# DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Re: DOF and Cropping take 2

James O'Neill wrote:

awaldram wrote:

mm lets try another 'wrong source'

http://www.luminous-landscape.com/tutorials/dof2.shtml

Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case. In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

That's wrong . You only need to look at the tower in the background behind the doll he has in the foreground to see it looks less and less out of focus the shorter the focal length gets.

as said I most in another branch

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 [using the h = f^2/ca formula] 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.

Yep they forgot to put 'nearly' I think (could be wrong ) what happens is as you approach infinity the object compress hence giving fractional magnification changes  now they mean nothing but when multiplied by infinity can have a massive affect.

Either that or the formula is an approximation fro something considerably more complex

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