# Focal Length - Depth of Field?



## EchoingWhisper (Dec 27, 2011)

I thought: Aperture controls depth of field, then why does focal length control depth of field too?

I used to think that the longer the focal length, the lesser the depth of field, but not anymore.
Since longer focal length compresses the distance between objects, it increases the size of everything further away. Thus, the circle of confusion (bokeh/blur) gets larger too, which makes the background LOOKS more blurry. In reality, the depth of field is just the same, only that the defocused area has larger circle of confusion. The defocused area in shorter focal lengths is not able to be seen because of diffraction/anti aliasing etc.

I might be wrong so, point out any mistakes of my point of view. I don't really understand what I'm talking... lol


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## EchoingWhisper (Dec 27, 2011)

Wait, I seem wrong.


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## 480sparky (Dec 27, 2011)

Focal length DOES affect DOF.  A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'.  A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"


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## EchoingWhisper (Dec 27, 2011)

480sparky said:


> Focal length DOES affect DOF.  A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'.  A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"



I think I understand what I am talking about now, if your subject (point of focus) is the same size at all focal length, the depth of field should be the same. Circle of confusion in defocused area of longer focal lengths are larger, but depth of field should be the same.


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## 480sparky (Dec 27, 2011)

EchoingWhisper said:


> 480sparky said:
> 
> 
> > Focal length DOES affect DOF.  A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'.  A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"
> ...



How does 5'-8" = 9'-11" and 41' = 10'-1"?

If you have the 20mm focused at 10', an object at 20' will be in focus.  Switch to a 200mm, and it won't be.


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## Rephargotohp (Dec 27, 2011)

Why, focal length "doesn't matter" is because YOU MOVE. To get the same framing on your subject as you change focal lengths, you move the reciprocal distance.

So the three things that affect DOF, Aperture, Focal Length and Distance to subject, when you* frame the subject the same,* Focal Length and Distance to subject cancel each other out. ( there actually can be a slight change in DOF but not one that is noticeable.\

The reason a Long lens seems to have a shallower dof is perspective compression


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## Big Mike (Dec 27, 2011)




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## Rephargotohp (Dec 27, 2011)

QUOTE=480sparky;2434341]Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"[/QUOTE]
ONLY if your distance to subject stays constant

Put in your DOF calculator f/4 50mm 10 feet to subject, then put in 100mm 20 feet to subject. DOF will remain the same

Focal length Does affect DOF, But in practice, It doesn't because we move


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## EchoingWhisper (Dec 27, 2011)

Rephargotohp said:


> Why, focal length "doesn't matter" is because YOU MOVE. To get the same framing on your subject as you change focal lengths, you move the reciprocal distance.
> 
> So the three things that affect DOF, Aperture, Focal Length and Distance to subject, when you* frame the subject the same,* Focal Length and Distance to subject cancel each other out. ( there actually can be a slight change in DOF but not one that is noticeable.\
> 
> The reason a Long lens seems to have a shallower dof is perspective compression



Yep, that is what I am trying to say.


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## 480sparky (Dec 27, 2011)

Since when did we start moving?


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## Rephargotohp (Dec 27, 2011)

480sparky said:


> Since when did we start moving?



When mom said we couldn't stay home anymore, I drank all her beer

I have a cardboard box outside Starbucks now, It's nice


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## Crollo (Dec 27, 2011)

Rephargotohp said:


> The reason a Long lens seems to have a shallower dof is perspective compression



I believe he already stated that about 3 times now.


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## Rephargotohp (Dec 27, 2011)

Crollo said:


> Rephargotohp said:
> 
> 
> > The reason a Long lens seems to have a shallower dof is perspective compression
> ...



Actually, no
He said the CoC gets larger, It doesn't

I'm sorry I tried to clairify his statement..that you needed to take offense to it


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## Iron Flatline (Dec 27, 2011)

Rephargotohp said:


> He said the CoC gets larger, It doesn't


Hehehe... Beavis, he said CoC.


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## table1349 (Dec 27, 2011)

Understanding Depth of Field in Photography


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## Ysarex (Dec 27, 2011)

Focal length does play a role in DOF, but many (most) photographers are confused about it.

Think of it this way: *DOF is a function of f/stop and magnification.*  At a constant magnification you can change focal lengths without having  much effect on DOF. Magnification is a function of focal length,  distance to subject and sensor size. Because an immediate and obvious  result of changing focal lengths (without moving) is a magnification  change it became very easy to conclude that the DOF change that resulted  was due to the focal length change -- fair enough, but then the next  step was to start claiming that long lenses have inherently less DOF and  short lenses inherently more -- now we're getting into possible  trouble. If you have a multi-format camera for example that can switch  between 4x5 sheet and 6x7 roll is your 90mm lens wide angle or normal --  different DOF from the same lens?

