Luminous Landscape Forum
Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: dwdallam on April 06, 2008, 02:04:24 am
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This is the link to Michael's esdsay on DoF.
http://www.luminous-landscape.com/tutorials/dof2.shtml (http://www.luminous-landscape.com/tutorials/dof2.shtml)
The conclusion of Michael's essay is that "If the subject image size remains the same, then at any given aperture all lenses will give the same depth of field."
I must have gotten something wrong, or I'm misunderstanding what I have read--to the point of embarrassment, and I admit it.
But I have gotten some different results today using my 24-70 and 70-200 lenses.
What seems to be left out here is that "If the subject image size remains the same, then at any given aperture all lenses will give the same depth of field" --given that the comparison is of an object at infinity, not objects between the subject and infinity (Michael uses the distant tower as a comparison point). Like I said, the more I write, the more I most likely make an ass of myself, but give me some charity here for missing the point of something integral.
My results showed a clear and decisive increase in DoF using a wide angle lens and my 200mm at the same aperture even though the subject remained the same size in the frame, and the comparison object is not at infinity.
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I believe that Michael was cropping and enlarging in his comparison, not changing shooting position to reframe.
The real "take away" from Michael's comparison should be that enlarging print size reduces depth of field.
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In the attached picture, I've enlarged and lightened (due to different exposure) a crop from the 24-70 to match the relative size of the background on the 70-200 shot. They look pretty similar to me with respect to focus.
Remember that the wide angle appears to have better depth of field due to the wider field of view (different perspective) just as a small print seems to have better depth of field and focus than a large print.
When I first started shooting digital I was very dissapointed that shots on my D30 appeared to be in focus but were not once I got them to my monitor. Most were due to motion on either my part or the subjects. I've since learned what I can handhlod and what I can't.
joe
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There have been lengthy discussion on this topic in the past, on this forum. I'm not sure if they are still in the archives. You could try a search.
The concept that Michael appears to be addressing in this article relates to the absolute resolution of the out-of-focus parts, rather than the appearance of sharpness.
If the distance to the subject is varied with the focal length so that the main subject occupies the same area on the film or sensor, the out-of-focus objects in the background will have approximately the same resolution, irrespective of the focal length used. That means, if we had a test chart of fairly widely spaced lines in the OoF background, we'd still be able to count the same number of lines whatever focal length lens we used, provided the main subject is the same size on the sensor.
However, DoF is not just a matter of resolution, but appearance. The appearance of DoF will vary depending on the size of the print we are viewing and the viewing distance from the print.
Although the OoF parts in the image taken with the longer focal length will actually possess the same resolution as the same parts in the image that was taken with the shorter focal length, the background will appear sharper in the image taken with the shorter FL lens because those parts will be less magnified or smaller in relation to the main subject. The background will also be more extensive as a consequence of the smaller magnification.
Small images always appear sharper than big images when both images are of equal resolution.
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I believe that Michael was cropping and enlarging in his comparison, not changing shooting position to reframe.
The real "take away" from Michael's comparison should be that enlarging print size reduces depth of field.
[a href=\"index.php?act=findpost&pid=187420\"][{POST_SNAPBACK}][/a]
I think he says in his essay that he moved the camera to get closer with the wider lens.
"In the 28mm and 17mm frames the hand puppet is no longer visible, because to keep the gremlin doll the same size I had to get closer than 3 feet — inside the distance to the puppet."
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There have been lengthy discussion on this topic in the past, on this forum. I'm not sure if they are still in the archives. You could try a search.
The concept that Michael appears to be addressing in this article relates to the absolute resolution of the out-of-focus parts, rather than the appearance of sharpness.
clipped. . .
Small images always appear sharper than big images when both images are of equal resolution.
[a href=\"index.php?act=findpost&pid=187536\"][{POST_SNAPBACK}][/a]
That sounds technically correct. However, as the eye sees it, you will get a much nicer "appearing" OOF background when you shoot subjects close in with a longer lens than will "appear" with the shorter lens. So it's a little misleading because people start to think that as long as the object is the same size in the viewfinder, the background will "appear" the same. no matter the lens length. Obviously it does not.
There are two reasons I became curious of these two positions: (1) I read a lot of people photography books on all aspects of people, including fashion. In many of the shoots, the fashion photographers are outside using 400 and even 600MM lenses. Why, when a 50 will do? One reason is that you can shoot at f22 and get a blurry background and the entire face is tack sharp, no matter where you focus on the face. (2) I was reading yet another book on photography by a well known author who holds the position that three things dictate what the background looks like: Aperture, focal point, and lens length.
So I remember reading M's DoF essay years ago and wanted to test it, since the book I was reading gave examples like the one I gave and I saw the difference immediately. So I guess technically true, but practically (as the eye sees it) false? Just to make clear, I'm not challenging the laws of physics here--only the way we see them, which is really the important aspect of photography (I think?).
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In the attached picture, I've enlarged and lightened (due to different exposure) a crop from the 24-70 to match the relative size of the background on the 70-200 shot. They look pretty similar to me with respect to focus.
Remember that the wide angle appears to have better depth of field due to the wider field of view (different perspective) just as a small print seems to have better depth of field and focus than a large print.
When I first started shooting digital I was very dissapointed that shots on my D30 appeared to be in focus but were not once I got them to my monitor. Most were due to motion on either my part or the subjects. I've since learned what I can handhlod and what I can't.
joe
[a href=\"index.php?act=findpost&pid=187501\"][{POST_SNAPBACK}][/a]
I still think the F16 example is softer on the 200mm, and as far as the f5.0 goes, it "seems" to me a slam dunk simply because there is no detail on the 200mm side--no matter how much you blow it up. Or, am I missing something here?
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I think he says in his essay that he moved the camera to get closer with the wider lens.
"In the 28mm and 17mm frames the hand puppet is no longer visible, because to keep the gremlin doll the same size I had to get closer than 3 feet — inside the distance to the puppet."
[a href=\"index.php?act=findpost&pid=187549\"][{POST_SNAPBACK}][/a]
My bad... I was working from memory. I should have re-read the original article.
I think I just was remembering the last crop of the 17mm shot where he enlarged it to show that the tower was the same.
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My bad... I was working from memory. I should have re-read the original article.
I think I just was remembering the last crop of the 17mm shot where he enlarged it to show that the tower was the same.
[a href=\"index.php?act=findpost&pid=187563\"][{POST_SNAPBACK}][/a]
True, but the tower is at infinity for all focal lengths. I'm not sure of why this may be important, but I think it is.
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True, but the tower [in Michael's images] is at infinity for all focal lengths. I'm not sure of why this may be important, but I think it is.[a href=\"index.php?act=findpost&pid=187568\"][{POST_SNAPBACK}][/a]
It is indeed.
In Michael's shots, the magnification of the distant tower in the image is purely proportial to the lens' focal length---in the 400 mm image it's twice as big as in the 200 mm image, four times as big as in the 100 mm image, eight times as big as in the 50 mm image etc. The fact that he moved a little closer with the shorter lenses hardly affects the magnification of the tower since it is so far away---one mile is virtually the same distance as one mile and 30 ft.
But in the 400 mm image, the tower is not only eight times bigger than in the 50 mm image; it also has circles of confusion eight times bigger. In other words, in is eight times more blurred. So the ratio of size and blur is kept constant ... in other words, the relative sharpness is the same. And that's Michael's point.
In your example, the background objects are fairly close. So while you took the 200 mm images from a focusing distance five times longer than the 40 mm images in order to keep the foreground object's size constant, you significantly changed the distance to the background objects, too. So in the 200 mm image, the red post is not five times bigger than in the 40 mm image (as an object at infinity would be); it's only three times bigger because you backed up significantly. For the 200 mm shot, you increased the distance to the Aladdin object by a factor of 5, and to the red post by a factor of 5/3.
So generally speaking, Michael's statement is wrong. Depth-of-field is not the same with all lenses when adjusting the focus distance according to focal length. It is approximately true for a few special cases but wrong in general. The special cases include the background at infinity, or comparing a long lens with an even longer lens (like 200 mm telephoto vs. 300 mm telephoto) at not too short focusing distances.
