DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
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James O'Neill
Veteran MemberPosts: 4,045Gear list
And this proves
In reply to Ian Stuart Forsyth, Feb 12, 2014

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.

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.

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