# Focal length, focus distance, and depth of field



## robertwsimpson (Feb 10, 2010)

There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field.  People have been arguing that a photo taken with a longer photo length (ie the long side of a zoom lens) will have a smaller DoF, given the same f-stop setting.  As I learn best by doing, I have some exhibits that should help show what is actually happening.  Hope this helps.  Keep in mind that the basis of this is "similarly framed photographs."

Exhibit 1:





This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 18mm and f/3.5.  The first thing that everyone should take notice of is that a blurred background is very achievable with one of the worst and cheapest lenses that canon makes.  

Exhibit 2:




This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 55mm and f/5.7.  This is the widest aperture setting available at this zoom.

Exhibit 3:




This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 18mm and f/5.7.  This is the same aperture value as the maximum available at 55mm.

First, lets dispel the myth that on a variable aperture lens, the DoF becomes smaller at the long end of the lens.  This is clearly not the case.  The easiest way to tell this is by looking at the coffee cup behind the stuffed animal.  At 55mm and 5/5.7, the black line at the top of the cup is distinct.  At 18mm and f/3.5, it is barely distinguishable at all.  Another way to tell is by looking at the dark spot above the hind leg of the stuffed animal.  In the 18mm shot, the spot is blurry.  In the 55mm shot, some of the fibers are distinguishable.  In this instance, logic prevails.  A shot at 18mm and f/3.5 does in fact have a smaller DoF than a similarly framed shot at 55mm and f/5.7.

Next, lets compare the two shots with the same f-stop values.  Comparing the coffee cups reveals similar levels of detail.  The black ring is distinguishable, although blurred.  The spot on the stuffed animal's leg is blurred, but some of the fibers are visible.  I would confidently assert that these two photos have very similar depths of field.

Thus ends the data dissection of this experiment.  I do have some other observations.  First, Depth of Field has far more to do with focus distance than focal length.  Let me explain.  Everyone is familiar with the focus graph.  If not, it looks something like this:




So, the closer the object is to the minimum focus distance of the lens, the smaller the DoF becomes.  We see this with macro lenses.  That is why giant f-stops (tiny apertures) are used in macro photography.  The farther away an object is from a lens' minimum focus distance, the larger the DoF becomes.  That is why the maximum focus distance on a lens is infinity, rather than some arbitrary distance.  The farther away one focuses, the DoF becomes so gigantic that for intents and purposes, everything far away is in focus.  

One reason that I think people assume longer focal lengths give a shallower depth of field is that when compared to shorter focal lengths, longer ones act to compress the apparent distance between an object and its background.  This can be seen in the size of the coffee cup in the 18mm shots vs the 55mm shot above.  I did not move the cup, but it appears closer to the camera in the 55mm shot.  Since it appears closer to the camera, and it is still blurred by the same amount as the 18mm shot, it appears that the 55mm shot has a shallower DoF than the 18mm shot.

I hope that this helps everyone.  Also, I hope that when people make assertions on the internet, no one just believes them without reading up or doing his own experiments.  Not only is it silly to just accept what other people say, but you can learn a lot by checking things out for yourself.


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## robertwsimpson (Feb 10, 2010)

By the way, comments and questions are very welcome.  We're all here to learn.  If you think I've gotten something wrong here, please let me know.  Just be respectful.


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## robertwsimpson (Feb 10, 2010)

also, please excuse my tax documents and our business plan, which are sitting there for the world to see.


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## Dao (Feb 10, 2010)

Longer focal length do have a shadower DoF

i.e.
Camera with crop factor of 1.6

Focal Length = 50mm
Aperture = F/5.6
Subject Distance = 10ft
DoF = 2.62 ft

Focal Length = 200mm
Aperture = F/5.6
Subject Distance = 10ft
DoF = 0.15 ft


Now try another experiment.
Use a 50mm Lens and take a portrait shot with background far enough to create a blur.  Set the Aperture at F/5.6

Then use a lens with 200mm focal length and take a portrait shot with the same background.  But this time, the camera need to move back further away from subject so that the framing is about the same as 50mm.

You will see the result.


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## robertwsimpson (Feb 10, 2010)

Dao said:


> Longer focal length do have a shadower DoF
> 
> i.e.
> Camera with crop factor of 1.6
> ...



I just did that experiment.  The results are posted above?

I'm not arguing that a 50mm lens and a 200mm lens both have the same DoF at a focus distance of 10 feet.


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## Rosshole (Feb 10, 2010)

Play around with different setting on this page, then duplicate the settings with your camera.

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


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## robertwsimpson (Feb 10, 2010)

How are you guys not getting that I just did all of this in real actual life and posted the photos above so that you can see them?  No need for theoretical calculations.  Just look at the pictures.


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## Rosshole (Feb 10, 2010)

Ok, I understand your testing, but you didn't post how far you wer from the focus point, which is an important part of the equation.

Your 18 mm shot seems VERY close to the subject. The shorter the distance to the subject, the shallower the depth of field. These principals are easier to see when comparing a 70 mm shot to a 200 mm shot.

EDIT: because there are three variables in this equation, a 3d graph would be in order.... the 2d graph doesn't express all of the proper values.  Also, you are changing two of the variables in each photo (distance to focus point, focal length), try again and change only one at a time.  The results should be apparent then


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## robertwsimpson (Feb 10, 2010)

Well, clearly, to frame a photo the same, one must be farther away from it at 55mm than at 18mm.  I even talked about this in my original post.  Did you read it all?


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## robertwsimpson (Feb 10, 2010)

Rosshole said:


> Ok, I understand your testing, but you didn't post how far you wer from the focus point, which is an important part of the equation.
> 
> Your 18 mm shot seems VERY close to the subject. The shorter the distance to the subject, the shallower the depth of field. These principals are easier to see when comparing a 70 mm shot to a 200 mm shot.
> 
> EDIT:  because there are three variables in this equation, a 3d graph would be in order....   the 2d graph doesn't express all of the proper values.



3d graph is not needed, as the aperture value remained constant for the main part of the test.


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## Rosshole (Feb 10, 2010)

Your DoF will remain essentially the same if you increase the length to focal point while increasing the focal length.


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## robertwsimpson (Feb 10, 2010)

Rosshole said:


> Your DoF will remain essentially the same if you increase the length to focal point while increasing the focal length.



Exactly.  That is why I said that the DoF is essentially the same for similarly framed photos with equal aperture values at different focal lengths.  It's all about focus distance.


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## Rosshole (Feb 10, 2010)

I guess that I missed your whole point of starting this thread then...


