DOF and Cropping take 2

Started 7 months ago | Discussions thread
James O'Neill
Veteran MemberPosts: 3,931
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Re: Back to basics
In reply to awaldram, 7 months ago

awaldram wrote:

Hi James

This site provides pretty pictures to further expand on what your saying

http://www.normankoren.com/Tutorials/MTF6.html

And fro MC an

And for MC if he hasn't quite got it yet with his Nikon fora buddies.......

It is well known that short focal length lenses have large apparent depths of field and long telephoto lenses have small apparent depths of field. There are some very practical reasons for this conception, but it isn't quite true. DOF is much more closely related to magnification and f-stop; DOF expressed in distance is nearly independent of focal length. It appears smaller with telephoto lenses because it is smaller when expressed as a fraction of the lens-to-subject distance, s.

Thanks for the diagram, I toyed with the idea of drawing something similar.

I'm not sure that the other bit is right though.

Lets say I focus on infinity and my hyperfocal distance is 10M. There is an object at 10 M which takes up a certain amount of the frame. And appears just about in focus. Or I can focus on the object and have infinity just in focus.

I change my to one of double the focal length. c hasn't changed, and I set the same aperture, so now my hyperfocal distance is 40M. To make the object the same size I double the distance between it and the camera. It is now 20M from me and 20M in-front of the hyperfocal distance, so if I focus on infinity my object is out of focus and if I focus at 20M D.o.F doesn't go all the way to infinity.

I have seen an awful lot of mis-explanation which seems to be work with
H = F/ca instead of F^2/ca
is that what is happening here ?

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