Consider this real story. I'm standing behind the counter in a camera  store where I sell the pro gear. In walks a photographer wannabe who has  done some weddings and wants to move up to illustration. He has a print  in his hands (show off) of open end wrenches shot with the camera at a  45 degree angle. Says he's not getting the DOF he needs from the 80mm  (medium format film) and maybe it's time to break the piggy and get the  50mm. That 50mm is serious money and so I say, "Aren't you going to just  move in closer with the 50mm so you have the same crop?" "Yeah," he  says. "Then the DOF will be the same," I say (and I was right). Since he was the  photographer and I was just a sales clerk he put me in my place and  explained DOF to me. I thanked him and sold him the lens.

There's always infinity. Although the above story illustrates that magnification and not focal length is the active factor you have to come back to the reality that every focal length  has a unique relationship with infinity. As such, at any given f/stop,  the hyperfocal distance in a landscape is going to keep moving closer to  the camera as the lens focal length shortens. Magnification is likewise  being reduced (see bold above), but it is nonsense to suggest to a  landscape photographer if s/he just backs up far enough with the 300mm  the DOF will be the same as with the 30mm. In photography there's a  limit to infinity (always wanted to say that). So it is also reasonable to say that practically speaking very wide lenses typically produce photos with broad DOF and very long lenses typically produce photos with shallow DOF.

Joe


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## EchoingWhisper (Dec 27, 2011)

Ysarex said:


> Focal length does play a role in DOF, but many (most) photographers are confused about it.
> 
> Think of it this way: *DOF is a function of f/stop and magnification.*  At a constant magnification you can change focal lengths without having  much effect on DOF. Magnification is a function of focal length,  distance to subject and sensor size. Because an immediate and obvious  result of changing focal lengths (without moving) is a magnification  change it became very easy to conclude that the DOF change that resulted  was due to the focal length change -- fair enough, but then the next  step was to start claiming that long lenses have inherently less DOF and  short lenses inherently more -- now we're getting into possible  trouble. If you have a multi-format camera for example that can switch  between 4x5 sheet and 6x7 roll is your 90mm lens wide angle or normal --  different DOF from the same lens?
> 
> ...



Yep, I understand what you are saying, as long as the depth of field doesn't reach infinity, at the same format, any focal length will have the same depth of field. But once you are reaching infinity, short lenses start to have advantage in depth of field. In short, use short lens in landscape, for any other uses, use any focal length you want, as long as you get the Bokeh you want and get near enough to the subject.


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## Ysarex (Dec 27, 2011)

EchoingWhisper said:


> Ysarex said:
> 
> 
> > Focal length does play a role in DOF, but many (most) photographers are confused about it.
> ...



With magnification held constant -- same framing for the subject.



EchoingWhisper said:


> But once you are reaching infinity, short lenses start to have advantage in depth of field....



It's still a simplified explanation, but yes!

The next factor you want to fold into the equation is the spatial relationship between the camera, subject and background. Think of it in terms of significant distances. An extreme example: The sun is 93 million miles from Earth give or take. Exposure at sea level is X and if you climb to the top of Mt. Whitney exposure is still X. That 15,000 feet (rounded) from sea level to the top of Mt. Whitney may seem significant to you if you try and climb it but relative to 93 million miles it ain't spit.

So, the relative significance of your subject to background distance changes with the distance between the subject and camera. Work an example: Subject is a couple seated 6 feet in front of a flowering shrub. You're 12 feet away with the camera (75mm lens on a 35mm full frame) and your DOF at f/8 is a total of 4 feet. How significant is the shrub 6 feet behind the subject? Does it appear out-of-focus? Is it possible the magnification reduction from the extra 6 foot distance will make the shrub appear fairly sharp? Oh no! Now back up to 24 feet and switch to a 150mm lens. The framing is the same. At f/8 the DOF is the same total 4 feet. BUT! You've reduce the significance of the distance to the background shrub and so reduced the significance of the magnification reduction. As a result the shrub appears blurrier in the second photo.

Again you can see how photographers over time have made spurious connections for causality here. In the second photo taken with the longer lens it's not really the focal length that's responsible for the blurrier shrub, it's the alteration in significance distances -- it's because you backed up and made the subject to background distance less significant.

