewfinder Magnification -Approx. *0.94x* with 50mm lens at infinity​



I suggest the following experiment:

1. Shoot a scene from a certain distance with a 50mm lens, develop the 35mm film (I suggest slide film so the colours are not inverted for the comparison in 2.). 

2. Put the camera into the same position, look at the scene through the viewfinder (50mm lens again), and compare what you see in the view finder with what you see when you directly look at the slide without a magnifying glass. The viewfinder image will appear larger. In the viewfinder image you will be able to differentiate between in focus and out of focus (within limits). On the slide you will most likely not be able to resolve it.
The viewfinder image will fill large part of your eye's FOV, whereas the slide you look at directly will only fill a tiny fraction of your eye's FOV. Hence the viewfinder image appears larger, and you can resolve much more detail. Without a magnifying glass you cannot get the slide close enough to your eye so it fills the same FOV as the viewfinder image does.

Really do that experiment, and you will understand.


----------



## Helen B (May 20, 2008)

I agree with Alex. Depth of field is defined solely in terms of perception.

There is no 'DoF and perceived DoF', there is only perceived DoF.

When you look at the ground glass of a 4x5 camera with the naked eye, the image can appear in focus. Put a loupe on the GG and you might see that what looked in focus to the naked eye no longer appears to be sharp. It's all a matter of enlargement.

I find that to get the most out of a film like Technical Pan (it's 2415, by the way) you have to use tighter DoF criteria than with lower resolution films, if you use good lenses and good technique. A high resolution system (lens, film, technique) can show more of a difference between the sharpness in the plane of focus and elsewhere than a lower resolution system would show.

Deep focus lenses were designed to use that property in reverse: if nothing in the image is bitingly sharp, you can get more apparent DoF.



JamesD said:


> Realize that depth of field is related to absolute aperture size (in, say, inches, not f-stops) and subject distance.  _If the subject is 10 feet away, depth of field will be the same with 100mm at f/4 and 50mm f/2._  In both cases, the aperture is 25mm wide.  If you don't believe that, try it.



That isn't in agreement with the usual formulae for DoF, the basics of which are given in my earlier post (#34).

Best,
Helen


----------



## JamesD (May 21, 2008)

Helen B said:


> That isn't in agreement with the usual formulae for DoF, the basics of which are given in my earlier post (#34).



After running numbers, looks like you're absolutely right.  I need to dig up my source on that and find out what I misread or misremember, and also reexamine the photos I took when I tried it.


----------



## Alex_B (May 21, 2008)

Helen B said:


> There is no 'DoF and perceived DoF', there is only perceived DoF.



Thank you, this one sums it up pretty much.


----------



## Ben-71 (May 21, 2008)

Helen B
I agree with Alex. Depth of field is defined solely in terms of
perception.

​
Agreed. We're here for the photographs. However, when we talk about 
the definition of a term, we have to stay within the requirements for what 
a definition is.  

There is no 'DoF and perceived DoF', there is only perceived DoF.


See my next post.

When you look at the ground glass of a 4x5 camera with the naked
eye, the image can appear in focus. Put a loupe on the GG and you
might see that what looked in focus to the naked eye no longer
appears to be sharp. It's all a matter of enlargement.

​
Of course it all starts from human perception. That's how the 0.02mm, 
*in the camera,* was agreed on in the first place. 

The very beauty of the in-camera DOF definition stems from what you say*:*

The DOF is NOT influenced by anything outside the camera, not by
enlargement with all its variables and not by viewfinders, not by ground
glasses and not by how we look at them.
If all these variables were included in the definition of DOF, the definition
would have taken a whole book, to cover every variable, and to set a
standard to every variable.

It seems to me that you ignore the simple fact that there is a number
  a specific size of a dot (0.02mm)  that sets the line between sharp or 
un-sharp, and that this number is NOT a dot on an enlargement, or in a 
view finder, or on a ground glass  it is on film or sensor, in the camera.

