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10-01-2008, 03:50 PM   #1
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DOF

In this post I reacted on some statements by Wheatfield.
Since it seems he isn't going to answer, and the subject is more suited for this forum here are my questions again:

QuoteOriginally posted by Wheatfield Quote
Depth of field is a relationship between the size of the aperture and the reproduction ratio, nothing more.
Focal length is not part of the relationship.
Seems you have invented a whole new defintion of DOF. Perhaps you can explain what the relationship is. It's also unclear to me what you mean with reproduction ratio.

Perhaps someone can enlighten me?

On the test with the 50 and 85mm lenses by Stevebrot the following:
if you dial the numbers in a DOF calculator, you'll find the DOF to be allmost the same (within a few centimeters), so concluding that DOF isn't depending on focal length based on this test doesn't make sense to me.

10-01-2008, 04:23 PM   #2
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Not this again...

I believe he is just saying that for any given composition of a photograph, what determines the DOF is the aperture and the magnification (=subject distance, =reproduction ratio, all same thing really).

So if you take a picture of something with a 50mm lens, and also with a 100mm lens where you stepped backwards to get the same framing, the DOF will be the same.

If you are interested in the depth of field at a particular aperture, at a fixed subject distance, then the focal length matters. But of course then the photos from various focal length lenses will be quite different. (different "reproduction ratio" and hence consistent with his statement)
10-01-2008, 07:23 PM   #3
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QuoteOriginally posted by baw Quote
In this post I reacted on some statements by Wheatfield.
Since it seems he isn't going to answer, and the subject is more suited for this forum here are my questions again:

Seems you have invented a whole new defintion of DOF. Perhaps you can explain what the relationship is. It's also unclear to me what you mean with reproduction ratio.

Perhaps someone can enlighten me?

On the test with the 50 and 85mm lenses by Stevebrot the following:
if you dial the numbers in a DOF calculator, you'll find the DOF to be allmost the same (within a few centimeters), so concluding that DOF isn't depending on focal length based on this test doesn't make sense to me.
sorry, I took some pictures to illustrate it, but then haven't had time to process them. I will get back to this though. It is an interesting, and oft misunderstood topic by all levels of photogrpaher
10-01-2008, 07:40 PM   #4
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May I suggest that you Google "depth of field"? It's a very common term.

An explanation and examples can be found here.

Plus, there's tons of other links.

...my 2 cent$...

10-01-2008, 09:34 PM   #5
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QuoteOriginally posted by baw Quote
Seems you have invented a whole new defintion of DOF.
Don't think of what he posted as a definition. It's oart of an equation that will yield the right answer, but of course the definition of the term itself involves what's in focus and not in focus. It just so happens tha you can calculate it the way he alluded to.

QuoteQuote:
It's also unclear to me what you mean with reproduction ratio.
What's relevant in determining DOF is how big the subject is within the field of view, but also how big you are going to view the image. That is, something might look in focus if you view it as a thumbnail, but view it full screen, and it might not look so sharp any more.

QuoteQuote:
On the test with the 50 and 85mm lenses by Stevebrot the following:
if you dial the numbers in a DOF calculator, you'll find the DOF to be allmost the same (within a few centimeters), so concluding that DOF isn't depending on focal length based on this test doesn't make sense to me.
I'm not sure what you are talking about. There were severla things going on in the thread you referenced. What specific numbers are you talking about plugging in?
10-01-2008, 10:27 PM   #6
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according to wikipedia DOF is this:

"The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number (relative aperture). Except at close-up distances, DOF is approximately determined by the subject magnification and the lens f-number. For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF"

so, they are saying that focal length is a part of the formula except in close-up situations...
10-01-2008, 11:03 PM   #7
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QuoteOriginally posted by debbie Quote
according to wikipedia DOF is this:
so, they are saying that focal length is a part of the formula except in close-up situations...
Yep.

It's just that very often, people seem to think that they can get different depth of field effects in the SAME picture (that is, where the subject appears to be the same size) by using a different focal length lens and either moving closer or further away (as necessary). This is incorrect.

