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Boyd McCollum
Re: reply
on Dec 17, 2007 at 8:11:55 pm

There are really only 4 factors in depth of field for the same camera (film/sensor size) based on specific constants (there are also specific mathematical formulae to describe these various relationships):

1. At any given focal length, DOF increases as the aperture gets smaller.
2. At the same aperture and focal length, DOF increases as the distance to the subject increases.
3. At the same aperture and distance to subject, DOF is greater for a shorter focal length than for a longer one.
4. At the same aperture and image size (the subject framing remaining constant), DOF remains approximately constant for all focal lengths.

In this thread, it appears Bob is talking about situation number 4, whereas Todd is talking about number 3. Both are making valid points.

The situation many people encounter is too much DOF for the framing/composition they want - for example, in an interview. The common advice given is to move the camera back and zoom in. However, when you do this, to achieve the same frame composition, you have to move the camera back to a point where the DOF of the new focal length matches what you previously had. This is demonstrative phenomenon.

However, using a longer focal length can help with the illusion of a shallower DOF by narrowing the field of view and magnifying the background blur of the image (making the background larger in relationship to the foreground).

An excellent article with specific photographic examples and reference material can be found at:

http://www.vanwalree.com/optics/dof.html

So yes, focal length does affect DOF. But there are areas where you achieve the same DOF with different focal lengths - same subject framing being one of them.



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