Let me try and relay it from another angle  your 'view-angle' of perception'*:*

You look at an enlargement. On it, there're yellow dots on black background.
Their size is the smallest that your individual eyes can resolve.
Now, on the same enlargement, there're also other dots, of exactly the same
size, but they are blue, on green background.
While you can see sharp yellow dots, the blue ones will not look sharp to
your individual eyes.
In order for your individual eye to perceive the blue dots as sharp, they must
be larger.

What does this do to a definition of DOF as the 'perceived DOF on an
enlargement'?
The 'perceived DOF on an enlargement' is elusive. Even how tired you are
changes your perception.
The dot size in the camera, however, is a constant, which allows you to
predetermine DOF.

Technical Pan (it's 2415, by the way)

Thanks... I used it years ago, and just a few times, for very specific job
requirements   for those enlargements from 35 on location, and for large
B&W prints of 1:1+ Macro.
I took a whole day of developing tests to surpass Kodak's resolution
specifications... 
but then, the 2415, with a Nikon F3, bellows and a 20mm (or so) reversed,
gave better resolution than the available films for the larger formats that 
I tested. It was sharp at x15 enlargements. Incredible, except for its 'thing'
with red...

Back to the subject...  Two prints, one from that 2415, one from a regular
film. One is sharp, the other is not. Did the DOF change? 
Should we redefine DOF for each film? The DOF in the camera was the
same. it just wasn't materialized in one of the prints. 

Apparent (to use your word) DOF on an enlargement is a changing thing,
even for the same person at different times. 
It is NOT a technical definition.
The definition of DOF, in the camera, is simple and constant. 
It IS a technical definition.​


----------



## Ben-71 (May 21, 2008)

Originally Posted by *JamesD* 
_The fact is that DOF is about "appearing in acceptable focus." That phrasing is in the definition. _
_If, with a 35mm lens at f/2.8, your slightly out of focus blob is .03 inches _*across on the film or sensor*,
_then it's _*.03 inches*_ across. Period. If the sensor or film is smaller rather than larger, it takes up _
_more area in the frame. That makes the blob bigger in the final 8X10. Because it's physically _
_bigger on the paper, it may no longer be in "acceptable focus."_​ 
1) It is 0.03mm, and not 0.03 inch.
2) It says that DOF is defined in the camera, and not in the enlargement,
which is exactly my claim.​ 
*Alex_B *
Exactly, DOF is defined by "appearing in acceptable focus". That is the photographic definition 
(which can of course be refined by talking of circles of confusion and so on). 
And it is the only definition I am aware of. ​ 
Right. It also defines the size of the dot, in the camera. 
This definition disregards enlargements.​ 
Give us an alternative definition which makes sense, then we can discuss further.​ 

I will, in a moment.​ 
After checking definitions on the Internet, I think that I now see where the
DOC ("Definition Of Confusion" ) comes from.​ 
Obviously, any definition of 'sharp' -- in our context -- must start from the
human eye's resolution.
The dot size, which is the smallest that the best eye-sight can see as sharp,
was chosen for defining 'sharp'.
This dot is about 0.16mm across.​ 
Here. I think, enters the reason for the confusion*:*​ 
Camera manufacturers calculated the DOF *backwards, from the print,*
*back to the camera :*​As most people typically enlarged from 35mm negatives to ~5x7" prints (x5),
in order for a dot to be sharp on this print size, it had to be ~0.033mm in the
camera.

The fact that the calculation was done backwards, from a print to camera,
does NOT mean that the definition of DOF regards the enlarged print.
On the contrary*:*
The x5 enlargement was just a tool that was used, because of its
commonness, to define DOF *on the recording media in the camera.*

Indeed, the 0.02mm or 0.03mm numbers that we see everywhere in
connection with DOF, relate to the *camera,* and not to enlargements.