Of course there really is NO "depth of field" in an ideal sense. It's just that in usual situations your eye can't tell the difference between things that are perfectly in focus and very slightly out of focus. There is only one plane that is in perfect focus. So how big you print out your picture, how close you stand to it, how good your eyes are all play a role in determining the "depth of field."
10-02-2008, 04:43 AM   #8
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so, how does it work if you change sensor- or filmsize?? Does the DOF get shorter if you have a larger film format??

10-02-2008, 06:18 AM   #9
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Sensor size

QuoteOriginally posted by debbie Quote
so, how does it work if you change sensor- or filmsize?? Does the DOF get shorter if you have a larger film format??
If you leave everything else the same going from eg FF sensor to APS-C size sensor, you'll end up with a smaller image (crop factor ), but the exact same DOF.
If you print/view the APS-C image at the same size as the Full frame image, you change the magnification as well, and consequently the DOF.

(with everything else the same I mean: same lens and settings, same sensor resolution!!, magnification, viewing distance etc.etc.)
10-02-2008, 06:21 AM   #10
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Sorry, I should have stated that I feel I have a grip on this whole DOF thing
10-02-2008, 06:28 AM   #11
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QuoteOriginally posted by Marc Sabatella Quote
I'm not sure what you are talking about. There were severla things going on in the thread you referenced. What specific numbers are you talking about plugging in?
The two focal lengths, f/stop and an estimated focussed distance, like 1 meter for the 50mm and 2 meter for the 85mm lens. Using a maxCoC like 0,025mm you'll find that the range of acceptable sharpness around the same focussed object for both lenses is practicaly the same, so no surprise that the DOF in both images look the same as well.
Concluding that the focal length has no influence on DOF based on these two images makes no sense to me.
10-02-2008, 06:33 AM   #12
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Everyone always told me that sensor/film size makes a huge impact on dof variation.
Dammit. what a confusing thread.
10-02-2008, 07:24 AM   #13
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QuoteOriginally posted by baw Quote
If you leave everything else the same going from eg FF sensor to APS-C size sensor, you'll end up with a smaller image (crop factor ), but the exact same DOF.
If you print/view the APS-C image at the same size as the Full frame image, you change the magnification as well, and consequently the DOF.

(with everything else the same I mean: same lens and settings, same sensor resolution!!, magnification, viewing distance etc.etc.)
ok, but if I use, lets say a 35mm on a aps-c camera to make a picture and make the same one on a FF with a 50mm (lets asume that would give the same object size in the image) and print them out in the same size (and use the same f-stop)
Would the DOF be the same??
10-02-2008, 07:59 AM   #14
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One basic problem is that DOF is just an approximation. It's not like there is some magic point before which everything is in focus and then after that nothing is - it's just a gradual blurring as you get further from the one point that is most in focus, and a sort of arbitrary line we draw between what's "acceptably" sharp and what isn't. The formulas allows us to draw that arbitrary line with precision, but it's still pretty arbitrary.

The other problem is that the factors that influence DOF are inter-related.

For example, sensor size *by itself* is irrelevant: use the same lens at the same focal length, shoot the same picture at the same distance, and DOF will be the same. But of course, we wouldn't use the same lens on a smaller sensor - we'd use a shorter one, so the image will look the same size within the frame. But now it's being captured physically smaller on the smaller sensor. And of course, we don't stop once we've taken the picture - in order to see the image (which on the sensor is tiny, even if we could somehow see the pixels there), we end up blowing it up bigger to view on screen or print. And a smaller sensor means we need to blow it up more to view it the same size.

It's not like it's impossible to account for all those factors and yield - as I said - a very precisely computed but still quite arbitrary number. But in practice, why worry about calculating anything here? We've got eyes, let's use them to see for ourselves what the DOF is in any given picture. Set it up how we like. If we want more, stop down. If we want less, open up. it really need not be much more complicated than that.
10-02-2008, 08:38 AM   #15
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QuoteOriginally posted by debbie Quote
ok, but if I use, lets say a 35mm on a aps-c camera to make a picture and make the same one on a FF with a 50mm (lets asume that would give the same object size in the image) and print them out in the same size (and use the same f-stop)
Would the DOF be the same??
Depth of field is directly related to the reproduction ratio, i.e. the relation of the image and its size on the sensor/film. It is not related to the focal length of the lens - two different focal length lenses at the same f no. will give the same DOF for the same reproduction ratio. The best way is to use the DOF button which allows one to have some indication of the DOF before shooting.
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