Joe


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## Dao (Dec 28, 2011)

I read this article awhile back and it is quit useful.

DOF2


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## bp4life71 (Dec 28, 2011)

480sparky said:


> Focal length DOES affect DOF.  A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'.  A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"



Where is the math on this?  Id like to be able to calculate this!  Good stuff!


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## table1349 (Dec 28, 2011)

bp4life71 said:


> 480sparky said:
> 
> 
> > Focal length DOES affect DOF.  A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'.  A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"
> ...



How to Calculate Depth of Field | Camera Dojo - Take better pictures with our photography tutorials and podcast


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## zcar21 (Dec 28, 2011)

EchoingWhisper said:


> I thought: Aperture controls depth of field, then why does focal length control depth of field too?
> 
> I used to think that the longer the focal length, the lesser the depth of field, but not anymore.
> Since longer focal length compresses the distance between objects, it increases the size of everything further away. Thus, the circle of confusion (bokeh/blur) gets larger too, which makes the background LOOKS more blurry. In reality, the depth of field is just the same, only that the defocused area has larger circle of confusion. The defocused area in shorter focal lengths is not able to be seen because of diffraction/anti aliasing etc.
> ...



You're right, aperture and distance are the only two factors affecting depth of field. People get confused because at lower focal lenght objects become smaller and _appear_ in focus, at longer focal length objects become larger and blurry objects _look _even more blurry.

I was confused myself until I rethought the whole thing.

I mostly use f stops 4-8, so the main aspect affecting depth of field in my case is distance to subject.


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## JClishe (Dec 28, 2011)

Aperture, focal length, and distance to subject affect DoF. Additionally, distance between subject and background affect bokeh. Done and done.


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## Ysarex (Dec 29, 2011)

zcar21 said:


> EchoingWhisper said:
> 
> 
> > I thought: Aperture controls depth of field, then why does focal length control depth of field too?
> ...



Lenses focus on a 2D flat plane. Objects in front of or behind that focus plane _appearing_ in focus *IS* DOF.



zcar21 said:


> I was confused myself until I rethought the whole thing.
> 
> I mostly use f stops 4-8, so the main aspect affecting depth of field in my case is distance to subject.



If you used f/stops 2-4 or 8-16 then distance wouldn't be the main aspect affecting DOF?

Joe


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## Ysarex (Dec 29, 2011)

JClishe said:


> Aperture, focal length, and distance to subject affect DoF. Additionally, distance between subject and background affect bokeh. Done and done.



Not done. If you take the same photo with a full frame versus a crop sensor camera you'll have different DOF between the two.

Joe


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## Rephargotohp (Dec 29, 2011)

zcar21 said:


> You're right, aperture and distance are the only two factors affecting depth of field. People get confused because at lower focal lenght objects become smaller and _appear_ in focus, at longer focal length objects become larger and blurry objects _look _even more blurry.
> 
> I was confused myself until I rethought the whole thing.
> 
> I mostly use f stops 4-8, so the main aspect affecting depth of field in my case is distance to subject.



Nope  peolple still aren't understanding this.

Aperture, Focal Length and Distance to subject ALL affect DOF

BUT, in the _Pratical Application, Where the foreground subject is framed the same,_ Aperture is the only thing that affects DOF
Focal Length doesn't because we move
Distance to Subject doesn't because we change focal Lengths


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## EchoingWhisper (Dec 29, 2011)

Ysarex said:


> JClishe said:
> 
> 
> > Aperture, focal length, and distance to subject affect DoF. Additionally, distance between subject and background affect bokeh. Done and done.
> ...



It's the same, only you get more of the sides (it's not actually wider), because the subject reflected on the sensor is the same size at the same distance and same focal length. But again, in practical application, a larger sensor will get less DOF, cause' you move.


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## jake337 (Dec 29, 2011)

JClishe said:


> Aperture, focal length, and distance to subject affect DoF. Additionally, distance between subject and background affect bokeh. Done and done.



The lens you choose defines your bokeh.  Bokeh cannot be changed.


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## zcar21 (Dec 29, 2011)

Ysarex said:


> zcar21 said:
> 
> 
> > EchoingWhisper said:
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Objects appear focused, but enlarge it to full size you'll notice it Is out of focus. About aperture I either use f4, f5.6, or f8 only three different DOF, but if I add all the different distances to subject the DOF is ilimited.