It is no coincidence that manufacturers defined DOF in the camera rather
than in an enlargement, as too many variables interfere and change the
*perceived DOF in an enlargement.*
Including those variables in the definition of DOF, would have eliminated the
possibility to calculate DOF, and would have made DOF scales on lenses
worthless.

As the 0.02mm or 0.03mm numbers relate* to the camera, *all the other
variables that influence the perceived DOF on the enlarged print* are eliminated.*
Otherwise, there would have been a* standard *for a print that defines DOF.
E.g.: Type of paper (matt or glossy, for instance), contrast, etc'.
*There is no such standard.*

Even if such a standard existed, it couldn't be applied to other techniques of
enlargement, such as projecting slides on screen, computer screens, etc'. 

By definition, a definition should be kept simple, and it is:
DOF is defined by a dot size of 0.02mm ~ 0.03mm in the camera.

This definition kicks out yet another variable*:* The resolution of film or sensor.
The smallest dot that is considered as sharp, in the camera, had been
arbitrarily defined, once.
This is a good definition also because, not only it is not dependant on
enlargement variables, but it is also not affected by future resolution
improvements in the recording media in the camera.
With such improvements, DOF stays the same, while larger & sharper
enlargements will become possible.

You asked for a definition*:*​ 
*DOF is the section of a photograph, on the recording media in a*
*camera, where the details of the nearest to the furthest objects *
*are made of dots not larger than 0.02mm~0.03mm.*
(I say "0.02mm~0.03mm", because both numbers are in wide use. 
I'd rather prefer just one size.)​ 
The *perceived* DOF in an enlargement, depends on many variables, such
as viewing light characteristics, how smooth or how white should the paper be, 
is the dot in B&W or color, which color, and quite a few more.
One of those variables is the in-camera DOF. ​ 
Unlike the simple definition of DOF in the camera, the perceived DOF in an
enlargement *has never been really defined. *​ 
It hasn't been defined, because there's no point in defining it.
Such a definition should have included too many variables, and would have
been too long & complicated, to make it practical.​


----------



## AndrewG (May 21, 2008)

Alex_B said:


> But "appear in focus" always relates to print size and viewing distance.


 
Surely the area in focus, both in front of and behind the subject, will remain the same regardless of print size and viewing distance-it is only grain size and noise/pixellation which becomes more evident as you increase print size. You can't vary DOF in a hard copy once the image is fixed.
Or am I misunderstanding you?


----------



## Alex_B (May 21, 2008)

AndrewG said:


> Surely the area in focus, both in front of and behind the subject, will remain the same regardless of print size and viewing distance-it is only grain size and noise/pixellation which becomes more evident as you increase print size. You can't vary DOF in a hard copy once the image is fixed.
> Or am I misunderstanding you?


well, if you step back while viewing a hard copy, DOF will increase, if you look at a small print from a large enough distance, DOF will be infinite, since all will be acceptably sharp for that viewing distance.


----------



## AndrewG (May 21, 2008)

Alex_B said:


> well, if you step back while viewing a hard copy, DOF will increase, if you look at a small print from a large enough distance, DOF will be infinite, since all will be acceptably sharp for that viewing distance.


 
Surely what you are seeing is increased/decreased perceived sharpness in the print; the closer you are the more grain you see but what is printed cannot move therefore DOF must be constant.


----------



## Alex_B (May 21, 2008)

AndrewG said:


> Surely what you are seeing is increased/decreased perceived sharpness in the print; the closer you are the more grain you see but what is printed cannot move therefore DOF must be constant.



But if you use a magnifying glass on your print, and grain is fine enough, print resolution is high enough, and you lens is a sharp lens, then you will realise DOF will get smaller, the more you magnify.

Of course, if the lens is not sharp anyway or if the grain is too large, then you won't realise this since the effect is mostly swallowed by the limited physical resolution of your system including the print.


----------



## Alex_B (May 21, 2008)

Ben-71 said:


> Camera manufacturers calculated the DOF *backwards, from the print,*
> *back to the camera :*​As most people typically enlarged from 35mm negatives to ~5x7" prints (x5),
> in order for a dot to be sharp on this print size, it had to be ~0.033mm in the
> camera.