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## robertwsimpson (Feb 10, 2010)

Rosshole said:


> I guess that I missed your whole point of starting this thread then...



it was to dispel a myth that is rampant on this forum.


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## Stosh (Feb 10, 2010)

There are 2 different kinds of blurriness and many people don't recognize that.  There is blurriness from depth of field and there is another called background blur.  

Depth of field rules apply to objects "near" the subject (or wherever the lens is focused).  As long as you use the same field of view (or framing as you say), depth of field is determined only by f-ratio, and your pictures clearly show that.  In your pics, depth of field can be found by looking mostly at the stuffed animal.  When both are shot at f/5.7 with different focal lengths, they have the same depth of field.

Background blur rules apply to objects "distant" beyond the subject.  Distant can be loosely defined as anything beyond twice subject distance.  You don't have many details in your examples to show how background blur changes among different lenses, but the amount of background blur is determined by physical aperture.  Larger aperture makes more background blur.  Lots of people think that aperture means f-ratio, but it doesn't.  Physical aperture is calculated by focal length/f ratio.  So in your examples 18mm/3.5=5mm.  55mm/5.7=10mm.  18mm/5.7=3mm.

Part of the reason bokeh and blurriness is such a mystery is because it's usually a combination of both types of blur.  Most people aren't even aware of the rules of background blur, only depth of field.  When people say"that's nice bokeh", they're usually seeing background blur.  And since background blur is attained by large apertures and distant backgrounds, longer focal length lenses *usually *have larger physical apertures, hence all the talk about using longer focal lengths.

Do some more experimenting and put more distance between your subject and background.  Then you'll start seeing how larger apertures will blur the background more.  Very nice pics illustrating depth of field.


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## Derrel (Feb 10, 2010)

A couple of comments would be that the focus distance calculated from the film/sensor plane, and not the front of the lens. Second, in macro shooting depth of field is not a function of lens focal length, but is almost entirely based upon image magnification: using a 60mm macro lens and a 180mm macro lens and achieving the same,exact size images, I was able to prove that strange fact to myself some years ago.So, to re-state, at macro ranges, the focused distance has hardly any impact whatsoever,and depth of field is entirely a function of image magnification; that, however changes at some point. also, and this is weird, but at macro magnifications, and indeed throughout most of the close-up range, depth of field is almost the same in front of the focus point and behind the focus point; this firmly goes against the old simple "rule" that DOF is 1/3 in front of the focused point and 2/3 behind the focused point. A thoughtful examination of that old,simple "rule" will tell us something, and that something is in this next paragraph.

Under "close to normal" focusing distances, let's say roughly two feet to 15 feet, as in your tiger and travel coffee cup experiment, depth of field is "reasonably shallow" when the focusing distances are short. However, one depth of field principle that your experiment does not show is that as camera-to-subject distances move beyond about 15 feet to 20 feet, the depth of field on a small-sensor or small-format camera approach the hyperfocal distance EXTREMELY rapidly, and depth of field increases at an amazing,almost exponential rate. And the depth of field at these longer camera-to-subject distances extends roughly one third in front of the focus point, and 2/3 of the total depth of field lies BEHIND the focusing point. At longer focusing distances, longer than this indoor-kitchen table-to kitchen wall range, depth of field increases EXTREMELY RAPIDLY BEHIND THE FOCUS POINT. In other words, depth of field is not linear: it kind of "snowballs" as camera to subject distance increases.

So, depth of field is actually not described accurately by your indoor experiment. What your experiment shows is close-range depth of field behavior that is nether macro-range, nor medium to long range. If, for example, your stuffed animal had been positioned at 22 feet from the camera, the background would be rendered much,much more clearly with both the 18mm setting and also the 55mm setting. So when you wrote, "No need for theoretical calculations. Just look at the pictures." that comment does not actually describe the true behavior of depth of field; your testing scenario is missing two of the major "different" behaviors of depth of field, because your do not have DOF scenario 1) an extremely close situation, nor do you have Scenario 3) a situation where the focused distance approaches within 50% of the hyperfocal focusing distance.

A good article on this subject can be found buried somewhere within the many pages in the Bob Atkins web pages.


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## robertwsimpson (Feb 10, 2010)

you can see background blur by looking at the division in the wall behind the two "subjects" that I've already mentioned.  

That wall is way over twice the focus distance in both of those photos.  The background in the photo shot at 55mm appears "blurrier" because it appears larger, due to the compression of distance effect that I mentioned in my original post.  Just because it is larger in the 55mm photo does not mean that it is blurrier.  To me, it appears to be the same as in the 18mm photo as far as blur.  Of course, it is harder to tell on the 18mm photo because of the much smaller apparent size of the object in question.  

Try to discard what you "know" or have been told, and evaluate these images based on what you see.  If you indeed see something different than I do, please let me know.  As I said, this should be a learning process.  I like learning as much as the next person (probably moreso).


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## Gaerek (Feb 10, 2010)

*Brings out his handy dandy iPhone, with his handy dandy DoF calculator app*

Focal Length: 50mm
Aperture: f/3.5
Subject Distance: 10'
Gives a Total DoF of: 1'8"

Focal Length: 100mm
Aperture: f/3.5
Subject Distance: 10'
Gives a Total DoF of: 0' 5"

With all else being equal, longer focal length = less DoF.

I'll trust the calculations in this app more than I'll trust your unscientific test where you have variables flying all over the place (different apertures, different focal lengths, AND different distance to subject).


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## Gaerek (Feb 10, 2010)

robertwsimpson said:


> you can see background blur by looking at the division in the wall behind the two "subjects" that I've already mentioned.
> 
> That wall is way over twice the focus distance in both of those photos. The background in the photo shot at 55mm appears "blurrier" because it appears larger, due to the compression of distance effect that I mentioned in my original post. Just because it is larger in the 55mm photo does not mean that it is blurrier. To me, it appears to be the same as in the 18mm photo as far as blur. Of course, it is harder to tell on the 18mm photo because of the much smaller apparent size of the object in question.
> 
> Try to discard what you "know" or have been told, and evaluate these images based on what you see. If you indeed see something different than I do, please let me know. As I said, this should be a learning process. I like learning as much as the next person (probably moreso).


 
Set your camera on a tripod and shoot the subject at different focal lengths keeping everything else equal (aperture, subject distance, etc). I promise you, the longer focal length will give you _much_ shorter DoF.

I believe what physics tell me should happen. Not what an uncontrolled, random test with variables flying all over the place. Unless you've stumbled upon a way to break the laws of physics, you should probably just give up your crusade here.