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## zcar21 (Dec 29, 2011)

The link dao provided proves that focal length doesn't affect dof. This is true provided that magnification stays the same, which is not always the case. the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant. At very short focal length almost 2/3 of the depth of field is in the rear of the focal point, 50mm or longer the difference begins to even out.


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## jake337 (Dec 29, 2011)

Rephargotohp said:


> zcar21 said:
> 
> 
> > You're right, aperture and distance are the only two factors affecting depth of field. People get confused because at lower focal lenght objects become smaller and _appear_ in focus, at longer focal length objects become larger and blurry objects _look _even more blurry.
> ...




I don't agree with your statement of Practical Application.  At least for portraiture, I'm using different focal lengths for different framing.  I'm not gonna take a full body with my 50mm lens, swap to 135mm, then back up to achieve the same framing.  I'm switching focal lengths for different framing. 50mm for full body, 85mm for 3/4 to chest up, and 100mm for neck and shoulders.

Ideally I would be using:
50mm for full body
85mm for 3/4
105mm for 1/2
135mm for chest up
200mm for shoulder up


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## jake337 (Dec 29, 2011)

zcar21 said:


> The link dao provided proves that focal length doesn't affect dof. This is true provided that magnification stays the same, *which is not always the case*. the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant. At very short focal length almost 2/3 of the depth of field is in the rear of the focal point, 50mm or longer the difference begins to even out.



I would say it is most often not the case.  I wouldn't go out and take an exposure at each focal length, keeping the same magnification of my subject, then go home and choose the perspective I like best.  I would plan out the perspective I am seeking beforehand, then choose a lens which best suits that perspective.


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## Ysarex (Dec 29, 2011)

zcar21 said:


> The link dao provided proves that focal length doesn't affect dof. This is true provided that magnification stays the same, which is not always the case. the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant. At very short focal length almost 2/3 of the depth of field is in the rear of the focal point, 50mm or longer the difference begins to even out.



You seem to be quite confused. As for a link proving anything; I can direct you to a link that proves Obama is a Muslin, George Bush was a space alien and we never really landed a man on the moon. (Not to say Dao's link contains bad info.)

Read what you just wrote. You start with this: "The link dao provided proves that focal length doesn't affect dof." That's a clear factual statement. Then you say this: "This is true provided that magnification stays the same, which is not always the case." In this second statement you're saying your first statement is wrong.

Next you say this: "the link gryhonslair99 provided shows interesting things to notice about dof, even though dof stays constant." What is that dangling clause, "even though dof stays constant," modifying? Are you trying to attach that as a qualifier to the beginning of the next sentence? 

And your last assertion: "At very short focal length almost 2/3 of the depth of field is in the  rear of the focal point, 50mm or longer the difference begins to even  out." That's simply not accurate. It is correct that DOF tends to distribute unevenly around the focus plane, but not the way you describe it here. You are groping in the right direction however. And if in fact you are correct that with shorter focal lengths the uneven distribution of DOF is more uneven than with longer focal lengths (that is correct) then you're providing evidence to prove your first statement false.

Joe


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## Ysarex (Dec 29, 2011)

EchoingWhisper said:


> Ysarex said:
> 
> 
> > JClishe said:
> ...




When I said "same photo" I meant the exact same content in each. So in the case of a portrait, the same subject framing using either camera. Sorry I wasn't more clear. In that case the smaller sensor camera will reproduce the subject at a smaller magnification so that at the same f/stop, the smaller sensor camera will produce a photo with more DOF.

You don't have to move to frame the subject the same way with different sensor size cameras. You can put say a 5D and a 7D side by side to take a photo and have both cameras frame the subject identically. The lens focal lengths in this case will be different.

Again we look to magnification as the active agent. With a smaller sensor everything is photographed at smaller magnifications. The circle of confusion for the smaller sensor is smaller and DOF is greater.

Joe


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## Ysarex (Dec 29, 2011)

jake337 said:


> Rephargotohp said:
> 
> 
> > zcar21 said:
> ...



So you're not suggesting that Rephargotohp's assertion is inaccurate, only that it doesn't resonate with the way you like to work taking portraits. You'd agree then in fact that his assertion is basically accurate; that with magnification held constant, a focal length change will have a negligible effect on DOF and only aperture can effect a DOF change.

It's worth noting that the OP didn't qualify the original question "as applied to portrait photography." So if you instead considered Rephargotohp's practical application in the context of say product illustration, it may in fact be spot on and resonate perfectly with the way a product photographer might work photographing say cosmetics.