And this is the definition of DOF assuming a ~5x7" print being made from a 35mm negative seen from the usual viewing distance. for 5x7 prints.

If you print larger and look at the print, you would normally step back so the print fills about the same fraction of your eye's FOV, and you can approximately remain in the .03mm range for the circle of confusion/sharp dot size or whatever name you want to give it. This is why often the variation with printing size can be neglected since viewing distance will vary accordingly and both variables are thus coupled.

However, if you record on a smaller medium, or on a larger medium, then the .03mm have to be adjusted accordingly to a smaller or larger value accordingly.



> *DOF is the section of a photograph, on the recording media in a*
> *camera, where the details of the nearest to the furthest objects *
> *are made of dots not larger than 0.02mm~0.03mm.*
> (I say "0.02mm~0.03mm", because both numbers are in wide use.
> I'd rather prefer just one size.)​



yes, 0.03 or something of that magnitude for 35mm film and assuming "standard viewing conditions".


for a smaller sensor it will be a smaller circle though, and for a large format camera it will be a much larger circle/dot.


----------



## AndrewG (May 21, 2008)

Alex_B said:


> But if you use a magnifying glass on your print, and grain is fine enough, print resolution is high enough, and you lens is a sharp lens, then you will realise DOF will get smaller, the more you magnify.
> 
> Of course, if the lens is not sharp anyway or if the grain is too large, then you won't realise this since the effect is mostly swallowed by the limited physical resolution of your system including the print.


 
So what we are talking about here is our perception of DOF relative to viewing distance as well as what was captured and printed and which is immovable?


----------



## Alex_B (May 21, 2008)

AndrewG said:


> So what we are talking about here is our perception of DOF relative to viewing distance as well as what was captured and printed and which is immovable?



the discussion is (by now) all about whether there is any absolute DOF which is not related to final visual perception. And I say there is not such a thing.

I almost forgot what the original question of the thread was all about


----------



## Helen B (May 21, 2008)

Most calculations of DoF come from an original idea that a CoC of 0.2 mm is acceptable for an 8x10 print. The acceptable CoC for the film or sensor format is calculated from that. It isn&#8217;t the other way round.

  The value of 0.2 mm can be arrived at by considering visual acuity at the &#8216;comfortable near distance of distinct vision&#8217;, which is about 250 mm. At that distance a high contrast line of 0.075 mm width can just be distinguished. In practice this is relaxed a little, to give a practical value of 0.1 mm. A &#8216;line&#8217; is a cycle between dark and light, so the corresponding acceptable circle of confusion is 0.2 mm (one cycle).

  Here are the commonly used values for film formats based on the 0.2 mm criterion:

  8x10 (inches): 0.2 mm
  4x5 (inches): 0.1 mm
  6x7 (cm): 0.05 mm
  24x36 (mm): 0.025 mm 

  These assume that an 8x10 print made from them is viewed at 250 mm, or a 16x20 at 500 mm etc in proportion &#8211; ie keeping the angular size of the print the same. When you print large, people have a tendency to approach the print more closely, especially when there is fine detail to be found. This raises the required definition in the print, making DoF calculations based on 0.2 mm inadequate. Because CoC and f-number are both linear in the simplified DoF equation you can make an allowance for a smaller CoC by using a smaller f-number. For example, if you have a lens that has a DoF scale based on a CoC of 0.033 mm you can use it for a CoC of 0.025 mm if you use the f-number markings for the next smaller f-number in full stops. If you were using the lens at f/16, you could use the DoF markings for f/11.

  Over the years the CoC used to compute the DoF markings for Zeiss lenses intended for 35 mm still photography has decreased from 0.033 mm to 0.030 mm to 0.025 mm. This has been documented by Zeiss. The decrease is largely due to improved film and lens resolution. The same has happened in the movie world &#8211; we are now using smaller CoCs that we did when I started over thirty years ago.