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## robertwsimpson (Feb 10, 2010)

Derrel said:


> A couple of comments would be that the focus distance calculated from the film/sensor plane, and not the front of the lens. Second, in macro shooting depth of field is not a function of lens focal length, but is almost entirely based upon image magnification: using a 60mm macro lens and a 180mm macro lens and achieving the same,exact size images, I was able to prove that strange fact to myself some years ago.So, to re-state, at macro ranges, the focused distance has hardly any impact whatsoever,and depth of field is entirely a function of image magnification; that, however changes at some point. also, and this is weird, but at macro magnifications, and indeed throughout most of the close-up range, depth of field is almost the same in front of the focus point and behind the focus point; this firmly goes against the old simple "rule" that DOF is 1/3 in front of the focused point and 2/3 behind the focused point. A thoughtful examination of that old,simple "rule" will tell us something, and that something is in this next paragraph.
> 
> Under "close to normal" focusing distances, let's say roughly two feet to 15 feet, as in your tiger and travel coffee cup experiment, depth of field is "reasonably shallow" when the focusing distances are short. However, one depth of field principle that your experiment does not show is that as camera-to-subject distances move beyond about 15 feet to 20 feet, the depth of field on a small-sensor or small-format camera approach the hyperfocal distance EXTREMELY rapidly, and depth of field increases at an amazing,almost exponential rate. And the depth of field at these longer camera-to-subject distances extends roughly one third in front of the focus point, and 2/3 of the total depth of field lies BEHIND the focusing point. At longer focusing distances, longer than this indoor-kitchen table-to kitchen wall range, depth of field increases EXTREMELY RAPIDLY BEHIND THE FOCUS POINT. In other words, depth of field is not linear: it kind of "snowballs" as camera to subject distance increases.
> 
> ...



funny that you wrote that entire paragraph, exactly explaining the exponential graph that I show in the original post.  You also re-explained to me that macro lenses don't depend on focus distance, when I was talking about macro photography as a whole (which exists in its entirety at the far left of the exponential graph, which as we all know, has nearly no slope, thus proving both of our points that the DoF is extremely shallow).  The far away stuff is explained at the other end of the graph, as well as in the paragraph directly under the graph.  I'm sorry if I did not make myself clear enough, sometimes I just expect other people to know exactly what I am talking about.  Thank you for your post, as I think that you more clearly explained my point.  I just wanted to make sure that I'm not disagreeing with you.


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## robertwsimpson (Feb 10, 2010)

Gaerek said:


> robertwsimpson said:
> 
> 
> > you can see background blur by looking at the division in the wall behind the two "subjects" that I've already mentioned.
> ...





> There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field. People have been arguing that a photo taken with a longer photo length (ie the long side of a zoom lens) will have a smaller DoF, given the same f-stop setting. As I learn best by doing, I have some exhibits that should help show what is actually happening. Hope this helps. *Keep in mind that the basis of this is "similarly framed photographs."*



please make sure to read the whole OP before commenting.


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## Stosh (Feb 10, 2010)

Gaerek said:


> *Brings out his handy dandy iPhone, with his handy dandy DoF calculator app*
> 
> Focal Length: 50mm
> Aperture: f/3.5
> ...



Yes, Gaerek, you said the key words - "with all else being equal".  Problem is that we're doing experiments that keep the field of view equal.  And to attain same fields of view, you have to double your distance when you double your focal length.  That's the only way to keep the same field of view.  Now plug that back into your formulas and you'll see that if you double both focal lengths and distance, you'll have the same depth of field.  Thus changing focal length has no effect on DOF.


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## Derrel (Feb 10, 2010)

Robert, it's not quite clear whose comments you are referring to in your post, post #17.

One thing most people might not be aware of is the point that Stosh made in his post, and that is that there is a difference between "depth of field" and "background blur".

The Bob Atkins pages have a really great example of how with a longer focal length lens that has a much larger-diameter aperture, ie, a 200mm lens with a  physically larger f/4 aperture, the degree of background blur will be greater with a lens that has a larger physical aperture, because background blur is NOT related to f/stop of f/ratio, but to actual, physical SIZE of the opening through which the light rays pass.

Once again, your kitchen experiment is not showing anywhere near the complexity of the subject; as I said,I don't know exactly who you were referring to when suggesting that we discard what we "know or have been told" and that we evaluate these images based on what we see, but I do know that these images have very,very little detail for comparative analysis, and they also do not show anything at long range; the test scenario
you chose is interesting, but it does not even begin to model the complexities of real-world depth of field behavior. I'd love it if your sample had a subject positioned at 10 feet from the camera, with a background 75 feet distant, and shot with 200,135,85,50,and 35mm lenses so that the foreground object was the same exact size in the frame, and all shots were done at f/4, and we could see the different degrees of background blur, not depth of field, that Stosh addressed in his post.

What we "see" in your samples is not "proving" much about DOF. It's a nice, but very limited example of depth of field at CLOSE range within the confines of a narrow focal length range of 18 to 55mm, or over a roughly 3x zoom ratio ALL of which lies within the wide-to-normal focal length range, and all of which is done within the confines of a kitchen's distance. I'm afraid your experiment fails to accurately describe the totality, or the real-world behavior of DOF in actual,normal, "real" photographic situations at focusing distances of say 10,12,15,20,25 feet. Your experimental set-up also lacks anything long in foal length and seriously lacks any lens with a truly,physically LARGE aperture, which goes back to Stosh's point about depth of field and background blur.


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## robertwsimpson (Feb 10, 2010)

Stosh said:


> Gaerek said:
> 
> 
> > *Brings out his handy dandy iPhone, with his handy dandy DoF calculator app*
> ...



Exactly.  


My whole point is this:
If someone comes into the forum and asks something like, "I want to take a picture of my niece with a blurred background."  The answer is not, "use the long end of your lens," since the DoF will be the same at the long and wide ends.  Further, on a variable aperture lens (like the ones that come with the camera that these people buy), a smaller DoF is achievable at the wide end.

That's my point.


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## robertwsimpson (Feb 10, 2010)

Derrel said:


> Robert, it's not quite clear whose comments you are referring to in your post.
> 
> One thing most people might not be aware of is the point that Stosh made in his post, and that is that there is a difference between "depth of field" and "background blur".
> 
> ...



Your experiment has 1 major flaw.  The subject wouldn't be 10 feet from the camera for a shot with a 200mm and a 35mm lens.  As mentioned, to keep the subject the same size in the frame, the distance between the camera and the subject changes greatly.