The reason why it's wrong in general is the fact that with different focal lengths at proportional focusing distances, the distances to the objects before and behind the plane of focus do not change proportionally. An object significantly behind the plane of focus (but not at infinity) will be rendered a little bigger and much more blurred with the longer lens. Objects at different distances will get imaged at different magnifications. And with the magnification being the same at the plane of focus, the magnifications will differ before and behind the plane of focus between lenses of different focal lengths in different ways. In Michael's words, that's a second-order effect (and he said we shouldn't bother with 2nd-order effects for practical intents and purposes) ... but this 2nd-order effect cannot be neglected because in many (not all) practical cases its effect *is* significant.
At constant image size DOF only depends on aperture and magnification, so wide-angle lenses generally have (a little) less foreground DOF and (much) more background DOF than telephoto lenses at the same aperture, at corresponding focus distances, and at the same image format. These differences, however, tend to diminish when the background is at infinity or when comparing long lenses to even longer lenses (as opposed to long lenses vs. short lenses); that's why Michael came to the wrong ... or let's say, over-simplified, conclusions.
-- Olaf
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It doesn't take a rocket scientist to figure it out if you shoot portraits close in, or you do macro or close up photography using a macro tube an a 400mm lens. It's obvious that a 200mm lens gives you a much shorter DoF. That's why I use it all the time at 150mm and over. I'm even getting an extension tube to increase its reach by 80mm.
I wonder if M will update his essay incorporating this information, with examples?
Thanks for those who replied and will continue to reply.
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I think one of the aspects of this ios that the longer the focal length, teh larger teh aperture is in comparison.
Something like "the physical aperture size of an f/2.8 setting on a 200mm lens is twice the area of the 100mm f/2.8 opening." So it's going to let more light in and have a shallower DoF.
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... or you do macro or close-up photography using a macro tube on a 400 mm lens.[a href=\"index.php?act=findpost&pid=187782\"][{POST_SNAPBACK}][/a]
Umm ... using a 400 mm lens on a 35-mm-format or APS-C-format camera at close distance is one of those situations where the 2nd-order effects mentioned above hardly show up. Here, Michael's statement would be true; when keeping the magnification constant then it doesn't matter whether you were using a 200 mm lens or a 400 mm lens, the resulting DOF would be the same.
It's obvious that a 200 mm lens gives you a much shorter DoF.[a href=\"index.php?act=findpost&pid=187782\"][{POST_SNAPBACK}][/a]
Huh!? Shorter than what? Than the 400 mm lens? Definitely not.
Everybody knows the three basic rules of depth-of-field, or DOF for short (on constant image format):
1. DOF is wider at smaller aperture; DOF is narrower at wider aperture.
2. DOF is wider with shorter focal length; DOF is narrower with longer focal length.
3. DOF is wider at longer focusing distance; DOF is narrower at shorter focusing distance.
When we eliminate the effect of rule 1 by keeping the aperture always the same and compensate for a longer focal length with a longer distance in order to keep the magnification of the main subject the same then rules 2 and 3 will cancel each other out. When using a longer lens then you'll have to back up in order to keep the magnification the same. The longer focal length will reduce DOF, and at the same time the longer focusing distance will increase DOF. These two effects annihilate each other, and they do so not approximately but exactly---in a first-order approach.
This is what Michael's essay is about, and it's nothing new. It has been a well-known fact for ages. Actually, we can boil the three rules of DOF down to two rules:
1. [Same as rule 1 above]
2. DOF is wider at smaller magnification; DOF is narrower at larger magnification.
Focal length and focusing distance per se don't matter; it's the combination of the two, i. e. the magnification, that matters---in our first-order approach.
If we'd stop here then we're in perfect accordance with Michael's essay. Unfortunately we just can't stop here. Second-order effects raise their ugly head, and they will affect DOF to a degree that is not just academic but makes a real difference in many (not all) real-life situations.
The 2nd-order effect I am talking about is the fact that across the depth of the field, magnification is not constant. For close objects it's larger than for distant objects ... that's a very trivial fact of life. So when trying to compensate for a longer focal length through backing-up then you can achieve the same magnification as before only for one single distance. You'd back up so the main subject will appear at the distance presicely corresponding to the increased focal length, yielding the same magnification as before. However, now objects before and behind the plane of focus will appear at different magnifications as before---that is what we call perspective, and it also affects actual DOF. With the longer focal length, background objects appear larger than with the shorter focal length at shorter distance, and so, according to rule 2 in the modified set of DOF rules above, background DOF becomes narrower (and foreground DOF becomes wider (!) but the gain in the foreground is less than the loss in the background).
And that's not just academic nitpicking; it often (not always) will make a visible difference ... as Doug's examples easily show. It tends not to make a significant difference only when the background is at infinity or when DOF is very small in relation to the focusing distance (i. e. with very long lenses or at close-up range).
There are finer points that also can affect actual DOF, like focusing methods, the physical length of the lens, the ratio of exit to entry pupils, residual lens aberrations, diffraction, and others. Michael called them 2nd-order effects that should be neglected. In fact those are not 2nd but 3rd and 4th and higher-order effects ... and yes, for most practical intents and purposes those can be neglected indeed. The 2nd-order effect described above, however, mustn't be neglected generally.
-- Olaf
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I still think the F16 example is softer on the 200mm, and as far as the f5.0 goes, it "seems" to me a slam dunk simply because there is no detail on the 200mm side--no matter how much you blow it up. Or, am I missing something here?
[{POST_SNAPBACK}][/a] (http://index.php?act=findpost&pid=187555\")
Yes, you are missing something; the CoC is the same for both focal lengths even at f/5. (http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Untitled-2%20copy.jpg)
[a href=\"http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Focal%20Length_DOF.jpg]Here[/url] are 100% crops taken at various focal lengths illustrating the lack of any differences in inches (3,6,9 and 12). There are some minor variations from the different lenses I used (ironically, the 17mm lens appears to be the most out of focus of all the focal lengths tested), but the only things that fundamentally change are the perspective and the FOV of the background. Changing perspective and background FOV are not insignificant, but changing them neither adds nor subtracts "DOF".
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Yes, you are missing something; the CoC is the same for both focal lengths even at f/5. [a href=\"index.php?act=findpost&pid=188125\"][{POST_SNAPBACK}][/a]
That's what I believe is generally correct. I tried the experiment myself a few years ago. The actual resolution of the detail in the OoF parts is never exactly the same, but it's close enough for me. But maybe not for Olaf who seems greatly concerned about such subtle differences in OoF parts. I'm still waiting to see, from Olaf, a practical demonstration of differences in DoF between different camera formats, after adjusting FL and aperture to take account of the different format size.
Michael's demonstration of this DoF similarity in the article which is the subject of this thread, fails not so much in that it is mathematically imprecise but because the appearance of the DoF is vastly different in the examples he compares of images from different lenses, when viewing the entire image.
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Yes, you are missing something; the CoC is the same for both focal lengths even at f/5. (http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Untitled-2%20copy.jpg)
Here (http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Focal%20Length_DOF.jpg) are 100% crops taken at various focal lengths illustrating the lack of any differences in inches (3,6,9 and 12). There are some minor variations from the different lenses I used (ironically, the 17mm lens appears to be the most out of focus of all the focal lengths tested), but the only things that fundamentally change are the perspective and the FOV of the background. Changing perspective and background FOV are not insignificant, but changing them neither adds nor subtracts "DOF".
[a href=\"index.php?act=findpost&pid=188125\"][{POST_SNAPBACK}][/a]
Like I said, I'm not arguing the physics behind it. That's a done deal.
I think the point is that if you don't blow up the shorter focal lengths to match the longer, your background will look more out of focus using a longer focal length given the parameters of the test I did. I mean practically, how are you going to get the background as soft as the 200mm when you cannot blow it up because you have a face in it?
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I mean practically, how are you going to get the background as soft as the 200 mm when you cannot blow it up because you have a face in it?[a href=\"index.php?act=findpost&pid=188161\"][{POST_SNAPBACK}][/a]
You don't.