  Here&#8217;s what David Samuelson (who designed one of the more popular DoF calculators for use with motion picture cameras) has to say:

_Cameramen should be selective in choosing the depth of field they work to, using 0.05 mm or more when the circumstances are easy, 0.025 mm as a middle-of-the-road value and 0.0125 mm, or even less, when tolerances need to be tight. _

  He&#8217;s referring to 35 mm motion picture formats.


Best,
Helen


----------



## JamesD (May 21, 2008)

I had a reply typed up for Ben-71's post, complete with usage of the block-quote feature.  Then I decided that I'm not going to bother with all that, I'll just sum it up.

Ben, I'd like to borrow your dictionary, and I'm curious about the sources of your figures and methodologies, because they don't seem to match up very well.

Additionally, you seem to be referring to circles of confusion as depth of field, which doesn't make sense.  Depth of field is a phenomenon occurring along the axis of the optical system opposite the viewing/recording side.  The circle of confusion is a phenomenon occurring perpendicular to the axis of the optical system on the viewing/recording side.

Also, even if your methodologies and figures are accepted for photography, they fail to explain the phenomenon in any non-recording optical system--for instance, binoculars.

Beyond any of that, large prints are intended for viewing from large distances.  That's rather the point of having a 16X20 print rather than a 4X6.  You can hang it on the wall and look at it from the couch.

One more thing...


> 1) It is 0.03mm, and not 0.03 inch.
> 2) It says that DOF is defined in the camera, and not in the enlargement,
> which is exactly my claim.


Thanks for trying to change my meaning for me, but no thanks.  I said .03 inches, and I meant .03 inches.  I'm fairly certain of this size, as I measured it with my ruler.


----------



## Ben-71 (May 22, 2008)

​
*JamesD*
Depth of field is a phenomenon occurring along the axis of the 
optical system opposite the viewing/recording side. 
The circle of confusion is a phenomenon occurring perpendicular 
to the axis of the optical system on the viewing/recording side.​ 
​Breaking news:
The world is 3D.
Optical phenomena occur along the axis of the optical system.
But, a photograph is on a flat plane that &#8211; believe it or not &#8211; is 
perpendicular to the axis of the lens... ​ 
​
....you seem to be referring to circles of confusion as depth of field, which doesn't make sense.​I suggest that you read my words again*...*
Indeed, circles of confusion are not DOF... they are, though, 
a little bit perpendicular to the perception of it... ​ 
This is just one of many places where you can check it 
(inch or mm included) -
http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml​
I said .03 inches, and I meant .03 inches. I'm fairly certain of this 
size, as I measured it with my ruler.​Well... if you insist... but if your lens/sensor give circles of confusion that 
are *that* big (*0.76*mm*!*), I wonder why you keep it&#8230; ​​​


----------



## Ben-71 (May 22, 2008)

Helen,​ 
It seems that we talk on parallel lanes.
Some others here 'volunteered' words into my mouth.​ 
Of course COC is different for different formats. 
I've been sticking to 35mm as a case study.
Of course it's all about perception and the final enlargement. 
This is all common knowledge.​ 
What's missing is reference to my claim, which is*:*​ 
DOF is just one of the variables that influence our perception of depth 
in an enlarged photograph.​ 
It doesn't matter if the tables are for enlargement format & lens, or for 
sensor format & lens.​ 
It was practically pointless to try and isolate a DOF definition from the 
numerous factors that influence the perception of depth in an enlargement. ​ 
Therefore, the definition of DOF per se, was moved to the camera level, 
because only here there can be a simple, calculable definition of DOF, 
isolated & free of many other factors.

So far, are we on the same wave length?

As you well know, a bad composition can completely spoil the perception 
of depth, if the darks & lights, for instance, are wrongly placed.
A photograph can include DOF according to the definition of DOF per se, 
but at the same time it can give no perception of depth.