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## Stosh (Feb 10, 2010)

robertwsimpson said:


> you can see background blur by looking at the division in the wall behind the two "subjects" that I've already mentioned.
> 
> That wall is way over twice the focus distance in both of those photos.  The background in the photo shot at 55mm appears "blurrier" because it appears larger, due to the compression of distance effect that I mentioned in my original post.  Just because it is larger in the 55mm photo does not mean that it is blurrier.  To me, it appears to be the same as in the 18mm photo as far as blur.  Of course, it is harder to tell on the 18mm photo because of the much smaller apparent size of the object in question.
> 
> Try to discard what you "know" or have been told, and evaluate these images based on what you see.  If you indeed see something different than I do, please let me know.  As I said, this should be a learning process.  I like learning as much as the next person (probably moreso).



I did look at your examples.  Both shots at 18mm are easy to compare because they the same backgrounds.  Look at that white "ghost" in the center.  It's definitely more blurred in exhibit 1.

Now on to exhibit 2.  If the white ghost is still there (hard to tell), it's so blurred it's pretty much gone.  Do you agree with this observation?

I think your experiments are great.  So far, they agree 100% with what we know about depth of field and background blur.  I'd love to hear your comments too.


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## Stosh (Feb 10, 2010)

robertwsimpson said:


> Exactly.
> 
> 
> My whole point is this:
> ...


But the answer usually *is* use the longer end of lens because of background blur, not depth of field.  You need to separate the 2 types of blur.  Purely for depth of field, you're exactly right - it's ONLY related to f-ratio, not focal length.  But because longer focal lengths have larger physical apertures, they WILL have more background blur than the same f-ratio at shorter focal lengths.

What most people don't understand is that it really has nothing to do with the focal length, only that longer focal lengths usually have a larger physical apertures.  That's why they'll make more background blur.


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## robertwsimpson (Feb 10, 2010)

Stosh said:


> robertwsimpson said:
> 
> 
> > you can see background blur by looking at the division in the wall behind the two "subjects" that I've already mentioned.
> ...



lol why would I compare two shots at 18mm with different f-stop values for background blur?  I know that a smaller f-number is going to blur the background more.  No one would ever debate that.

The "ghost" is a light reflection in the translucent glass of the back wall that went away because I pulled the camera back for the 55mm shot, so it does not serve us any good for the purposes of this experiment.


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## robertwsimpson (Feb 10, 2010)

Stosh said:


> robertwsimpson said:
> 
> 
> > Exactly.
> ...



That is something that I will have to test and get back to you on.  Clearly, it requires more space than the confines of my office at work.


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## Stosh (Feb 10, 2010)

robertwsimpson said:


> That is something that I will have to test and get back to you on.  Clearly, it requires more space than the confines of my office at work.



An interesting experiment I plan to do is to take different pics at different focal lengths, same FOV, but *same apertures* (which means different f-ratios).  For example, same subject taken with all these setups:
50mm at f/2.0
100mm at f/4.0
200mm at f/8.0
All of these shots would have 25mm of physical aperture.  As long as the background was more than double the subject distance in all cases, and the subject was framed the same, the background blur should theoretically be the same in all shots, although the DOF will be obviously quite different.

I really enjoy this stuff.  Understanding why things happen is the key to making it happen in your photographs.


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## Derrel (Feb 10, 2010)

What this kitchen example shows is that at close range, a small stuffed animal can be made to span the frame, and the background can be blurred, when 1) the object being photographed is small in size and the lens angle of view is narrow and 2) when the focused distance is realtively close and WELL short of approaching the hyperfocal focusing distance.

Robert, to show how skewed this stuffed animal and 18-55mm lens example is, I'd like to ask you to take your camera to work at the auto dealership, and photograph a car in side-view, so that the entire driver's side of a car is shown in the frame, using the 18-55mm zoom lens, and a lens aperture of f/5.6. At the shooting distances required, we will have a VERY,very different set of example photographs to look at. I'd like to see a full side viw at 18mm and one done at 55mm, and then look at how blurry the background in THOSE photographs is.


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## robertwsimpson (Feb 10, 2010)

Stosh said:


> robertwsimpson said:
> 
> 
> > That is something that I will have to test and get back to you on.  Clearly, it requires more space than the confines of my office at work.
> ...



That is something that I find hard to believe... I anxiously await your results!


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## robertwsimpson (Feb 10, 2010)

Derrel said:


> What this kitchen example shows is that at close range, a small stuffed animal can be made to span the frame, and the background can be blurred, when 1) the object being photographed is small in size and the lens angle of view is narrow and 2) when the focused distance is realtively close and WELL short of approaching the hyperfocal focusing distance.
> 
> Robert, to show how skewed this stuffed animal and 18-55mm lens example is, I'd like to ask you to take your camera to work at the auto dealership, and photograph a car in side-view, so that the entire driver's side of a car is shown in the frame, using the 18-55mm zoom lens, and a lens aperture of f/5.6. At the shooting distances required, we will have a VERY,very different set of example photographs to look at. I'd like to see a full side viw at 18mm and one done at 55mm, and then look at how blurry the background in THOSE photographs is.



I'm not interested in testing that.  What I wanted to test is exactly what I tested.  I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance.  This is where the smallest DoF is achievable.  I'm confused as to what you're getting at.  We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus.  In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically.  Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.


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## Stosh (Feb 10, 2010)

robertwsimpson said:


> That is something that I find hard to believe... I anxiously await your results!


No experiments real soon.  Have to wait until the 2-3 feet of snow melt lol.


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## Gaerek (Feb 10, 2010)

I apologize, I misread what was being discussed. It was a bit early for me, and not totally awake when I posted. Carry on! Interesting discussion, for sure.


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## Derrel (Feb 10, 2010)

robertwsimpson said:


> I'm not interested in testing that.  What I wanted to test is exactly what I tested.  I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance.  This is where the smallest DoF is achievable.  I'm confused as to what you're getting at.  We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus.  In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically.  Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.



Robert, you began with this statement: "There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field" Sorry, but that statement is flat-out wrong: focusing distance has been calculated from the ***focal plane*** Since Day One, and ***not*** from the front of the lens. There is even a small circle with a line scribed through it, located immediately to the left of the hot shoe on most Canon cameras,showing the focal plane. On a 200mm macro lens, calculating the DOF from the front of the LENS would be a joke. So, your understanding of the most basic aspect of DOF starts off with an error of fact, and a misunderstanding. So, frankly, that alone made me suspicious of how much you really "know" about this technical subject.

