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12-28-2012, 10:04 AM   #1
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Question on depth of focus

The standard answer when you ask how to increase depth of focus is to either use a wider lens or to stop down.

I understand the stop down and have proved to myself that it works. However, I have been playing around with different lens focal lengths and I am puzzled because it seems to make no difference.

If I want to fill the frame with an object that is approximately 1 foot tall I can:
  1. Use a 200mm lens at 12 feet
  2. Use a 100mm lens at 6 feet
  3. Use a 50mm lens at 3 feet
  4. Use a 25mm lens at 1.5 feet
But according to the DOF calculator all of those combinations produce exactly the same DOF.


So why is using a shorter focal length lens always recommended for increasing DOF?


What am I missing?

12-28-2012, 10:34 AM   #2
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Depth of focus is a function of shooting distance relative to focal length. A while back, we had a discussion thread going about this very topic. If you look at a DOF calculator, and adjust your shooting position to get the same primary image size, relative to focal length, you will see that the depth of focus remains largely unchanged, and the blurr of background at any distance is also largely unchanged.

It was a real eye opener for some.
12-28-2012, 10:47 AM   #3
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hmmmm.... I would like to see and experiment on this and how the out of focus areas compare... I don't think 25mm is gonna give you the "blur" of 200mm.... I understand dof is a different subject but this is very interesting..

I may have to do this myself ....
12-28-2012, 01:16 PM - 1 Like   #4
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Background blur and depth-of-field are different things. Background blur is a function of the absolute size of the aperture and the distance from the focal point to the background as well as the working distance. DOF is a function of relative aperture size (f-stop) and working distance. 50mm/1.4 and 100mm/2.8 will blur backgrounds similarly because they have the same aperture size. However, the 50mm will have much smaller DOF because the wide relative aperture and it would be closer to the subject.

12-28-2012, 01:19 PM   #5
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This might help: Depth of field
12-28-2012, 01:23 PM   #6
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Just to show focus depth if you're close enough to the subject with a fast 20mm:

12-28-2012, 08:25 PM   #7
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QuoteOriginally posted by Lowell Goudge Quote
It was a real eye opener for some.
+1 it certainly is for me. Every article I read on DOF says somewhere to just use a wider lens. But that just doesn't work.

So why does that keep getting repeated if it is so obviously wrong?
12-29-2012, 06:00 AM   #8
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QuoteOriginally posted by jatrax Quote
+1 it certainly is for me. Every article I read on DOF says somewhere to just use a wider lens. But that just doesn't work.

So why does that keep getting repeated if it is so obviously wrong?
I think the reason is a function of magnification

With wider lenses the real effect is to make the foreground larger than the background, with the size of the background reduced, it appears sharper, when in fact it is not

12-29-2012, 06:34 AM   #9
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QuoteOriginally posted by jatrax Quote
+1 it certainly is for me. Every article I read on DOF says somewhere to just use a wider lens. But that just doesn't work.

So why does that keep getting repeated if it is so obviously wrong?

It is true and not true. It is not true when one does a frame filling shot like the hat above. Probably never true when you are close to the minimum focus point of the lens.

It is true for average shots where the subject is at a small distance and one wants objects in front of and behind in focus. There are some absolutely fantastic images in the DA-15 thread right here at the Pentax forums that illustrate this point very well.

I am much too lazy to plug all the combinations in and present the results, but the interested can go here Online Depth of Field Calculator and play with the different combinations.
12-29-2012, 06:54 AM   #10
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You are getting different perspectives with the different focal lengths as well as different ratios between camera-to-subject and camera-to-background distances. These affect both the subject rendition as well as the background blur even though the DoF remains constant between the wide and telephoto focal lengths. Experiment with this yourself with a subject separated from its background and see how it alters the results despite the same DoF.
12-29-2012, 07:21 AM - 1 Like   #11
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QuoteOriginally posted by L33tGreg Quote
This might help: Depth of field
And from the same site, the nuts and bolts of The DOF equations In particular, equation (13) shows precisely how magnification works its way into the DoF equations and the following paragraph discusses conditions where DoF depends solely on magnification and f-stop (or rather approximately depends solely).


ps. "Depth of Focus" is usually used to mean something different from "Depth of Field". You can think of Depth of Focus as Depth of Field in the film plane, or how much wiggle room your sensor/film has for the image to still be recorded sharply. See the link above, or it's also covered in this relevant Cambridge in Colour article Understanding Depth of Field in Photography
12-29-2012, 12:57 PM   #12
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QuoteOriginally posted by BrianR Quote
Cambridge in Colour article Understanding Depth of Field in Photography
Excellent article, thank you.

The pertinent part is I think:
QuoteQuote:
If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length!
However:
QuoteQuote:
Even though the total depth of field is virtually constant, the fraction of the depth of field which is in front of and behind the focus distance does change with focal length
This part is likely what causes the recommendation of a wide angle lens for more DOF. The wide angle lens does not actually increase the DOF but it does increase the portion of the DOF that is behind the focus point. Changing from 52% @ 100mm to 70% at 10mm.

So for product photography using a wide angle makes more of the DOF usable rather than wasting it on foreground that might be blown out or thrown away. And the same for landscapes where you are trying to get a sharp foreground as well as the background, you waste less of your DOF on objects in front of the focus point.
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