Therefore, the perceived DOF, in an enlargement, is not identical with the 
definition of DOF per se, so these are two different things.

Best,

Ben​​


----------



## Helen B (May 22, 2008)

Ben-71 said:


> Therefore, the definition of DOF per se, was moved to the camera level,
> because only here there can be a simple, calculable definition of DOF,
> isolated & free of many other factors.​



Ben,

While agreeing in principle, I would put a slightly different slant on that: The practical formulae for the _calculation_ of depth of field, which are not a definition of depth of field in themselves, use the circle of confusion for the camera image because that is the easiest way to do it. The choice of circle of confusion is, however, based on the final image and the way in which it is intended or expected to be viewed.

The discussion appeared to be about whether or not there was an absolute value for DoF as well as the perceived value. We were trying to say that there was no such thing as an absolute value for DoF because it was all perceived, and you appeared to be arguing with that.

Best,
Helen

Here, for interest, are the words of Rudolf Kingslake, the former Director of Optical Design for Eastman Kodak who later became an Emeritus Professor at the University of Rochester:

_"In order to derive formulae with which the depth of field of a lens can be calculated in any particular case, we must first standardize the manner in which the final picture is to be viewed, since obviously a slight blurring of out-of-focus objects may be quite invisible to someone standing at the other side of the room and yet be distressingly evident when the print is examined closely. As this factor is by far the most important in any depth calculation, we shall treat it in three different ways: (a) we can take into account the actual distance of the observer from the final print or projected image, (b) we can assume that the final picture will be observed from the correct center of perspective, or (c) we can adopt a fixed circle of confusion on the film, the value of which will, of course, depend on the dimensions of the negative."_


----------



## Ben-71 (May 22, 2008)

Sorry Helen, I must go now.

Please note the adition at the end of my post.

I'll be back tonight (my time...) or tomorrow.

Best,

Ben


----------



## Helen B (May 22, 2008)

OK. The whole thing appears to have been a misunderstanding over what you meant by 'DoF and perceived DoF'.

What you call 'perceived DoF' I would call 'appearance of depth' or 'depth perception' or somesuch, and I would reserve DoF for a more geometrical optics-based phenomenon related solely to the appearance or otherwise of blur. 

'DoF and depth perception' maybe, because they are two slightly different phenomena worthy of two slightly different descriptions, I think.

Best,
Helen


----------



## Sark (May 23, 2008)

Ben-71 said:


> Unlike the simple definition of DOF in the camera, the perceived DOF in an
> enlargement *has never been really defined. *


Not defined, but described. It&#8217;s called image resolution. This is how I have always understood it. DOF and image resolution (_image_ as in the final output image), are considered separately. 




Alex_B said:


> yes, 0.03 or something of that magnitude for 35mm film and assuming "standard viewing conditions".


 Apparent size appears in direct proportion to viewing distance. Double the true size and then double the viewing distance, and apparent size remains constant. This is the basis of perceived DOF.

Nobody can dispute the above statement, but I personally question the relevance of perceived DOF in the practical application. If you accept there is a true &#8220;standard viewing distance&#8221;, then in theory at least, the only format you would ever need to use is 35mm. You could enlarge to your hearts content. Viewing distance would take care of the perceived DOF. This begs the question; Why the popularity of medium format, or 5x4. Why did the great landscape photographers use 10x8. The answer is &#8220;image resolution&#8221;. We don&#8217;t view from a &#8220;standard viewing distance&#8221;, or even a single viewing distance. Yes, you can set values that function for the maths, but they&#8217;re rarely relevant in the practical application of photography.

I guess this is now just a discussion about how you perceive the concept. 




JamesD said:


> I said .03 inches, and I meant .03 inches. I'm fairly certain of this size, as I measured it with my ruler.


Is it one of those wooden rulers, with the really thick markings. You have to be careful with those. 




