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Author Topic: Michael's DoF Essay Question and Examples  (Read 18134 times)

dwdallam

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Michael's DoF Essay Question and Examples
« on: April 06, 2008, 02:04:24 am »

This is the link to Michael's esdsay on DoF.

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.
« Last Edit: April 06, 2008, 02:06:10 am by dwdallam »
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Sheldon N

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Michael's DoF Essay Question and Examples
« Reply #1 on: April 06, 2008, 11:16:09 am »

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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Sheldon Nalos
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joedevico

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Michael's DoF Essay Question and Examples
« Reply #2 on: April 06, 2008, 05:31:51 pm »

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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Ray

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Michael's DoF Essay Question and Examples
« Reply #3 on: April 06, 2008, 09:25:51 pm »

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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dwdallam

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Michael's DoF Essay Question and Examples
« Reply #4 on: April 06, 2008, 10:55:08 pm »

Quote
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."
« Last Edit: April 06, 2008, 10:56:33 pm by dwdallam »
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dwdallam

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Michael's DoF Essay Question and Examples
« Reply #5 on: April 06, 2008, 11:07:09 pm »

Quote
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?).
« Last Edit: April 06, 2008, 11:21:52 pm by dwdallam »
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dwdallam

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Michael's DoF Essay Question and Examples
« Reply #6 on: April 06, 2008, 11:12:56 pm »

Quote
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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Sheldon N

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Michael's DoF Essay Question and Examples
« Reply #7 on: April 06, 2008, 11:40:33 pm »

Quote
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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Sheldon Nalos
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dwdallam

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Michael's DoF Essay Question and Examples
« Reply #8 on: April 07, 2008, 12:28:19 am »

Quote
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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01af

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Michael's DoF Essay Question and Examples
« Reply #9 on: April 07, 2008, 01:55:44 pm »

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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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dwdallam

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Michael's DoF Essay Question and Examples
« Reply #10 on: April 07, 2008, 09:03:18 pm »

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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dwdallam

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Michael's DoF Essay Question and Examples
« Reply #11 on: April 08, 2008, 02:50:19 am »

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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01af

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« Reply #12 on: April 08, 2008, 07:07:26 am »

Quote
... 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.


Quote
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
« Last Edit: April 08, 2008, 09:17:00 am by 01af »
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Tony Beach

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Michael's DoF Essay Question and Examples
« Reply #13 on: April 09, 2008, 12:48:43 am »

Quote
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]


Yes, you are missing something; the CoC is the same for both focal lengths even at f/5.  

[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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Ray

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« Reply #14 on: April 09, 2008, 03:32:45 am »

Quote
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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dwdallam

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« Reply #15 on: April 09, 2008, 04:28:33 am »

Quote
Yes, you are missing something; the CoC is the same for both focal lengths even at f/5. 

Here 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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01af

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« Reply #16 on: April 09, 2008, 06:51:49 am »

Quote
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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Tony Beach

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« Reply #17 on: April 09, 2008, 11:08:54 am »

Quote
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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dwdallam

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« Reply #18 on: April 09, 2008, 10:17:38 pm »

Quote
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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Tony Beach

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Michael's DoF Essay Question and Examples
« Reply #19 on: April 09, 2008, 11:07:23 pm »

Quote
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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