Then your transitioned into this gem: "First, Depth of Field has far more to do with focus distance than focal length. Let me explain. Everyone is familiar with the focus graph. If not, it looks something like this:" [graph]

Well, let's plug in some REAL examples,and see how your words actually square with the facts. What you stated as a fact, as a principle, does not square with the real world.  Online Depth of Field Calculator

55mm lens, focused at 15 feet,aperture f/16. DOF near limit 10.3 feet, far limit 27.5 feet, total DOF band is 17.2 feet deep. In front of subject 4.7 feet or 27%, behind subject 12.5 feet or 73%. Hyperfocal distance for 55mm lens on 1.6x Canon 32.8 feet.

200mm lens, focused at 15 feet, aperture f/16. DOF near limit, 14.5 feet, far limit 15.5 feet, total DOF band, 1.0 feet deep. In front of subject 0.48 feet or 48%, behind subject 0.52 feet (52%). Hyperfocal distance for a 200mm lens on 1.6x Canon, 423.3 feet.

So, your words are clearly wrong. Erroneous. You wrote, "First, Depth of Field has far more to do with focus distance than focal length. Let me explain." Okay---then please explain, correctly this time, what you meant to say. Because as it is obvious, at the identical focus distance of 15 feet, the depth of field from a 55mm lens stopped down to f/16 produces a deep, expansive band of depth of field---the total depth of field with the 55mm lens focused at 15 feet is 17.2 feet deep from front to back. Now, with a 200mm lens, also set to a 15 foot focusing distance, the total depth of field with the lens stopped down to f/16 is 1.0 feet deep from front top back.

So, when you wrote, ""First, Depth of Field has far more to do with focus distance than focal length.Let me explain." what you wrote was wrong. If you wish to give us a dissertation on how to calculate depth of field, then begin 1) with the correct location from which DOF is calculated, and then please use some of the on-line resources to check the veracity of your second premise, which as the numbers have proven here, was simply incorrect. There is a reason peer review is called for in serious academic work, and so when other people try and correct your mis-statements and oversimplifications, I think perhaps you ought to consider that what you "know" might not be provable, and that what "you know" and what you stated as facts in this discussion began with two big errors, right off the bat.


----------



## robertwsimpson (Feb 10, 2010)

Stosh said:


> robertwsimpson said:
> 
> 
> > That is something that I find hard to believe... I anxiously await your results!
> ...


BLAST!  Let me know when you actually do that, because I would be interested to see what you come up with.


Gaerek said:


> I apologize, I misread what was being discussed. It was a bit early for me, and not totally awake when I posted. Carry on! Interesting discussion, for sure.


no worries.  I love discussing stuff like this, since theoretical and real world application seem only loosely related with each other sometimes.  It's fun to see if you can make real world results show what theoretical calculations say should be happening.


----------



## robertwsimpson (Feb 10, 2010)

Aw man, if you put someone on your ignore list, it still shows that they've made an inane comment in your thread?  Lame.


----------



## Stosh (Feb 10, 2010)

Derrel said:


> robertwsimpson said:
> 
> 
> > I'm not interested in testing that.  What I wanted to test is exactly what I tested.  I intentionally picked a subject that I could fit to the frame at nearly minimum focus distance.  This is where the smallest DoF is achievable.  I'm confused as to what you're getting at.  We all know that the blurring is more slight when the subject (no matter what size) is closer to infinite focus.  In fact, I have a whole paragraph in my OP explaining this, as well as a graph that represents it, well, graphically.  Maybe I am not getting what you are trying to say, but it appears to me that you are repeatedly explaining stuff to me that I already know.
> ...



Woah guys, take it easy.  Although technically Derrel's correct in what he wrote, I gave Robert some leeway, based on the rest of his post, that he actually did understand DOF, but was ignoring background blur.  The argument about focal distance to front of lens instead of focal plane is definitely correct, but irrelevant in most cases.  Knowing that Robert specified that the FOV be the same for his experiment, I knew that he didn't mean it how you interpreted it.  He meant that you can achieve out of focus backgrounds if you choose a distance that's close to minimum focus, then go from there with your other lenses to achieve the same FOV.


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## SoonerBJJ (Feb 10, 2010)

robertwsimpson said:


> Aw man, if you put someone on your ignore list, it still shows that they've made an inane comment in your thread? Lame.


 
Just curious... were you trying to ignore Derrel's last post?


----------



## robertwsimpson (Feb 10, 2010)

SoonerBJJ said:


> robertwsimpson said:
> 
> 
> > Aw man, if you put someone on your ignore list, it still shows that they've made an inane comment in your thread? Lame.
> ...



Yes, I find most of what he says useless to me. (no offense, Derrel.)

I'd rather just not even know it's there.


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## robertwsimpson (Feb 10, 2010)

Stosh said:


> Derrel said:
> 
> 
> > robertwsimpson said:
> ...



Exactly.  If you're trying to take a photo of a particular object, then it doesn't matter that changing the focal length at a particular distance from the subject makes the DoF smaller, because now your subject is not correctly in the frame anymore.


----------



## robertwsimpson (Feb 10, 2010)

I apologize to everyone for not knowing the correct verbiage to convey my point.  I didn't realize that exact wording was so important.


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## SoonerBJJ (Feb 10, 2010)

robertwsimpson said:


> I apologize to everyone for not knowing the correct verbiage to convey my point. I didn't realize that exact wording was so important.


 
If you are presuming to educate others then using the correct verbiage IS important.

You say you are open to learning and wanted this to be an open discussion yet you "ignore" the participants who question your lesson?  Hmmm...  note to self.


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## robertwsimpson (Feb 10, 2010)

SoonerBJJ said:


> robertwsimpson said:
> 
> 
> > I apologize to everyone for not knowing the correct verbiage to convey my point. I didn't realize that exact wording was so important.
> ...



You're questioning the person when you should be questioning the data.  Unfortunately, there isn't much to question, as I have shown concrete examples of what I am asserting.  

I am learning the correct words, and I am discussing this with people in this thread.  I am not going to read comments that are disrespectful or sarcastic, and I am doing my best not to type comments that are that way.  I'll thank you to do the same.


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## SoonerBJJ (Feb 10, 2010)

If you don't know the correct terminology then perhaps your assertions also bear closer scrutiny.

I have to acknowledge the irony of someone making a thread professing to bring light to the forum's "confusion" on a certain subject when it seems the OP is just as confused.

I, for one, am grateful for those who stepped in to give some clarification to an obviously complex subject.


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## SoonerBJJ (Feb 10, 2010)

BTW I think rigorous peer review is good for the forum.


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## robertwsimpson (Feb 10, 2010)

Of course it is good.  That's why I made the thread.  There aren't enough people in the world who questions commonly held beliefs.  I found out what I expected.  If you guys don't like the results, I just won't share next time.


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## inTempus (Feb 10, 2010)

Robert, you're not only wrong but your insistence that you're right despite ample evidence debunking your false premises is somewhat aggravating to others (or so it would seem).  

