Focal length, focus distance, and depth of field

[Derrel]

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.

 

[robertwsimpson]

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).

 

[Gaerek]

*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).

 

[Gaerek]

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.

 

[robertwsimpson]

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.

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.

 

[robertwsimpson]

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.

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.

 

[Stosh]

*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).

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.

 

[Derrel]

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.

 

[robertwsimpson]

*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).

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.

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.

 

[robertwsimpson]

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".

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. Itg'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.

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.

 

[Stosh]

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.

 

[Stosh]

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.

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.

 

[robertwsimpson]

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.

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.

 

[robertwsimpson]

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.

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.

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.

 

[Stosh]

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.