Ben-71 said:


> Well... if you insist... but if your lens/sensor give circles of confusion that are *that* big (*0.76*mm*!*), I wonder why you keep it&#8230;


Ben, just so this doesn&#8217;t get petty or personal. The 0.03inch JamesD  referred to was in relation to the &#8220;blob&#8221; in his own experiment, not the 0.02mm being discussed in relation to CoC.

Sark


----------



## Alex_B (May 23, 2008)

Sark said:


> Apparent size appears in direct proportion to viewing distance. Double the true size and then double the viewing distance, and apparent size remains constant. This is the basis of perceived DOF.




Exactly.

I am aware that "standard viewing" is something rather soft, not easy to define with numbers without an uncertainty margin  

Any human, given the freedom to move around freely, will normally watch a photograph or a painting from a distance such that it fills a certain fraction of his personal field of view. You can see this in museums, that people stand really close in front of small images and at quite a distance from those huge wall-sized images.

However, I agree, there are situations when people deviate from this rule:

1. You cannot step back enough, since there are limitations in available space (small room, etc.)

2. An image as a piece of art contains several smaller scenes, which are interesting to look at closer. 

3. There are fine structures in the image, which make people get closer to investigate.

4. We should not forget that landscape photography plays a special role, since unlike most other types of photography it is not focussed so much on objects (and here I also count people as objects), but on large scenes. When looking at a "large scene", preferentially shot in wide angle, then people tend to step into the scene, they want to become part of it. Then the standard viewing conditions do not apply, since you will be much closer to the large image and hence the image will be much larger than your personal FOV. To see the whole image you will have to actually move your head sometimes. So this is a special case which does not apply to portrait, wildlife, fashion and other types of photography. Sometimes it plays a role in architectural photography though.



> Nobody can dispute the above statement, but I personally question the relevance of perceived DOF in the practical application.


Oh, I think it is the only relevant type of DOF in practical application.
 Just for some you will have to adjust the numbers since your print size to viewing distance ratio will be totally different from what is usually assumed as standard (see my explanation above).


----------



## RyanLilly (May 23, 2008)

Alex_B said:


> well, if you step back while viewing a hard copy, DOF will increase, if you look at a small print from a large enough distance, DOF will be infinite, since all will be acceptably sharp for that viewing distance.



Just like when you look at a picture on your cameras LCD and it looks sharp and in focus, only to find out that its blurry and out of focus when you view it on your computer.


----------



## Sark (May 24, 2008)

RyanLilly said:


> Just like when you look at a picture on your cameras LCD and it looks sharp and in focus, only to find out that its blurry and out of focus when you view it on your computer.


Exactly, but I would considered the enlared imaged had poor image resolution, due possibly to focusing, etc. I wouldn't consider it had bad DOF.

As I have already stated. This is really just a discussion about how you perceive the concept.

*Sark said...* _Nobody can dispute the above statement, but I personally question the relevance of perceived DOF in the practical application. _


Alex_B said:


> Oh, I think it is the only relevant type of DOF in practical application.
> Just for some you will have to adjust the numbers since your print size to viewing distance ratio will be totally different from what is usually assumed as standard (see my explanation above).


Firstly, let me just clarify that when I said "perceived DOF in the practical application", I was referring to the print. Which I think you understood. What I'm suggesting is that DOF as an in focus, or out of focus consideration at the capture stage is of course very important. And it is this interpretation of DOF most photographers tend to consider. I'm just not convinced that perceived DOF in the print is a consideration during capture. Not for most photographers. Image resolution in the final image, yes. 
To quote your own words...


Alex_B said:


> It is always a compromise. But I would say composition and the effect you want to achieve with your image comes first. So to me that also includes control of DOF. If DOF however does not really matter, then you can think about peak sharpness.


Maybe I have misunderstood you, but you appear to be agreeing with me here.

Like I say, this may now just be a discussion about how we differ in our perception of the same basic principles.

Sark


----------



## Ben-71 (May 26, 2008)

Alex_B ​And this is the definition of DOF assuming a ~5x7" print being made
from a 35mm negative seen from the usual viewing distance. for 5x7
prints.
​Choice of terms, I think, may lead to misunderstanding.