It's plain stupid to enlarge two images by different magnification factors and then say, look, the blur suddenly is the same! To compare blur, or DOF, between images taken with different focal lengths and corresponding distances, of course you have to look at prints enlarged the same way because DOF gets affected by enlargement.
DOF is defined through the maximum diameter of the circle of confusion that can be considered "sharp enough." And the range where the COC's diameter stays below that limit---aka DOF---changes with focal length when keeping the magnification at the plane of focus the same. And it changes again when printing at different sizes.
So of course you can compensate for different DOF by printing at different sizes. But that's just plain pointless. And furthermore, the concept of relative sharpness is not the same as DOF. DOF depends on (among other factors) print size; relative sharpness does not.
-- Olaf
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Like I said, I'm not arguing the physics behind it. That's a done deal.
I think the point is that if you don't blow up the shorter focal lengths to match the longer, your background will look more out of focus using a longer focal length given the parameters of the test I did. I mean practically, how are you going to get the background as soft as the 200mm when you cannot blow it up because you have a face in it?
[a href=\"index.php?act=findpost&pid=188161\"][{POST_SNAPBACK}][/a]
This is a common issue where someone suggests increasing DOF by using a shorter focal length, or when they suggest the opposite. Either way it is wrong. On the one hand you agree that it is wrong, but then you insist on writing that you get a "softer" background with a longer focal length. What you get is a bigger background with a smaller FOV (that is also well illustrated in my link when you compare the 200mm and 17mm crops of the 12 inch targets).
Go to the link and look at the 100% crops, they are all enlarged exactly the same. Take 250x250 pixel crops from your own shot and compare them (that would have been my preferred approach, but your example shot was too small). Also look at the full frame images in the images I linked to and you will see with your own eyes that DOF is exactly the same for 17mm up close as it is for 200mm farther away -- the 200mm background is not softer.
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This is a common issue where someone suggests increasing DOF by using a shorter focal length, or when they suggest the opposite. Either way it is wrong. On the one hand you agree that it is wrong, but then you insist on writing that you get a "softer" background with a longer focal length. What you get is a bigger background with a smaller FOV (that is also well illustrated in my link when you compare the 200mm and 17mm crops of the 12 inch targets).
Go to the link and look at the 100% crops, they are all enlarged exactly the same. Take 250x250 pixel crops from your own shot and compare them (that would have been my preferred approach, but your example shot was too small). Also look at the full frame images in the images I linked to and you will see with your own eyes that DOF is exactly the same for 17mm up close as it is for 200mm farther away -- the 200mm background is not softer.
[a href=\"index.php?act=findpost&pid=188229\"][{POST_SNAPBACK}][/a]
If you can make the background look the same with a 17mm lens as with a 200mm lens shooting people close in, please post those shots. I'd much rather use a small lens than a large and heavy one given I can get the same effect , and without distortion of the subject's body or face.
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If you can make the background look the same with a 17mm lens as with a 200mm lens shooting people close in, please post those shots. I'd much rather use a small lens than a large and heavy one given I can get the same effect , and without distortion of the subject's body or face.
[a href=\"index.php?act=findpost&pid=188344\"][{POST_SNAPBACK}][/a]
No, of course you cannot make the background look the same with different focal lengths, but that is not the topic you started. Using a longer focal length does not make the background "softer"; it changes the perspective of the background. What is important to understand for the purposes of this thread is that longer focal lengths have the same DOF as shorter focal lengths at the same aperture and magnification. Increasing focal length will not decrease DOF and beyond a reasonable point actually distorts perspective as much as using a too short focal length -- you can make someone appear too flat just as you can make them appear to tall, too top or bottom heavy, etc.
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No, of course you cannot make the background look the same with different focal lengths, but that is not the topic you started. Using a longer focal length does not make the background "softer"; it changes the perspective of the background. What is important to understand for the purposes of this thread is that longer focal lengths have the same DOF as shorter focal lengths at the same aperture and magnification. Increasing focal length will not decrease DOF and beyond a reasonable point actually distorts perspective as much as using a too short focal length -- you can make someone appear too flat just as you can make them appear to tall, too top or bottom heavy, etc.
[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
Honestly, some people could argue under water. I don't know what technical point of view you are coming from, but the reality is that DOF is a perceptual phenomenon. The only thing that matters in the determination of DOF for a human viewer is a determination of acceptable sharpness. Whether the background of a telephoto shot is "softer" or not, it is certainly less acceptably sharp. Using the OP's original images, I can make certain parts of the middle background of the shorter focal length appear acceptably sharp just by walking back from my monitor a bit. For the same distance from my monitor, this isn't the case for the longer focal length. Therefore, in this case, DOF IS greater in shorter focal length images.
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Using a longer focal length does not make the background "softer"; it changes the perspective of the background.[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
Tony, this statement is wrong. Actually, using a longer focal length (and backing up accordingly) will make the background softer and change perspective, i. e. make the background objects appear larger. With regard to relative sharpness of the background objects, these two effects cancel each other out, i. e. relative sharpness remains the same (more or less). But DOF does change, because DOF is about absolute sharpness, not relative. You, and Michael R., mustn't confuse these two things.
If with the shorter focal length an object in the background that is just outside DOF can be recognized despite the blur, then with a longer focal length (from an accordingly longer distance) it will be recognizable, too. It will be rendered more blurred but also bigger, i. e. relative sharpness will be the same. But this fact doesn't entitle you to say DOF was the same! DOF is not relative sharpness. DOF is defined via the diameter of the circles of confusion, and those diameters will increase due to the larger magnification of the background objects so DOF will decrease.
What is important to understand for the purposes of this thread is that longer focal lengths have the same DOF as shorter focal lengths at the same aperture and magnification.[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
This statement is wrong again, generally! It's true only as a first-order approximation so it will hold only in those cases where DOF is small in relation to focusing distance, i. e. when comparing long lenses to longer lenses, or in the macro range. But when comparing a, say, 28 mm lens to a 100 mm lens (for 35-mm format) then the difference in DOF, caused by the second-order effect of non-constant magnifications across the range of DOF, will become obvious.
Increasing focal length will not decrease DOF ...[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
It will---particularly when starting out with a short focal length. Less so when starting out with a lens that already is fairly long.
Let's just use a depth-of-field calculator (there are several floating around on the Internet) and compute a few examples:
35-mm format, COC = 0.030 mm
17 mm lens, f/11, focus distance 0.61 m:
DOF is 1.35 m, from 0.37 m to 1.72 m (or -0.24 m to +1.11 m);
28 mm lens, f/11, focus distance 1.00 m:
DOF is 0.92 m, from 0.72 m to 1.64 m (or -0.28 m to +0.64 m);
50 mm lens, f/11, focus distance 1.79 m:
DOF is 0.81 m, from 1.47 m to 2.28 m (or -0.32 m to +0.49 m);
100 mm lens, f/11, focus distance 3.57 m:
DOF is 0.78 m, from 3.22 m to 4.01 m (or -0.35 m to +0.44 m);
300 mm lens, f/11, focus distance 10.71 m:
DOF is 0.77 m, from 10.33 m to 11.10 m (or -0.38 m to +0.39 m);
1,000 mm lens, f/11, focus distance 35.71 m:
DOF is 0.77 m, from 35.33 m to 36.10 m (or -0.38 m to +0.39 m).
The focus distances are adjusted to the focal lengths so at the plane of focus the magnification will be always the same. As you can see, DOF decreases rapidly in the wide-angle range (high influence of the 2nd-order effect) und then slower in the telephoto range (2nd-order effect hardly shows up here). Between 300 mm and 1,000 mm the change is only millimeters---or fractions thereof. So for practical intents and purposes, we can ignore the 2nd-order effect for lenses longer than portrait telephoto ... or maybe even for anything longer than a standard lens if we're not too finicky. But we definitely cannot for wide-angle lenses.
Contrary to intuition, foreground DOF increases with longer focal lengths but background DOF decreases, and at a faster rate so the sum of the two also decreases. (By the way, those who still believe DOF always extends 1/3rd towards the camera and 2/3rds into the background should give this (wrong) notion a thought or two.)