I know Derrel is a pain in the rear end, and he's often times flat out wrong or makes up "facts" out of thin air... but in this instance he's actually right.

Why don't you do some of your own more exhaustive testing vs. waiting on someone else to prove you wrong?  You seem to have the time and interest in doing it.  I think you might be surprised by what you find.  If someone else shows you to be wrong, given your attitude in the thread thus far, you'll probably just dismiss their post.


----------



## robertwsimpson (Feb 10, 2010)

inTempus said:


> Robert, you're not only wrong but your insistence that you're right despite ample evidence debunking your false premises is somewhat aggravating to others (or so it would seem).
> 
> I know Derrel is a pain in the rear end, and he's often times flat out wrong or makes up "facts" out of thin air... but in this instance he's actually right.
> 
> Why don't you do some of your own more exhaustive testing vs. waiting on someone else to prove you wrong?  You seem to have the time and interest in doing it.  I think you might be surprised by what you find.  If someone else shows you to be wrong, given your attitude in the thread thus far, you'll probably just dismiss their post.



I'm confused.  Can you give me an instance of where I was wrong?  I tested something and came out with the result that I expected.  Darrel things I am wrong because he is missing my point completely.  I am sure that is due in part to my inability to communicate my point clearly, and also to his misreading what I am saying.  I am losing interest in trying to tell him that we're both saying the same thing in different ways.


----------



## SoonerBJJ (Feb 10, 2010)

robertwsimpson said:


> I tested something *IN ORDER TO COME* out with the result that I expected.


 
I corrected this statement for you.


----------



## robertwsimpson (Feb 10, 2010)

SoonerBJJ said:


> robertwsimpson said:
> 
> 
> > I tested something *IN ORDER TO COME* out with the result that I expected.
> ...



Hey, if it weren't true, I wouldn't have been able to do it.


----------



## inTempus (Feb 10, 2010)

Robert, therein lies the problem.  Your mistakes have been explained ad nauseum by several other posters but you don't seem to register what they have tried to explain.  I'm sure my jumping in and repeating what they've said will be of little benefit.

Just trust that you're assumptions and conclusions are wrong.


----------



## robertwsimpson (Feb 10, 2010)

inTempus said:


> Robert, therein lies the problem.  Your mistakes have been explained ad nauseum by several other posters but you don't seem to register what they have tried to explain.  I'm sure my jumping in and repeating what they've said will be of little benefit.
> 
> Just trust that you're assumptions and conclusions are wrong.



Sorry, but I'll trust my actual data long before I'll trust anyone on the internet.


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## Bitter Jeweler (Feb 10, 2010)

robertwsimpson said:


> Sorry, but I'll trust my actual data long before I'll trust anyone on the internet.


 
:lmao:

Dude, you are on the internets, and therefore, you shouldn't trust yourself.

:lmao:


----------



## robertwsimpson (Feb 10, 2010)

Bitter Jeweler said:


> robertwsimpson said:
> 
> 
> > Sorry, but I'll trust my actual data long before I'll trust anyone on the internet.
> ...



oh crap, you're right


----------



## inTempus (Feb 10, 2010)

robertwsimpson said:


> inTempus said:
> 
> 
> > Robert, therein lies the problem.  Your mistakes have been explained ad nauseum by several other posters but you don't seem to register what they have tried to explain.  I'm sure my jumping in and repeating what they've said will be of little benefit.
> ...


And it's exactly that attitude that makes trying to help you an utterly futile endeavor.  

So, continue to be wrong.  Just do yourself (and the other noobs out there) a favor and not preach to them about what you think you know about DoF and get them all twisted around with their understanding of such important concepts.


----------



## robertwsimpson (Feb 10, 2010)

inTempus said:


> robertwsimpson said:
> 
> 
> > inTempus said:
> ...



You should probably just put me on your ignore list


----------



## Overread (Feb 10, 2010)

I wish you lot would learn to debate rather than just keep sniping at each other. It gets very wearing reading though threads where its only sniping at each other rather than a proper healthy debate (as such its taking me a while to get through this thread) 

A quick observation though is could you repeat the test where this time you shoot a subject away from the minimum focusing distance of the lens and keep the camera to subject distance and the aperture but vary the focal length.


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## kundalini (Feb 10, 2010)

A little humility goes a l-o-n-g way


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## inTempus (Feb 10, 2010)

robertwsimpson said:


> You should probably just put me on your ignore list


Nah, I enjoy reading threads where folks impale themselves.  

I'll let ya have your thread back.  I'll keep checking in to see how things progress... hopefully they do progress.


----------



## Derrel (Feb 10, 2010)

robertwsimpson said:


> By the way, comments and questions are very welcome.  We're all here to learn.  If you think I've gotten something wrong here, please let me know.  Just be respectful.




Hmmm, that's interesting that you would say that.

"You got some things wrong."


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## robertwsimpson (Feb 10, 2010)

Overread said:


> I wish you lot would learn to debate rather than just keep sniping at each other. It gets very wearing reading though threads where its only sniping at each other rather than a proper healthy debate (as such its taking me a while to get through this thread)
> 
> A quick observation though is could you repeat the test where this time you shoot a subject away from the minimum focusing distance of the lens and keep the camera to subject distance and the aperture but vary the focal length.



I know what will happen when I do that, because I have zoom lenses.  When you zoom in on an object without moving yourself or that object, your DoF becomes smaller.  That's not what I'm debating.  The fact that people keep trying to debate this fact is the thing that is getting annoying in this thread.


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## robertwsimpson (Feb 10, 2010)

kundalini said:


> A little humility goes a l-o-n-g way



so does making a contribution to a debate instead of dropping by to say something that doesn't have to do with anything being talked about.


----------



## robertwsimpson (Feb 10, 2010)

inTempus said:


> robertwsimpson said:
> 
> 
> > You should probably just put me on your ignore list
> ...



given the last 3 "contributions," not likely.


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## SoonerBJJ (Feb 10, 2010)

robertwsimpson said:


> kundalini said:
> 
> 
> > A little humility goes a l-o-n-g way
> ...


 
I think the debate ended awhile ago.  You're the only one who hasn't caught on.

"Impaling" is a good descriptor.  I was thinking it was like watching a trainwreck in slow motion but I like impaling better.

:thumbup:


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## kundalini (Feb 10, 2010)

robertwsimpson said:


> so does making a contribution to a debate instead of dropping by to say something that doesn't have to do with anything being talked about.


WOW, I thought I was on your ignore list. 