And so then in the context of general photography as asked by the OP, you'd agree that Rephargotohp is accurately describing the phenomena in question. OK.

Joe


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## EchoingWhisper (Dec 29, 2011)

Ysarex said:


> EchoingWhisper said:
> 
> 
> > Ysarex said:
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Circle of confusion doesn't depend on sensor size, it depends on lens resolution, camera resolution and AA filter. You are confused. At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same, Bokeh is also the same. I don't know how changing to DX will 'increase' depth of field. Maybe in practical application yes, but in your argument... sorry I have to say No.


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## o hey tyler (Dec 29, 2011)

Actually, I am pretty sure CoC does depend on sensor size, or frame size.


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## EchoingWhisper (Dec 29, 2011)

o hey tyler said:


> Actually, I am pretty sure CoC does depend on sensor size, or frame size.



Do you realise that larger sensor often have larger pixels?


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## Ysarex (Dec 29, 2011)

EchoingWhisper said:


> Ysarex said:
> 
> 
> > EchoingWhisper said:
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First of all, in practical application to calculate DOF, standard circles of confusion are based on the format (sensor/film size) with smaller sensors being assigned smaller circles of confusion. You can see that's the case in this chart at DOFMaster:

Circles of Confusion for Digital Cameras

You're not hearing what I said. You said this: "At the same distance, the same aperture, the same focal length, the same lens, any sensor size, the DOF is the same..." I completely agree. You are right and I've never asserted otherwise.

I'm saying this: Use different sensor size cameras to make the same photograph -- content in each photo is exactly the same. So if it's a tight portrait it begins 2 inches above the subject's head and extends down to the second button of his her shirt/blouse, both cameras produce the same content. To do that they would have to use different focal length lenses from the same distance or be at different distances if they used the same lens.

Because of the decreased magnification from the smaller sensor camera that photo will have more DOF at the same f/stop.

I'm rarely confused.

Joe


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## EchoingWhisper (Dec 29, 2011)

Also, the larger the circle of confusion, the higher the depth of field.


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## EchoingWhisper (Dec 29, 2011)

Ysarex said:


> EchoingWhisper said:
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> 
> > Ysarex said:
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During the film days, yes, because pixel size at the same ISO is normally the same, but not in the digital.


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## Ysarex (Dec 29, 2011)

EchoingWhisper said:


> Ysarex said:
> 
> 
> > EchoingWhisper said:
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Don't believe me -- run the math. Use a DOF calculator and run some simulations and then believe the math.

Joe


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## zcar21 (Dec 29, 2011)

Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.

Focal length does not affect depth of field.


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## jake337 (Dec 29, 2011)

zcar21 said:


> Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.
> 
> Focal length does not affect depth of field.



LOL, ok. Only when your subject magnification remains the same and the camera to subject distance changes to keep that magnification the same. 

This is not usually the case. Let me ask you this, when doing portait work do you keep moving back everytime you swap to a lens of a longer focal length? Or do you keep a relative distance (10-15 feet) and change lens based on how you want to frame your next exposure?

When your camera to subject distance remains constant, while using a constant aperture, the focal length you choose WILL greatly effect your DOF.  Period.


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## zcar21 (Dec 29, 2011)

o hey tyler said:


> Actually, I am pretty sure CoC does depend on sensor size, or frame size.



Agree.


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## zcar21 (Dec 29, 2011)

jake337 said:


> zcar21 said:
> 
> 
> > Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.
> ...



If you have compared the two pictures after enlarging one of them I'll might believe it, but my eyes tell it's just a different size.


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## EchoingWhisper (Dec 30, 2011)

zcar21 said:


> Depth of field: the distance between the nearest and farthest part with relatively shap detail. What is sharp for some may not be as sharp for somebody else, specially if you change the size and enlarge it. But I'll stick to my statement.
> 
> Focal length does not affect depth of field.



What is sharp may not be sharp for somebody else - exactly, the higher resolution, the more likely you are gonna perceive the lack of sharpness, which means smaller COC = higher resolution = less depth of field.


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## EchoingWhisper (Dec 30, 2011)

Most calculation of COC is based on the d(iagonal)/1500, which isn't really accurate nowadays, because there wasn't such thing as sensor resolution last time and there wasn't any good lens resolution tests like DxOMark.


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## Ysarex (Dec 30, 2011)

Ysarex said:


> EchoingWhisper said:
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> 
> > Ysarex said:
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Here's the math using the DOFMaster calculator. I've checked their math in the past and it's good.