Relative sharpness of background objects will be the same, more or less, across all focal lengths (i. e. rendered more blurred but also bigger with longer lenses) ... but that's not what DOF is about.
-- Olaf
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Relative sharpness of background objects will be the same, more or less, across all focal lengths (i. e. rendered more blurred but also bigger with longer lenses) ... but that's not what DOF is about.
-- Olaf
[{POST_SNAPBACK}][/a] (http://index.php?act=findpost&pid=188408\")
I often put DOF in quotations because it is a concept and not an absolute. There is only one place where the image is in focus, and everything else is out of focus.
Despite the risk of being redundant, I find it necessary to repost the link demonstrating [a href=\"http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Focal%20Length_DOF.jpg]"DOF"[/url].
Basically, what we are discussing here is a magic trick. Claiming that the smaller focal length shows more detail behind the point of focus than the longer focal length does is the equivalent of claiming that the disappearing rabbit actually became invisible. The rabbit disappeared because the audience can no longer see it; but the magician and the magician's assistant(s) know where the rabbit is and it is not invisible. Likewise, you can see in the 100% crops below that the background on the bottom image taken at 105mm and f/11 appears "softer" than the image above it taken at 17mm and f/11, but close examination reveals nearly identical detail 25 inches behind the point of focus with both lenses (i.e. you can count just as many equal sized lines.
(http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Untitled-2_1.jpg)
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I often put DOF in quotations because it is a concept and not an absolute.
Oh---it is an absolute. How do you think DOF scales, DOF tables, DOF calculators, and DOF formulas are created? By magic? By cheating? Of course, the concept of DOF does contain some idealizing (as concepts always do) and furthermore an arbitrary convention of what we want to consider "sufficiently sharp." And of course you can argue about what "sufficiently" is supposed to mean. But this convention on "sufficiently" is clearly and unambiguously represented by a plain little number---the maximum diameter of the circle of confusion---and once you've settled on a value for that number, DOF is a clearly defined and precisely computable thing.
Basically, what we are discussing here is a magic trick.
Obviously, you have no idea what DOF really is. In case you're interested to learn about this surprisingly complex concept, steer clear from Michael R.'s articles about DOF (as they contain several errors and misunderstandings) and carefully read Paul van Walree's excellent article on DOF instead (see here (http://www.vanwalree.com/optics/dof.html)). After that, you may want to check out Harold M. Merklinger's ground-breaking article "The Ins and Outs of Focus" which is referred to as Reference #7 at the end of van Walree's article. That's the first essay about the concept of relative sharpness, or relative blur, that I am aware of (although Merklinger doesn't call it by that name).
Claiming that the smaller focal length shows more detail behind the point of focus than the longer focal length does ...[a href=\"index.php?act=findpost&pid=188483\"][{POST_SNAPBACK}][/a]
No-one claims such a thing. Instead, those who understand DOF claim that the shorter lens (from an accordingly shorter distance, and everything else being equal) has wider DOF. That's a different claim. Despite the risk of being redundant---you mustn't confuse these two concepts.
-- Olaf
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I often put DOF in quotations because it is a concept and not an absolute. There is only one place where the image is in focus, and everything else is out of focus.
Despite the risk of being redundant, I find it necessary to repost the link demonstrating "DOF" (http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Focal%20Length_DOF.jpg).
Basically, what we are discussing here is a magic trick. Claiming that the smaller focal length shows more detail behind the point of focus than the longer focal length does is the equivalent of claiming that the disappearing rabbit actually became invisible. The rabbit disappeared because the audience can no longer see it; but the magician and the magician's assistant(s) know where the rabbit is and it is not invisible. Likewise, you can see in the 100% crops below that the background on the bottom image taken at 105mm and f/11 appears "softer" than the image above it taken at 17mm and f/11, but close examination reveals nearly identical detail 25 inches behind the point of focus with both lenses (i.e. you can count just as many equal sized lines.
(http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Untitled-2_1.jpg)
[a href=\"index.php?act=findpost&pid=188483\"][{POST_SNAPBACK}][/a]
Your images show one possible combination of subject distance, background distance and aperture, out of infinite possible combinations. I'm glad you've found ONE that backs up your assertions. Believe me there are millions more that don't back it up. Just look at the OP's images, and 01af's dof calculation results. And besides, using line charts, which we already know contain lines from viewing the perceptually sharper 17mm image, is disingenous. If one was presented only with the 200mm f5 image from the OP, then one would have significant trouble describing what the background actually was. Not so with the shorter focal length.
Once again, DOF is a perceptual phenomenon. And it is ultimately defined by acceptable sharpness as determined by a particular viewer.
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Your images show one possible combination of subject distance, background distance and aperture, out of infinite possible combinations. I'm glad you've found ONE that backs up your assertions. Believe me there are millions more that don't back it up. Just look at the OP's images, and 01af's dof calculation results. And besides, using line charts, which we already know contain lines from viewing the perceptually sharper 17mm image, is disingenous. If one was presented only with the 200mm f5 image from the OP, then one would have significant trouble describing what the background actually was. Not so with the shorter focal length.
Once again, DOF is a perceptual phenomenon. And it is ultimately defined by acceptable sharpness as determined by a particular viewer.
[a href=\"index.php?act=findpost&pid=188558\"][{POST_SNAPBACK}][/a]
The link I provided to the comparisons also compared 3 inches, 6 inches, 9 inches and 12 inches at f/5.6, in addition to the above at 25 inches at f/11. What I see in this thread is a lot of verbage and a bit of confusion, but I know what my experience is and what I see with my own eyes. I have done this comparison in a number of ways, including shots of tree outside my front door taken with a 35mm and with a 200mm lens with the background being over a block away and the results are always the same -- the rendition of out of focus areas between whatever focal lengths is always comparable. If you, Olaf, or anyone else has a pair of images taken in a manner that allows us to see two objects of equal distance in the background taken at the same aperture with the same magnification but at different focal lengths that demonstrates that I am wrong -- then I would be happy to see them.
As for Olaf's claim that DOF is something that is absolute; he contradicts himself in pointing out that the very definition of what constitutes an acceptable CoC is debatable. Since the most widely accepted definitions upon which most tables and formulas are based on is 4x6 inch prints viewed from 10 inches away, and many choose to define it more narrowly, this means that it is not absolute since two people can reasonably disagree about what is or is not within the "DOF". Anyway, I have read Merklinger and am well aware of all of this and find the both the discussion and the underlying math quite boring; what I see when I actually take the images and compare them is empirical proof and everything else is essentially a waste of time. So it comes down to that old saw, "Just go out and take some pictures."
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If you, Olaf, or anyone else has a pair of images taken in a manner that allows us to see two objects of equal distance in the background taken at the same aperture with the same magnification but at different focal lengths that demonstrates that I am wrong -- then I would be happy to see them.
[a href=\"index.php?act=findpost&pid=188579\"][{POST_SNAPBACK}][/a]
The OP has already provided them. Like I said earlier - step back from your monitor and you can make the red sticks (or whatever they are) in the background appear in focus for the shorter focal length shots, but for the same distance away, you can't do so for the longer focal lenght shots. Try it.
As for Olaf's claim that DOF is something that is absolute; he contradicts himself in pointing out that the very definition of what constitutes an acceptable CoC is debatable. Since the most widely accepted definitions upon which most tables and formulas are based on is 4x6 inch prints viewed from 10 inches away, and many choose to define it more narrowly, this means that it is not absolute since two people can reasonably disagree about what is or is not within the "DOF".
[a href=\"index.php?act=findpost&pid=188579\"][{POST_SNAPBACK}][/a]
I think what Olaf is trying to say is that once an acceptable CoC has been agreed upon, then it is absolute from that point on. However, I do seem to remember reading that at short and very long subject distances, the equations involved are only best approximations. Anyway, ultimately all the equations don't matter. What matters is the viewing of images and the determination of acceptable sharpness. As shown by my example above, the depth of field in the shorter focal length shot IS larger than the other.
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The OP has already provided [valid comparisons].