My contribution was a concern for cooler heads to prevail before this got really nasty (again) for no good reason. The fact that I haven't contributed to this point doesn't represent this was my first glance at this thread. The fact that I'm pretty comfortable with my gear to obtain the background blur that I desire doesn't mean that I can't still learn more. Stosh and Derrel (yes, there's an *e* in his name) made some very good comments on the technical side. It became quite boring with your unbending stance.... and lack of humility. The fact that someone else may_ know more _about a subject than you is life. How you deal with it shows character. You did a good thing to start this off for newbies, but this is a far cry from a debate.

Now, if you care to retort with another smartass comment, fire away.


----------



## NateWagner (Feb 10, 2010)

Robert, check out this site, it pretty much agrees with exactly what Stosh and Derrel have been saying. 

http://www.bobatkins.com/photography/technical/bokeh.html
Bokeh and Background Blur Calculator- Bob Atkins Photography

(the image was taken from the site linked, it is not mine. If this is against specific rules I will pull it down, but I thought it was a good example to show those interested).






As you can see, although they were taken from different distances in order to achieve the same FOV, the blurriness of the background is different. The primary reason for this is the physical size of the aperture.


----------



## Gaerek (Feb 10, 2010)

NateWagner said:


> Robert, check out this site, it pretty much agrees with exactly what Stosh and Derrel have been saying.
> 
> Bokeh and Background Blur - Bob Atkins Photography
> Bokeh and Background Blur Calculator- Bob Atkins Photography
> ...



So Robert's experiment had been tried before...they did a better job of it, in a MUCH more controlled environment, and got the results that everyone BUT Robert have been saying he should have gotten? Absolutely amazing...


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## robertwsimpson (Feb 11, 2010)

NateWagner said:


> Robert, check out this site, it pretty much agrees with exactly what Stosh and Derrel have been saying.
> 
> Bokeh and Background Blur - Bob Atkins Photography
> Bokeh and Background Blur Calculator- Bob Atkins Photography
> ...



Thank you for actually presenting data with your post.  It's refreshing!

I saw the same thing in my photos at 18mm and 55mm... I guess the way that I saw it was that the background becomes apparently larger with longer focal lengths, which in my mind didn't mean that they were necessarily more blurry, just that they were larger.  In this version with the trees, they do look more blurry though.  Interesting.


----------



## robertwsimpson (Feb 11, 2010)

Gaerek said:


> NateWagner said:
> 
> 
> > Robert, check out this site, it pretty much agrees with exactly what Stosh and Derrel have been saying.
> ...



Actually, my experiment was regarding depth of field, not background blur, which is a different matter (as already discussed in this thread).  Please take your sarcasm elsewhere.  Here, it is not appreciated.


----------



## Dao (Feb 11, 2010)

robertwsimpson said:


> NateWagner said:
> 
> 
> > Robert, check out this site, it pretty much agrees with exactly what Stosh and Derrel have been saying.
> ...



In real life, there is a definition of sharpness.  Read up on Circle of Confusion (CoC).  If you talks about DoF, you may want to know about CoC first.  

If there are 2 dots in the background, with the 18mm lens, the 2 dots may appear as 1 dot.  However, when you use a telephoto lens, the 2 dots may appear to be 2 blurry dots.  But of course, when you print that photo out and look at it at a distance, it may appear as one dot again.


In your example. the effect may not be too huge.  However, if you compare a 24mm photo with a 300mm photo, you may see a bigger difference.   When an out of focus object was enlarged, it may appear to have more blur especially when the background is further away from the subject.

You can do another experiment.  Subject in the foreground. And an big object such as  road sign (in a distance) in the background.  Take a photo with 24mm, then take another one with 200mm or 250mm with the framing and with aperture set as F/5.6.  You may notice that the road sign may appear pretty sharp (of course, it depends on the distance between the subject and the camera).  And the road sign in the one you take with the 250mm may appear to have more blur.

But I believe once you enlarge the road sign in the 24mm photo so that the size of the road sign is about the same as the one in 250mm, both sign may look about the same.

I think what that means is the DoF maybe the same, however, the one with the telephoto will have a better blur background and that is what people always want to suggest to other to use a telephoto lens when someone want to have a nice bokeh. (of course, there are other options as well such as aperture size, medium format, subject to camera and subject to background distance)


Of course, this is just my theory, I could be wrong.   I hope HelenB is here to explain in details!


----------



## robertwsimpson (Feb 11, 2010)

Dao said:


> robertwsimpson said:
> 
> 
> > NateWagner said:
> ...



That's what I am thinking actually.  It's very hard to judge background blurriness when there is such a giant size difference in the photo with different focal lengths... That's why I was mainly concentrating on Depth of Field.

When an object is larger in a photo, but the same amount of blur is present, it definitely appears "more" blurry, so maybe that is why people go for longer focal lengths.


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## Dao (Feb 11, 2010)

When you read up on Circle of Confusion, you will see the Depth Of Field is related to it closely.  Let's take another example, you take a photo and print it on a 4x6 and view it at the normal hand held distance.  Now print the same photo on a poster size and view it at the same distance.

You may notice some object in the background now appear to be out of focus in the poster because the object is blur.   However, same object in the other photo is in focus since it is decently sharp.  Do you think the Depth of field had changed?


----------



## robertwsimpson (Feb 11, 2010)

No, I think that when you view something in a larger print, it looks blurrier because it is larger.  Nothing about the photo has changed, you're just able to inspect it at a closer level, because it is larger.

Also, I have read a little about the circle of confusion.  I could definitely stand to read more, but on a basic level, I get what is happening.  It's the overlap in the circles that causes the shallow DoF and the background blur.  Larger aperture opening means larger circles means more overlap.


----------



## Gaerek (Feb 11, 2010)

robertwsimpson said:


> Actually, my experiment was regarding depth of field, not background blur, which is a different matter (as already discussed in this thread). Please take your sarcasm elsewhere. Here, it is not appreciated.


 
So, your experiment (hard to call it an experiment, as there were still far too many variables in it, but I'll give it to you anyway) proved that in a very specific circumstance, the apparent DoF doesn't change with focal length, given the same framing. 

Whether background blur is another issue is completely irrelavent, because for all practical purposes, in most circumstances, it's the blurry background people are looking for, not a shallower DoF. DoF is just another way to control the apparent blurriness of the background. Personally, I think it'd be great if there were a control that would allow me to control background blur independently of DoF. Oh wait, there is, it's called zooming in and stepping back. Why do you think most portraits are taken in the 80-100mm range?

