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# DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Re: And this proves

James O'Neill wrote:

In the first image you can't really see that the cereal behind the tomatoes is out of focus.

In the last image you certainly can.

A smaller image (or a crop) with the same focal length has less d.o.f. - even though the circle size has stayed the same - because the circle size where something looks out of focus is smaller - or you've magnified the out of focus bits until you can see the are out of focus whichever way round you care to think of it. .

A smaller image (or a crop) with same angle of view has more d.o.f. Because although the circle size which limits d.o.f. has gone down in proportion to in proportion to the image size, so has the focal length, and the size of the circle depends on square of focal length.

I think that's the same as me saying 'entry pupil diameter' as viewed from the lens front (f^2)

since the key formula is f^2 / c*a , you if halve the sensor size you halve f, and c. Now you either half the hyperfocal distance, or you have to have the aperture as well to keep h (and therefore all d.o.f. numbers) constant.

I did find this that explains some of the near field , far field issues we were discussing , as they say got a bit heavy for my ability/interest but suspect will be right up your alley

http://toothwalker.org/optics/dofderivation.html

What a great site they have some wonderful thesis's under the optics tab

This is very pertinent to this thread

http://toothwalker.org/optics/misconceptions.html

And covers some of the things you've been pointing out.

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