[a href=\"index.php?act=findpost&pid=188583\"][{POST_SNAPBACK}][/a]
Actually, he has not. Here is a valid comparison that meets all the parameters I insist on. Once again, it is yet another focus distance, aperture, and degree of magnification; but as with all the others, the results are the same -- anyone doubting my veracity or the validity of my tests should perform them for themselves.
(http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing//Untitled-3.jpg)
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All that is showing is that they are both out of focus and outside of depth of field range. Why is the OP's image not valid? Look at it and step back from your monitor and tell me what you see.
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All that is showing is that they are both out of focus and outside of depth of field range. Why is the OP's image not valid? Look at it and step back from your monitor and tell me what you see.
[a href=\"index.php?act=findpost&pid=188601\"][{POST_SNAPBACK}][/a]
Geez Bernie, you are the one arguing from underwater. I already told you why the OP's original image is not valid; there are no two things to compare in them. If I get close enough to what I'm focusing on with a long enough focal length then there will be very little left in the background (because of the reduced FOV) to make any comparisons to compared to using a shorter focal length which will provide a wider FOV and therefore more stuff (albeit rendered very small). The point is any two things you see in both images will be equally soft, presuming there's anything to see in the longer focal length shot.
What my examples show is that anything that is not in focus is equally out of focus and that the detail (or lack thereof) is the same regardless of focal length, the above example shows that 200mm is no "softer" than 35mm. Go out and take some shots, take them at any aperture, at any focal length, at any focus distance you like and bring them back here and show me that there is a greater or lesser degree of focus. I have shown you 3, 6, 9, 12 and 25 inches, and now I have shown you 113 feet -- the results are always the same.
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Ok, two points.
1. In the OP's images, as I have already said, the red sticks (?) are in both images. Step back from your monitor and tell me what you see.
2. I shouldn't have so easily dismissed your images. The EXACT same thing applies to your set of images too! Step back from your monitor to a point at which the sign in the background appears sharp (short focal length image), and then tell me whether you think the sign in the long focal length image appears sharp. You'd need corrective eye surgery if you did.
The problem with your argument is that you aren't being specific enough with your definitions. Whilst it is kinda true that they are blurred to the same extent, you need to go qualify that by saying they are blurred to the same relative extent. But the long focal lenght shot is blurred to a greater absolute extent. Hence the need to step back even further to make it appear sharp. And, once again, DOF is a perceptual phenomenon concerned with viewing images.
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Ok, two points.
1. In the OP's images, as I have already said, the red sticks (?) are in both images. Step back from your monitor and tell me what you see.
2. I shouldn't have so easily dismissed your images. The EXACT same thing applies to your set of images too! Step back from your monitor to a point at which the sign in the background appears sharp (short focal length image), and then tell me whether you think the sign in the long focal length image appears sharp. You'd need corrective eye surgery if you did.
The problem with your argument is that you aren't being specific enough with your definitions. Whilst it is kinda true that they are blurred to the same extent, you need to go qualify that by saying they are blurred to the same relative extent. But the long focal lenght shot is blurred to a greater absolute extent. Hence the need to step back even further to make it appear sharp. And, once again, DOF is a perceptual phenomenon concerned with viewing images.
[a href=\"index.php?act=findpost&pid=188607\"][{POST_SNAPBACK}][/a]
Yes, and to take this to it's logical extreme - "reductio ad absurdum" if you like - if you step back far enough everything will be sharp!
DOF is a perceptual phenomenon based around an arbitrarily defined COC. In a small print, DOF seems bigger because the COCs are less than whatever threshold you deem appropriate. On a big print, you have merely enlarged the same detail or COCs therefore the DOF seems less - ask any large format shooter (like me).
If it seems sharp, it is sharp. QED.
No doubt there are all sorts of optical calculations to be made but AFAICT they all depend on defining a COC size and, a print size and a viewing distance. If you set the COC big enough then everything could be said to be in focus, small enough and nothing is in focus.
It's all about what looks in focus - the eyes have it.
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Interesting technical discussion.
However, when I shoot a subject (a person) with my 200mm at 200mm, the background looks more out of focus than it does with shorter focal lengths, given that the focus point is close enough and the background close enough. So if I shoot a low neckline/head shot, the background appears much more out of focus than does the same shot with a 35mm focal length.
Also, in my examples, I can't see how backing up is going to give you detail in the f5 200m image because there IS NO REAL DETAIL!
Not only that, but if you back up away from one image, to be consistent, you have to view the other image at the same distance and the one always looks softer than the other, even at 1000 feet.
We're not talking about virtual zoom distances. Were talking about printing the image--or viewing it above (ok?)--and then backing up from BOTH images.
If people really think the backgrounds look the same, is the same, practically the same, in practice the same, then why not use 35mms for close up images instead of 200 or 400mms when doing things such as people shots?
It seems like we're going around in circles here. Technically, perhaps yes, no difference. But I'm not taking pictures of clients with a 17mm lens when I need the background wiped--as is the case in the f5 200mm example above--even if it is technically the same.
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What you need to understand is that what is wiping out the background using a longer focal length is not reduced DOF, it is reduced FOV. Reducing the FOV to isolate your subject is a legitimate consideration, but it needs to be understood so that you or someone else doesn't reverse this mistaken argument about DOF and make the mistake of thinking they can increase the DOF (for instance, to try and get two rows of people simultaneously in focus) by simply using a shorter focal length but still using the same aperture and magnification. Yes, there are secondary considerations like the absolute size of the aperture -- which is like painting with a smaller or larger brush; but soft is soft regardless of whether it is soft small brush strokes or soft large brush strokes.
Regarding the related question of "acceptable" DOF, these formulas and tables are subjective. Below is an image where the back piece is supposed to be "in focus" by an inch according to the tables (based on .02 CoC "acceptable" focus is 4" behind the front piece's arm, but it is only 3 inches back and is already soft IMHO). That is probably an acceptable DOF up to 8x10, but for a larger print it would start becoming noticeably soft.(http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing/_AB12299.jpg)
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forget about your tables and everything, because for this discussion they don't matter. What matters is comparing the DOF between two different focal length images with the same pair of eyes. It doesn't matter what you set the CoC at, or what arbitrary parameters you use to determine it, forget all about it. "The eyes have it" as Nick said. All that matters is what you see. Please do my test as I keep begging of you, and you will see what we are talking about.
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forget about your tables and everything, because for this discussion they don't matter. What matters is comparing the DOF between two different focal length images with the same pair of eyes. It doesn't matter what you set the CoC at, or what arbitrary parameters you use to determine it, forget all about it. "The eyes have it" as Nick said. All that matters is what you see. Please do my test as I keep begging of you, and you will see what we are talking about.
[a href=\"index.php?act=findpost&pid=188635\"][{POST_SNAPBACK}][/a]
Regarding your analysis of the original 40mm v. 200m comparison, the two "red sticks" are not even in the 200mm image. Look at the image and you will see that they were at center court, but you can only make out the far fence line in the 200mm image because the angle and FOV changed. As for comparisons of how out of focus 35mm is versus 200mm, they are as you said equally out of focus and if anything the sign in the 200mm image is sharper than the 35mm sign is -- especially if you back up to make the 200mm background the same size as the 35mm background.
(http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing/_AB12288.jpg)
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Regarding your analysis of the original 40mm v. 200m comparison, the two "red sticks" are not even in the 200mm image.[a href=\"index.php?act=findpost&pid=188641\"][{POST_SNAPBACK}][/a]
Umm... yes they are.
As for comparisons of how out of focus 35mm is versus 200mm, they are as you said equally out of focus
No, that's what you say. I say they are equally out of focus relative to their sizes, but in an absolute sense, which is what is important in DOF determination, they are certainly not equal.
and if anything the sign in the 200mm image is sharper than the 35mm sign is -- especially if you back up to make the 200mm background the same size as the 35mm background.
[a href=\"index.php?act=findpost&pid=188641\"][{POST_SNAPBACK}][/a]
back up? but then we aren't talking about apples vs. apples. For DOF comparison the subjects must be the same size in the two images.
Have you tried my test yet??