I selected a full frame 35mm camera and a 4/3 camera (2x crop factor). The subject is 4 feet wide. From the same distance both cameras are taking the exact same photo -- the content is identical (allowing any proportional variance). The DOF difference is due to the magnification reduction that results from using a smaller sensor.

Joe


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## jake337 (Dec 30, 2011)

zcar21 said:


> jake337 said:
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> > zcar21 said:
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See now I'm confused. So is it the magnification? When I read it, the statement seems to contradict itself. I need a nap.

Understanding Depth of Field in Photography
"On the other hand, *when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field *(even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect *due to higher magnification, not focal length*."

But to get that higher magnification, you need a longer focal length lens.....


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## Ysarex (Dec 30, 2011)

jake337 said:


> zcar21 said:
> 
> 
> > jake337 said:
> ...



Yes, it's the magnification.



jake337 said:


> When I read it, the statement seems to contradict itself. I need a nap.
> 
> Understanding Depth of Field in Photography
> "On the other hand, *when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field *(even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect *due to higher magnification, not focal length*."
> ...



To get the higher magnification you can use a longer lens or you can get closer or you can use a larger sensor or you can use any combination of the three. In the end it's the magnification. What you don't want to do is think that it's a fixed property of the lens. The reason for the confusion is this: Many photographers first learn this equation:

DOF = f/stop + focal length. Short lenses have more DOF and long lenses have less -- fair enough.

Eventually they learn this equation:

DOF = f/stop + focal length + subject distance. But they have that idea holding on from before about short and long lenses which, although not wrong, can cause confusion.

Hopefully they eventually learn this equation:

DOF = f/stop + focal length + subject distance + sensor size. But since they're artists they're now hopelessly confused and they fail to apply one of the foundation rules of algebra: simplify to solve the equation.

Magnification = (focal length + subject distance + sensor size) and therefore:

*DOF = f/stop + magnification.

==================================

*Next you get somebody come along (zicar21) who realizes that if you hold magnification constant, which by the way gives you the same content and that makes sense, then focal length and subject size drop out of the equation and hey look!! same DOF from two different focal lengths! What I first learned about short and long lenses isn't true!!

Not so fast. It's a bit more complicated than that*.  *You've got landscapes at infinity for one. It's just silly to say focal length doesn't effect DOF when shooting landscapes because you can't physically manipulate a 200mm lens to produce the same magnification as a 20mm with infinity in the photo.

It's still more complicated, because as I noted earlier in this thread, you've got to factor in the spatial relationship between camera to subject and subject to background distance.

And it's still more complicated because you'll next want to factor in the DOF distribution around the focal point.

================================

Fact: Focal length plays a roll in determining DOF.
Fact: With infinity in the photo it's a fact that a shorter lens will have a closer hyperfocal distance.
Fact: Without infinity in the photo and the magnification held constant, changing lens focal length will not significantly effect the total DOF.
Fact: Lens focal length will most certainly alter the DOF distribution as well as factor into the spatial relationship between camera/subject and subject/background distances and that will effect how blurry the background appears regardless of the total DOF.

OK now?

Joe


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## EchoingWhisper (Dec 30, 2011)

jake337 said:


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Not really, you could walk closer to get that higher magnification.


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## jake337 (Dec 30, 2011)

Ysarex said:


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Perfect! Thanks for the clarification in defintion.  Everything you stateed was in my brain, just not properly defined if that makes sense.


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## EchoingWhisper (Dec 30, 2011)

Ysarex said:


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Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.


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## Ysarex (Dec 30, 2011)

EchoingWhisper said:


> Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.



Put a US quarter on the table in front of you.

Now photograph that quarter so that it completely fills the frame edge to edge. Start with a 4x5 camera and work you way down to a micro 4/3s.

Your photo of the quarter taken with a 4x5 camera will be larger than life-size as a quarter is no where near 4 inches wide.

Your photo of the quarter taken with a 70mm camera will also be larger than life-size since a quarter is also less than 2.25 inches wide.

Your photo of the quarter taken with a 35mm camera will be almost exactly life-size since a quarter is just about 24mm wide.

Your photo of the quarter taken with a micro 4/3s camera will be less than life-size.

Note that you can't photograph a quarter at 1:1 using a micro 4/3s camera and include the entire quarter in the photo. The sensor is physically smaller than a quarter and so to include the entire quarter in the frame the sensor size physically forces you to reduce magnification -- and you'll have more DOF as a result.