The old adage of, "If you want that nice blurry background, bring out the telephoto." is still very relavant, which is what everyone has been trying to tell you. Your experiment really proves very little practical, as a telephoto will give you a more blurred background, given the same framing as a wider lens. Same DoF? Fine, but what does that prove, in most practical circumstances? You will still get a more blurred background using a telephoto.

So, since you're obviously wanting this, you're right! You're completely right! Focal length doesn't affect DoF given the same framing! There, I've admitted you were right. You weren't here to learn, you were here to prove others wrong. This was glaringly obvious from the start, especially the part where you ignored the person that was trying to explain what I explained above. So much for trying to learn from others here.

I hope I'm not on ignore since I was being sarcastic. God forbid I might have something to teach.


----------



## inTempus (Feb 11, 2010)

Sweet, progress.  Nate, you're a good guy for bringing forth the evidence necessary to set things right in this thread.


----------



## robertwsimpson (Feb 11, 2010)

Gaerek said:


> robertwsimpson said:
> 
> 
> > Actually, my experiment was regarding depth of field, not background blur, which is a different matter (as already discussed in this thread). Please take your sarcasm elsewhere. Here, it is not appreciated.
> ...



First, let me address the now infamous placing of darrel on the ignore list.  This is not the first time that I have wanted to ignore him.  He trolls the forum making stupid comments to new people and seldom attempts to help anyone.  I don't really care to read sarcastic or rude comments in this thread or anywhere else.  Problem solved.

Second, I was indeed testing DoF.  I'm not sure why you think this is useless, because people claim that DoF becomes smaller when using a telephoto lens.  That is incorrect.  What I didn't know is that people (in the know) consider background blur and DoF two different things.  I'd imagine I'm not the only one too.  Now I know, and I can more intelligently speak on the topic.  That was the whole point of this thread.  Now I can see why people say "bring out the old telephoto."  It's not because the DoF becomes smaller, but rather, the background blur is more pleasing.  Lesson learned.

Stop being rude, sarcastic or anything else that you think is funny or will get a rise out of people and talk about the task at hand.  It will keep things on topic and everyone will get more out of discussion.


----------



## robertwsimpson (Feb 11, 2010)

inTempus said:


> Sweet, progress.  Nate, you're a good guy for bringing forth the evidence necessary to set things right in this thread.



Indeed, this forum needs more Nates.


----------



## inTempus (Feb 11, 2010)

robertwsimpson said:


> inTempus said:
> 
> 
> > Sweet, progress.  Nate, you're a good guy for bringing forth the evidence necessary to set things right in this thread.
> ...


And not more inTempus's?

I'm crushed.

At least we agree on Derrel's "contributions" to the forum.  :thumbup:


----------



## NateWagner (Feb 11, 2010)

One other thing to remember is before the telephoto is truly useful with regards to background blur, vs. say a 50mm 1.4 the background has to be a ways away. I.E. if the background is close to the subject a 50 1.4 will actually create a more blurred background than say a 135 at 2.8. However, as the background approaches infinity distance that swings the other way. 

In other words, *if* you're able to separate the subject from the background the fast telephoto is typically your best bet.


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## Dao (Feb 11, 2010)

robertwsimpson said:


> No, I think that when you view something in a larger print, it looks blurrier because it is larger.  Nothing about the photo has changed, you're just able to inspect it at a closer level, because it is larger.
> 
> Also, I have read a little about the circle of confusion.  I could definitely stand to read more, but on a basic level, I get what is happening.  It's the overlap in the circles that causes the shallow DoF and the background blur.  Larger aperture opening means larger circles means more overlap.




What is the meaning of DoF? I believe it is the range of distance that appear to be sharp. 

Take a look at this
Image quoted from Understanding Depth of Field in Photography








When you look closer, I will say the Depth Of Field is only 3 dots.  When you look it at a distance, it seems to be 5 dots.


----------



## Gaerek (Feb 11, 2010)

Dao said:


> robertwsimpson said:
> 
> 
> > No, I think that when you view something in a larger print, it looks blurrier because it is larger. Nothing about the photo has changed, you're just able to inspect it at a closer level, because it is larger.
> ...


 
That's a very neat illustration. And it makes perfect sense. When I look close, only 3 dots are in sharp focus. When I step back from my screen, 5 look to be sharp. I think this is a perfect example of CoC, DoF, and how they relate to each other and with all types of blur in general.


----------



## robertwsimpson (Feb 11, 2010)

inTempus said:


> robertwsimpson said:
> 
> 
> > inTempus said:
> ...


lol 




NateWagner said:


> One other thing to remember is before the telephoto is truly useful with regards to background blur, vs. say a 50mm 1.4 the background has to be a ways away. I.E. if the background is close to the subject a 50 1.4 will actually create a more blurred background than say a 135 at 2.8. However, as the background approaches infinity distance that swings the other way.
> 
> In other words, *if* you're able to separate the subject from the background the fast telephoto is typically your best bet.


That's why I was trying to use a constant f-number.  Interesting things to think about though.




Dao said:


> robertwsimpson said:
> 
> 
> > No, I think that when you view something in a larger print, it looks blurrier because it is larger.  Nothing about the photo has changed, you're just able to inspect it at a closer level, because it is larger.
> ...



direct quote from the first paragraph of that article:

The depth of field varies depending on camera type, aperture and focusing distance, although print size and viewing distance can influence our perception of it.



Gaerek said:


> Dao said:
> 
> 
> > robertwsimpson said:
> ...



stepping back does not change the DoF of the photo.  It changes your perception of it.  Same thing with small prints vs large prints.

Think of it this way:

When you take a 800x600 resolution photo and print it out on a 8'x6' canvas and look at it close up, it will appear pixelated.  Step back far enough, and you won't see the individual pixels any more, making it appear clearer.  Is this because stepping back increases the resolution of the print?  No, it's because your eye can not perceive the fine detail of the print anymore.


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## robertwsimpson (Feb 11, 2010)

It is definitely an interesting thing to think about, though.  IE, if you're planning to use a specific DoF, it will appear deeper in small prints and shallower in large prints.  Something to keep in mind!


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## Dao (Feb 11, 2010)

Remember one important point here, the meaning of DoF.

"Range of distance that appear to be sharp"  And it is human perception as in Circle of Confusion.



From Wikipedia

"In photography, the circle of confusion (&#8220;CoC&#8221 is used to determine the depth of field, the part of an image that is acceptably sharp. A standard value of CoC is often associated with each image formathttp://en.wikipedia.org/wiki/Film_format, but the most appropriate value depends on visual acuity, viewing conditions, and the amount of enlargement. Properly, this is the _maximum permissible circle of confusion_, but it is commonly referred to simply as the _circle of confusion"
_


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