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I know what my experience is ...[a href=\"index.php?act=findpost&pid=188579\"][{POST_SNAPBACK}][/a]
No, I am afraid you don't. You can see something but you don't understand it. And then you refuse to listen to those who try to explain it to you.
I have done this comparison in a number of ways [...] and the results are always the same---the rendition of out-of-focus areas between whatever focal lengths is always comparable.[a href=\"index.php?act=findpost&pid=188579\"][{POST_SNAPBACK}][/a]
Yes, sure it is. Nobody can deny this. But what you need to understand is this: comparable rendition (i. e. equal relative blur) does not mean that DOF was the same. Got it? DOF on the one hand and relative blur on the other hand are two different things. If one of the two is equal in images taken with different focal lengths from corresponding distances then the other usually is not.
If you, Olaf, or anyone else has a pair of images taken in a manner that allows us to see two objects of equal distance in the background taken at the same aperture with the same magnification ...[a href=\"index.php?act=findpost&pid=188579\"][{POST_SNAPBACK}][/a]
You mean, with the same magnification at the plane of focus, don't you?
... but at different focal lengths that demonstrates that I am wrong---then I would be happy to see them.[a href=\"index.php?act=findpost&pid=188579\"][{POST_SNAPBACK}][/a]
No-one has such a pair of images. Because you are right with regard to relative blur. Still that doesn't mean DOF was the same. Because DOF being equal and relative blur being equal ARE TWO DIFFERENT THINGS! Please stop confusing the two!
As for Olaf's claim that DOF is something that is absolute; he contradicts himself ...
I think what Olaf is trying to say is that once an acceptable CoC has been agreed upon, then it is absolute from that point on.[a href=\"index.php?act=findpost&pid=188583\"][{POST_SNAPBACK}][/a]
Exactly.
I already told you why the OP's original image is not valid; there are no two things to compare in them.[a href=\"index.php?act=findpost&pid=188603\"][{POST_SNAPBACK}][/a]
Umm ... is there something wrong with your eyes?
What my examples show is that anything that is not in focus is equally out of focus ...[a href=\"index.php?act=findpost&pid=188603\"][{POST_SNAPBACK}][/a]
In relative terms---yes (i. e. same level of detail). In absolute terms---no (i. e. different DOF).
What you need to understand is that what is wiping out the background using a longer focal length is not reduced DOF, it is reduced FOV.[a href=\"index.php?act=findpost&pid=188632\"][{POST_SNAPBACK}][/a]
Actually, it is reduced FOV and reduced DOF.
... and make the mistake of thinking they can increase the DOF [...] by simply using a shorter focal length but still using the same aperture and magnification.[a href=\"index.php?act=findpost&pid=188632\"][{POST_SNAPBACK}][/a]
Actually, that's no mistake. Using a smaller focal length (at the same aperture and magnification, i. e. shorter distance) will increase DOF. Of course, it will not (or hardly) increase the level of detail in the background objects ... which obviously is confusing the hell out of some.
So, to answer Doug's original question: Yes, using a longer focal length from a longer distance (so that magnification at the plane of focus is the same) at the same aperture will render the background softer and more out of focus. That's a trivial and well-known fact for all photographers who have ever switched from a standard or wide-angle lens to a telephoto lens.
It will also render the background objects bigger so their level of detail will be the same as before---aproximately the same for closer background objects; exactly the same for background objects at infinity. But that's a different story.
-- Olaf
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Umm... yes they [the red sticks] are.
[a href=\"index.php?act=findpost&pid=188653\"][{POST_SNAPBACK}][/a]
Bernie, you are ignoring the effect of the reduced FOV at 200mm (illustrated below). The background tree just to the left of the subject serves as a reference point to draw an outline of where the 200mm background lines up within the 40mm shot -- it is not surprising to see that it lines up approximately in the middle. It appears to me that the red sticks are not in the 200mm shot.
(http://photos.imageevent.com/tonybeach/mypicturesfolder/sharing/index.php%20copy.jpg)
This entire discussion is confused by what represents more or less detail. There is more detail in a wider FOV inasmuch as I can see that the sign is on a corner; but I can no more read what the sign says at 35mm than I can at 200mm, but I can actually tell that are two prominent red areas in the sign in the 200mm shot which cannot be made out in the 35mm shot.
Olaf, if you are right about DOF decreasing at longer focal lengths then it stands to reason that the opposite is true.
Here's a simple exercise that has practical implications for portrait photographers. Take a couple with one person standing behind the other and take two shots of them; one farther back with a longer focal length and the other closer with a shorter focal length. Will the person in back be any more in focus (i.e. "sharper")? If the answer is no, and that is the answer, then there is no increase in the "DOF", unless you want to define "DOF" as something that allows the back person's eyes to be not as sharp as the front person's eyes.
I'm sure both of you and perhaps the OP as well will continue to tell me that I am wrong and just don't understand. That's fine, I don't care and will not reply any more to this thread since I am satisfied that I have communicated with reasoned argument and valid examples why focal length does not change "DOF".
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Bernie, you are ignoring the effect of the reduced FOV at 200mm (illustrated below). [...] It appears to me that the red sticks are not in the 200 mm shot.[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
Tony, your "illustration" is flawed in more than one regard, and illustrates nothing. Why don't you simply take a look at Doug's 200-mm shot? The red posts are there in the 200-mm shot which is obvious to anybody who bothers to open the eyes.
This entire discussion is confused by what represents more or less detail.[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
I can see absolutely no confusion whatsoever as to what "more or less detail" is supposed to mean in the context of this discussion.
... but I can actually tell that are two prominent red areas in the sign in the 200 mm shot which cannot be made out in the 35 mm shot.[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
They cannot? Well, I can see them easily in both shots. Actually, in the 35-mm shot I can see that the lower red area is bigger than the upper one while in the 200-mm shot, they appear almost equal in size. Exposure and contrast are different which makes comparison of sharpness tricky. I'd say the sign in the background of the 35-mm shot has slightly better resolution but much less contrast.
Olaf, if you are right about DOF decreasing at longer focal lengths then it stands to reason that the opposite is true.[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
I am right, and your opinion about what's true is distorted. Let me guess: you didn't bother to study Paul van Walree's article that I refered to above, did you? Neither did you get out your DOF calculator and checked out a few examples, did you? Sure, images are more compelling than formulas---the proof is in the pudding, as the British say---but do you really think formulas in text books are made up by evil creatures only to confuse and deceive good boys like you? Rather than misinterpreting your own flawed experiments, you should start reading, and learning, a bit or two about DOF. Poorly crafted test images can be very misleading.
Take a couple with one person standing behind the other and take two shots of them; one farther back with a longer focal length and the other closer with a shorter focal length. Will the person in back be any more in focus (i. e. "sharper")?[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
Assuming the person in front is the one we are rendering at the same magnification and focusing at then the answer is---yes, the person in back will be rendered sharper with the shorter focal length. Why don't you simply try it?
If the longer lens is, say, a 200 mm lens and the shorter a 100 mm then the difference in sharpness will be minuscule and hardly perceivable. If however the longer lens again is a 200 mm lens and the shorter a 35 mm then the gain in the second person's rendition's sharpness will be very obvious.
Of course, the person in back will also be rendered smaller ... so most likely the result won't appeal very much, no matter what the DOF may be.
If the answer is no, and that is the answer, then there is no increase in the "DOF", unless you want to define "DOF" as something that allows the back person's eyes to be not as sharp as the front person's eyes.[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
But DOF is something that allows this. Obviously you really have no idea what DOF is about. It's not about perfect sharpness; it's about sufficient sharpness---so yes, for two objects both within DOF, one may be still sharper than the other.
I'm sure both of you and perhaps the OP as well will continue to tell me that I am wrong and just don't understand.[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
Indeed! Maybe you should spend a minute to think about as to why we're doing so. Hint: it's not just to annoy you!
-- Olaf
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Bernie, you are ignoring the effect of the reduced FOV at 200mm (illustrated below). The background tree just to the left of the subject serves as a reference point to draw an outline of where the 200mm background lines up within the 40mm shot -- it is not surprising to see that it lines up approximately in the middle. It appears to me that the red sticks are not in the 200mm shot.
if i could work out how to upload images I could point out the red sticks in the 200mm. They are just above the "mm" in "200mm/f16". Surely you can see them?