Repeat process using Grand Tetons, 6 story building, 5.5 foot person, the family dog and the same holds true. To record the same subject on a smaller sensor you have to reduce magnification and as a result you get more DOF.

Joe


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## EchoingWhisper (Dec 30, 2011)

Ysarex said:


> EchoingWhisper said:
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> > Sensor size got nothing to do with magnification, it deals with field of view, not magnification. Does a 1:1 macro lens turn into a 2:1 macro lens when fitted on a DX body? No.
> ...



Maybe you don't know what is magnification eh? I remember Overread telling me something about it. Magnification is subject size to fill the frame divided by sensor size. To fill the frame of all those sensors, you have to change your distance to the subject, so... the magnification got to do with you moving (focus distance) and not the sensor size.


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## zcar21 (Dec 30, 2011)

I did the test and looked at looked at some pictures to compare. Ysarex is right. 

There is too much crap on the internet confusing people. Gotta be more careful.


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## Ysarex (Dec 30, 2011)

EchoingWhisper said:


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In this context, magnification is the size of the image of the object (directly on the sensor or film) relative to the actual size of the object. At a magnification of X1 (1:1 reproduction -- same thing) the size of the object and the size of the image of the object are the same. You can photograph an entire quarter using a full frame sensor camera at a magnification of X1. You can't photograph an entire quarter at a magnification of X1 with any crop sensor camera. To photograph the whole quarter with a crop sensor camera you'll have a magnification less than X1 and with less magnification you'll get more DOF.

Joe


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## EchoingWhisper (Dec 31, 2011)

Ysarex said:


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You're still not getting the point. Do the maths and you'll understand.


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## Ysarex (Dec 31, 2011)

EchoingWhisper said:


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I did that math over 30 years ago.

I think you're hung up here on the definition of magnification. I'm not saying that magnification is calculated using sensor size. Magnification is calculated using focal length and lens/subject distance.

Magnification = focal length/(subject distance - focal length).

My point is this: To photograph the same exact object that has a fixed physical size a smaller sensor camera is forced to use a smaller magnification factor. As a result you get more DOF. Smaller sensor cameras must use smaller magnification factors to take the same photos as larger sensor cameras.

You may have a big head, but odds are it's not bigger than 250mm x 200mm. That means we could photograph your head with an 8x10 sheet film camera, include your entire head in the photo, and achieve a magnification factor of X1. The image of your head would be the same size as your head. Take a photo of someone's head, include their entire head in the photo, and use a camera with a micro 4/3s sensor. That sensor is 18mm X 13mm. You won't be able to use an X1 magnification factor. If they have an average size head your magnification factor will be more like X.08 AND, *you'll get more DOF.* Yep, I just did the math (again) and yes I understand.

Joe


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## Ysarex (Dec 31, 2011)

I think I see the problem here.

You're probably reacting to my earlier post where I said DOF = f/stop + magnification and I noted that magnification was focal length + subject distance + sensor size.

My fault there for being sloppy; although the point is still correct. The role sensor size plays isn't in determining magnification, but rather in limiting magnification relative to the size of the object being photographed. So DOF = f/stop + (recording) magnification.

Joe


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## zcar21 (Dec 31, 2011)

In other words magnification is height of the object divided into height of its image.


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## zcar21 (Dec 31, 2011)

This is what got me confused about focal length. Read it if you have time to waste.
Background blur vs focal length, absolute aperture and relative aperture


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## EchoingWhisper (Dec 31, 2011)

Ysarex said:


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Smaller sensor cameras must use smaller magnification factors to take the same photos as larger sensor cameras is correct, but I'm just trying to say magnification doesn't change due to sensor size, a 1 cm long rice will still be 1 cm on a larger sensor.


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## Derrel (Dec 31, 2011)

Ysarex said:
			
		

> >SNIP>>As for a link proving anything; I can direct you to a link that proves Obama is a Muslin, George Bush was a space alien and we never really landed a man on the moon.



Okay, where can I see this Obama muslin link? Is he's a muslin, he would make a fantastic photography backdrop! I'd love to order the Old Master's Obama muslin backdrop in both brown and blue shades!


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## Ysarex (Dec 31, 2011)

EchoingWhisper said:


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Right -- we were talking past each other there for awhile.

Joe


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## Ysarex (Dec 31, 2011)

Derrel said:


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This is entirely off topic but: You know I just finished a drive to Nashville and back and I spent a couple hours listening to American Family Radio on the way down. It was quite an unsettling experience and I swear three different people called in over the course of a one hour program and commented that Obama was trying to destroy America because he was a Muslum -- woah!