I very well understand your line of thought &#8211; "The DOF is defined by the COF,​which was defined by eyesight, then calculated for a specific camera format,
so the COF can be multiplied back to enlargement size, to serve as the
criteria for the perceived sharpness and DOF".

There is a fundamental difference between the COF's definition and the very
possibility to even see it on prints.

When COF is applied back to enlargements, eye sight is no longer capable
of distinguishing all the details of that size, because of other variables, as I
elaborated before on this thread.

From 'usual viewing distance', for that magnification, there will be details that
some people can distinguish and other people cannot.
The definition at the camera still stands.

There may be details that no one will see, even though they are there​according to the definition at the camera (e.g.: when the COFs are of
slight variations of green foliage).
The definition at the camera still stands.

DOF can contribute to depth perception, but it is not a pre-requisite.
The sense of depth can sometimes exist although the photograph is not 
very sharp and every detail is larger than the COF.

The DOF at the camera is measured by a specific number of the COF that
applies to all.
Interpolate it back for an enlargement, and (due to variations in eyesight) 
there's no one viewing distance and one COF for everyone, there's no one 
size of COF for different colors on different backgrounds. 
Not to mention the varying lighting conditions that people use for viewing print.

The COF size also changes in size for different viewing lighting.​ 
The DOF at the camera is free from all that, so it can be a definition.​ 
Once it is interpolated to an enlargement, it is there, but practically, ​it becomes one of the variables that influence the way we see that
enlargement.

e.g.:
We take an oblique picture of a sheet of paper, so we get a blurred&#8211;sharp&#8211;
blurred DOF from bottom to top.
The paper is dotted or lined, COF size. On the right side of the paper, the
details are yellow, on black background. On the left side they are blue on
green background.
When we look at the enlargement from the 'usual viewing distance', and 
the yellow is sharp, the blue will not look sharp.
It'll play "now you see me, now you don't".
If the blue is at nearly the same density of the green, it would be hardly
noticed, maybe not at all.​ 
Both practically and conceptually, we have a definition of DOF at the camera,
and a useful application of the definition for enlargements.​ 
When the definition is applied to an enlargement, it is no longer a definition,​and becomes one of the variables that influence the way in which we
perceive that enlargement.

Best,

Ben


----------



## Ben-71 (May 26, 2008)

Helen
OK. The whole thing appears to have been a misunderstanding 
over what you meant by 'DoF and perceived DoF'.​ 
What you call 'perceived DoF' I would call 'appearance of depth' 
or 'depth perception' or somesuch, and I would reserve DoF for a
more geometrical optics-based phenomenon related solely to the
appearance or otherwise of blur. ​ 
'DoF and depth perception' maybe, because they are two slightly
different phenomena worthy of two slightly different descriptions, 
I think.​ 
​I said that it seemed to me that it's mainly a semantic question...
English is not my first language, and this may have played its part too.​ 
Using your terms*:*

While DOF can contribute to 'depth perception', an image can give
a lot of 'depth perception', without having even one sharp detail in it.​ 
An image can be sharp all over, or blurred&#8212;sharp&#8212;blurred, and look
as deep as the print on the paper.​ 
A sharp phot' taken with wide angle gives depth perception.
A phot' taken with a long Tele &#8211; with or without showing DOF &#8211;
will give much less depth perception, or not at all.​ 
Therefore, 'DOF' and 'depth perception' are different things, which are
sometimes connected, sometimes not.​ 
Best,​ 
Ben


----------



## JamesD (May 26, 2008)

Due to apparent misunderstandings of terminology leading to unnecessary comments, and debates going in circles, I'm going to drop out of this one.

I will, however, point out one fact which seems to have been missed in an earlier post of mine:  the "blob" in question was a street lamp, not a point source.  I don't generally take pictures of point sources of light.  They're rather dull.


----------