This entire discussion is confused by what represents more or less detail.
Confused is correct. Unfortunately, it is you who is confused. Resolution has nothing to do with DOF. DOF is concerned with acceptable sharpness. Of course resolution of distant objects is less with a shorter focal length lens. But acceptable sharpness (ie. DOF) is greater. Resolution has nothing to do with DOF. You are confusing these two terms.
I'm sure both of you and perhaps the OP as well will continue to tell me that I am wrong and just don't understand. That's fine, I don't care and will not reply any more to this thread since I am satisfied that I have communicated with reasoned argument and valid examples why focal length does not change "DOF".
[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
I hope you stick around, because if you do the test I said you will see it is true. Also you need to disabuse yourself of the incorrect belief that DOF is concerned with resolution. Once you do these two things, you will be free to enjoy the benefits of varying depth of field for creative effect.
I apologize if I got off on the wrong foot with the "underwater" comment. I had just come from a long forum post where one or two people where trying to argue something so counterintuitive, and they just wouldn't listen to reason. But having said that I am going to give you a little spray . I have been involved in forum posts about ETTR where some crusty old film guys stuck in the past, wouldn't accept examples and wouldn't do examples or tests themselves. And I'm sorry, you are like some of these guys. I have proposed a test, which explains the absolute essence of DOF, and you just resolutely won't do the test. This says to me, like the crusty old film guys, that you are stuck in your ways and no amount of evidence or anything is going to change your mind. So, if you are willing to approach this with an open mind, I urge you to stick around and we can try and explain each others point of view better. However, if you not willing to open your mind a bit, then perhaps it is best if you give up.
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Resolution has nothing to do with DOF.[a href=\"index.php?act=findpost&pid=188835\"][{POST_SNAPBACK}][/a]
Ouch! Bernie, I guess you want to say the right thing ... but the way you're saying it makes it wrong. DOF surely has to do with resolution! However it is not the same as resolution. And it mustn't get confused with relative blur.
I have been involved in forum posts about ETTR where some crusty old film guys stuck in the past ...[a href=\"index.php?act=findpost&pid=188835\"][{POST_SNAPBACK}][/a]
Huh!? How come? Conceptually, exposing to the right is just another word for exposing for the highlights which crusty old film guys should be very familiar with---at least those who shoot slide film. And the basic rule for negative film, exposing for the shadows, basically is the same again because the shadows in a positive are the highlights in the negative. The reasoning behind it is slighty different for film and digital but the basic concept is the same. So I guess something went severely wrong with that discussion ... well, not unlike here.
-- Olaf
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I also found this article by Michael to be contoversial and its conclusion misleading with regard to the way we use DoF in our photographs.
For example, often we use a shallow DoF to make objects in the background less distracting.
In the update to this article, which is a response to a reader's question about using a 300mm lens (from a greater distance) instead of a 200mm at f2.8 for shooting skiers, Michael safely offers the advice that the 300mm lens will provide exactly the same DoF as the 200mm lens.
However, in my view, if the intention of the photographer is to remove as much distracting detail as possible in the background, the longer focal length is a better choice.
If the reader's question had specified different lenses, for example, if the question had been, "I normally use a 300mm lens at f2.8 to photograph skiers, but next Saturday I've been offered a prime viewing position which is much closer to the action and which will allow me to fill the same amount of the frame (with a single skier) using my 85/1.2 at f2.8. Will DoF be the same?", I think Michael would have hesitated to answer in the same way.
The fact is, using the 85/1.8 for the same shots, would have introduced the risk of all sorts of distracting background skiers being visible and more prominent, not only because the background is more extensive (wider FoV) but also because smaller images (secondary skiers) would 'appear' sharper because they are smaller, yet retain the same absolute resolution as the blurrier looking objects (subjects) in the 300mm shots.
Nevertheless, we have to thank Michael for pointing out this phenomenon, that although a background object might look blurrier when using a longer lens from a greater distance, the actual detail in that blurry object is approximately the same as the detail in the smaller, and therefore sharper looking, object in the wider-angle shot taken from a closer distance.
I didn't know that when I first read this article. Of course, being a natural skeptic, I had to check this out for myself, and sure enough my own tests demonstrated that it is approximately true, but not precisely, as the OP's comparison demonstrates and as Olaf has mentioned several times.
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Ouch! Bernie, I guess you want to say the right thing ... but the way you're saying it makes it wrong. DOF surely has to do with resolution! However it is not the same as resolution. And it mustn't get confused with relative blur.
Ok, perhaps I am using the wrong terminology. When I talk about resolution I mean it in the sense of resolving detail. Clearly longer focal length lenses have greater resolving power than shorter focal lengths. So it is possible, and this has come up with some of the examples Tony has shown, that whilst some things might be within DOF, for example a sign, you may not neccessarily be able to read the sign (in a short focal length image). But this isn't a limitation of DOF, it is a limitation of the resolving power of the lens. But this is a red herring anyway. Because to be able to bring the longer focal length image into acceptable sharpness, one would have to go back so far, that one wouldn't be able to resolve the writing on the sign anyway.
Huh!? How come? Conceptually, exposing to the right is just another word for exposing for the highlights which crusty old film guys should be very familiar with---at least those who shoot slide film.
I don't want to open another can of worms, but it had to do with them not understanding the concept of sensor linearity and that if you half exposure you lose half your tonal values. I provided linear tiffs which showed just this, but their only reply to that was that I must have done something wrong. When I suggested that they do the test themselves... Silence...
So I guess something went severely wrong with that discussion ... well, not unlike here.
-- Olaf
[a href=\"index.php?act=findpost&pid=188841\"][{POST_SNAPBACK}][/a]
LOL.
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There are some interesting DOF articles online..
http://www.dofmaster.com/dof_imagesize.html (http://www.dofmaster.com/dof_imagesize.html)
http://www.vanwalree.com/optics/dof.html (http://www.vanwalree.com/optics/dof.html)
I won't disagree that DOF (overall DOF is very close if subject size is maintained...though not always the case..see above)
I will argue the following though..
Focal length "IS PART" of the formula, why do we still see people saying its got nothing to do with it..its there! Check your maths. I dont want to see any more nonsense about its nothing to do with it, its in black and white in the formula..!
The effect of focal length on DOF can be wiped out with distance to subject (which is what MR test was about..again, this is not set in stone though, take note of hyperfocal distances, macro etc)
You do not always "have" a subject in shots! (ala landcapes etc)
It is not practical to shoot an F1 race with a 20mm lens, or capture a bald mountain eagle flying ala close up shot with a 50mm lens.
Not all lenses can maintain subject size do to optical design. WA lenses are not good close up..limited in that regard, telel lenses have min focus distances too.
People tend to use longer focal lengths to "compress" the background more, even if you have the same DOF with a shorter FL one, the effect is not the same.
And the point that is not mentioned in the LL article is..
That the distribution of the DOF (front and rear) is NOT the same for different focal lengths, even if the overall DOF is the same or nearly the same. A long tele lens has a near 50/50 (almost) front to back DOF. A super WA lens 20mm say, has an average 1/3rd front to 2/3rds back DOF distribution. A very important point to remember...
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I'm sure both of you and perhaps the OP as well will continue to tell me that I am wrong and just don't understand. That's fine, I don't care and will not reply any more to this thread since I am satisfied that I have communicated with reasoned argument and valid examples why focal length does not change "DOF".
[a href=\"index.php?act=findpost&pid=188736\"][{POST_SNAPBACK}][/a]
No. I said before I'm not disagreeing with what is solid science, and you guys can hammer that out. In fact, if what is a fact is a fact, then someone is incorrect and someone is correct because there cannot be two truths to a single equation. So if anything is trivial, its debating a priori facts of math and science--that is, physics. It's just a matter of finding the information on the subject, a subject that has already been put to rest, and understanding that information.