Joe


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## Derrel (Dec 31, 2011)

Depth of field has proven to be a very difficult topic to discuss through forum posts. The Online Photographer blog did a week-long discussion of the factors at play. It's a difficult subject to discuss in part because there are SO MANY PEOPLE who have only a partial understanding of the subject. And a small subset of those people have written web-based articles about the subject, often spewing MIS-information about the subject. That mis-information is then repeated ad nauseum, all across the web. Over the last decade, with the web becoming a prominent place to discuss photographic issues, all too often often we see improper terms, poor examples, and examples that are shown purporting to show "truth", but which do not take into account the way depth of field behaves across the full spectrum of distances.

Depth of field at close distances and high image magnification is VERY, very shallow.In the macro- and close-up ranges, the focal length of the lens is nowhere near as large a factor as is the IMAGE MAGNIFICATION. Weird

Depth of field increases somewhat as camera-to-subject distances get longer than what I call the "close-up range". Then depth of field begins to increase at an EXTRAORDINARY rate as the focusing distance approaches the hyperfocal distance of whatever lens/sensor or lens/film SIZE combination a person is shooting with. As one can see by looking at depth of field charts or markings on lenses that still have them (!) there comes a distance at which depth of field at smaller apertures is QUITE deep, and even at wide apertures like f/4, at long ranges, depth of field can be measured in thousands of meters or more...extending from as close as say,a stone's throw away, and extending out to literally, Infinity.

In practical terms, depth of field issues can be grouped into the four types of scenarios. 1)macro-/extreme close-up  2)  close-range 3) medium-distance, and 4) long-range.

One odd thing about depth of field is that the charts and tables can delineate the DOF in distance units of feet and meters quite nicely, but the way the human visual system perceives DOF is not so clear-cut. It's easy to discuss DOF in a quantitative (measured) sense, but much more-difficult to discuss it in a qualitative sense. Just exactly *"how out of focus the background seems to be" *is something that the on-line DOF calculators and lens-barrel charts are not good at conveying. The same holds true in my opinion for the way many people tend to dismiss the depth of field "look" or "rendering" differences between small-format captures, like 1.5x APS-C d-slr photos, and those shot on say, medium-format rollfilm cameras. One of the things I see the most with small-format images is that many times background that is said to be out of focus is actually QUITE recognizable, despite the depth of field calculator's assurances that the background is "outside" of the DOF zone...Lewis Collard's article mentions this in his summary, where he states:"

*Smaller formats mean less background blur: *Given any relative aperture and any given framing of a subject, a larger film format will result in more background blur. This is because given any subject distance, background distance and relative aperture, increasing the focal length results in a _disproportionate_ growth of the defocus blur circle."  [http://lewiscollard.com/technical/background-blur/]
Again, this is a tough subject to write about...very challenging at times...and even then, when writing about DOF, it takes only one small misunderstanding, or one mis-statement, and an entire line of thinking can be brought down, or arguments caused,etc.,etc..

Background blur is another fun sub-issue. Take a look here: Bokeh and Background Blur Calculator- Bob Atkins Photography


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## JClishe (Dec 31, 2011)

Derrel said:


> Depth of field has proven to be a very difficult topic to discuss through forum posts. The Online Photographer blog did a week-long discussion of the factors at play. It's a difficult subject to discuss in part because there are SO MANY PEOPLE who have only a partial understanding of the subject. And a small subset of those people have written web-based articles about the subject, often spewing MIS-information about the subject. That mis-information is then repeated ad nauseum, all across the web. Over the last decade, with the web becoming a prominent place to discuss photographic issues, all too often often we see improper terms, poor examples, and examples that are shown purporting to show "truth", but which do not take into account the way depth of field behaves across the full spectrum of distances.
> 
> Depth of field at close distances and high image magnification is VERY, very shallow.In the macro- and close-up ranges, the focal length of the lens is nowhere near as large a factor as is the IMAGE MAGNIFICATION. Weird
> 
> ...



Best post in this thread.


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## Shoal (Dec 31, 2011)

> Hehehe... Beavis, he said CoC.



lol. Seriously though, this is a great thread. Thanks for starting this one


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## KmH (Dec 31, 2011)

zcar21 said:


> This is what got me confused about focal length. Read it if you have time to waste.
> Background blur vs focal length, absolute aperture and relative aperture


No ****. That guy is full of it. :cyclops:

Nice graphs though.


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