I think I already said also that I tend to agree prima facia about the Dof not changing--since I'm not interested in light and lens physics discussions--although I'm wondering how this works, when you have no detail to bring back, no matter how far you stand back. But I'll just give you that.
However, as you agree, using a longer lens, as in my examples, will render the background "more out of focus" given similar parameters as is the case in my test images. And that is what I'm talking about.
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What an amazing discussion.
People are accusing Tony for being blind, but yet they cannot see themselves that the 40mm and 200mm shots were framed differently; dwdallam didn't move straight backwards.
This can easily be seen from the base which the "Aladdin" whatsit is standing on, as well as the tilt of the scene, and the whatsit itself.
I guess you all need eye surgery! (Or perhaps stepping back into the next room and closing the door will help.)
dwdallam; what you observe is how it is, but your own description of what you see is at best imprecise, and it's difficult to be precise when discussing this unless you also have the technical vocabulary, as e.g. Olaf has. I don't have that vocabulary down pat, and I only very rarely need to use DOF tables/calculators for my own photography.
But I'm pretty sure we're all seeing the same thing, plus/minus needed eye surgery, retina defects, eyes overflowing over with tears and closed doors.
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What an amazing discussion.
[a href=\"index.php?act=findpost&pid=190818\"][{POST_SNAPBACK}][/a]
What a pointless post.
Putting eye surgery aside, I think it is clear some people need brain surgery! Do you not understand how perspective can change with focal length? If you can't get past this point, then you've got no hope of understanding DOF!
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I have already written and vowed to not engage in this discussion anymore, so I will not address the topic. However Bernie, I have a hard time deciding what I dislike more, your condescending remarks, your disingenuous apology, or your "surgery" insults. Disagreeing with someone does not give you a license to be abusive and if that is the tenor of this website then I will happily spend my time elsewhere.
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I have already written and vowed to not engage in this discussion anymore, so I will not address the topic. However Bernie, I have a hard time deciding what I dislike more, your condescending remarks, your disingenuous apology, or your "surgery" insults. Disagreeing with someone does not give you a license to be abusive and if that is the tenor of this website then I will happily spend my time elsewhere.
[a href=\"index.php?act=findpost&pid=190880\"][{POST_SNAPBACK}][/a]
I was actually replying to Jani's surgery remarks. I guess the tone of my last post was set by Jani's post which I found condescending. I suppose you didn't find it condescending because he was supporting you. Funny how it works out like that...
As for my apology, it actually was sincere. I'd buy you a beer if you were nearby
As for the avoiding the discussion, well that's your choice. But now that you've popped back in, you may as well give it another go. At least do the rest of us who didn't give up the fight the courtesy of trying and reporting back about the test I proposed. Is that crickets I hear??
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What an amazing discussion.
People are accusing Tony for being blind, but yet they cannot see themselves that the 40mm and 200mm shots were framed differently; dwdallam didn't move straight backwards.
This can easily be seen from the base which the "Aladdin" whatsit is standing on, as well as the tilt of the scene, and the whatsit itself.
I guess you all need eye surgery! (Or perhaps stepping back into the next room and closing the door will help.)
dwdallam; what you observe is how it is, but your own description of what you see is at best imprecise, and it's difficult to be precise when discussing this unless you also have the technical vocabulary, as e.g. Olaf has. I don't have that vocabulary down pat, and I only very rarely need to use DOF tables/calculators for my own photography.
But I'm pretty sure we're all seeing the same thing, plus/minus needed eye surgery, retina defects, eyes overflowing over with tears and closed doors.
[a href=\"index.php?act=findpost&pid=190818\"][{POST_SNAPBACK}][/a]
The shots were framed as closely as I could manually adjust my footsteps and ball head/tripod. I would say the frames are within 1" of each other in all ways.
"dwdallam; what you observe is how it is, but your own description of what you see is at best imprecise,"
And I think I've admitted that three or more times now.
Do you have any new information to add? If so, please share it.
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I have already written and vowed to not engage in this discussion anymore, so I will not address the topic. However Bernie, I have a hard time deciding what I dislike more, your condescending remarks, your disingenuous apology, or your "surgery" insults. Disagreeing with someone does not give you a license to be abusive and if that is the tenor of this website then I will happily spend my time elsewhere.
[a href=\"index.php?act=findpost&pid=190880\"][{POST_SNAPBACK}][/a]
Why do you think one person's comments represent the entire web forum community? It's a fallacy called "Specific to the General." I get your point, but don't let one person run you off (operative word being "one").
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I don't even know what the argument is.
If this statement is wrong, then we need a discussion. If it is right, then we've concluded what I set out to question, and we need a new thread on the physics of DoF regarding photography:
"Using a longer length lens will give a greater illusion of a blurry background when compared to a shorter lens and all things aperture, distance to subject, frame fill, and print size are equal."
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I don't even know what the argument is.
If this statement is wrong, then we need a discussion. If it is right, then we've concluded what I set out to question, and we need a new thread on the physics of DoF regarding photography:
[a href=\"index.php?act=findpost&pid=190902\"][{POST_SNAPBACK}][/a]
I think the statement is wrong in relation to DOF. And as I've said a number of times, the physics, charts, equations and what not don't really matter in this discussion. That is because we are comparing two situations where we (you) have minimised the differences between the two situations, to just the variables we are comparing. It seems to be the almost perfect experiment, where we don't have to bother with equations. In addition to all this, equations really don't matter anyway. And the reason for this is that ultimately the only important thing for DOF determination is the eyes. Once again for Tony et al, DOF is a visual perceptual phenomenon. If you see acceptable sharpness, then the image IS acceptably sharp. No amount of fussing over the FOV or level of blurriness in the background will change this fact. If you can see it with your eyes, then it is real. And I (and most others too) can see that DOF is larger with shorter focal length lenses (when all else that is important is equal).
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What a pointless post.
I'm glad you realise that, and decided put it in your introduction, but that still leaves the question why you bothered to post it.
Putting eye surgery aside, I think it is clear some people need brain surgery! Do you not understand how perspective can change with focal length?
Yes, I do, which should be clear from the part of the post which you didn't bother to read. You seem to obsessed with insulting other people.
However, perspective doesn't change as unevenly as illustrated in dwdallam's shots, if that's what you're implying.
I was actually replying to Jani's surgery remarks. I guess the tone of my last post was set by Jani's post which I found condescending.
Here's a mirror. Please make use of it and guess three times why the tone of my post was such, and why I thought it funny to read your own, condescending "surgery" remark, while missing what's right there in front of your own two eyes. Perhaps that's condescending, I thought it witty when I wrote it. I'm sorry that I insulted you then.
The shots were framed as closely as I could manually adjust my footsteps and ball head/tripod. I would say the frames are within 1" of each other in all ways.
I don't know where that 1" is - on your screen, in the full resolution image on your screen, but the point here was that ridiculing others for needing eye surgery when they themselves don't even bother looking at the examples you posted is slightly less than clever.
What you did was fine enough; it illustrated what you wanted to ask about.
"dwdallam; what you observe is how it is, but your own description of what you see is at best imprecise,"
And I think I've admitted that three or more times now.
Do you have any new information to add? If so, please share it
Yes, but please continue reading past the point which you quoted. It seemed, from your then-currently last post on the topic, that you still struggled with an apparent disconnect between what you saw and how the DoF "should" work according to physics.
I was trying to contribute by telling you that there isn't such a disconnect.
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To be honest, I don't know why I am replying to this dribble, but here goes.
However, perspective doesn't change as unevenly as illustrated in dwdallam's shots, if that's what you're implying.
So you're implying that he is lying then?? This seems to be the same type of response I used to get from the kooky old film guys when I presented evidence to them about sensor linearity.
Here's a mirror. Please make use of it and guess three times why the tone of my post was such
1. You're rude?
2. You have no idea?
3. You have nothing to contribute to the discussion?
How did I do?
Why don't you actually contribute something to this discussion and address some of the entirely valid and useful points myself and others have made from our side of the discussion? Or just throw more insults around. You decide.
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I haven't read this whole thread, but unfortunately it seems to have degenerated into a rude pissing contest.
It's therefore now closed.
Michael