DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
James O'Neill Veteran Member • Posts: 4,465
Re: Back to basics

awaldram wrote:

I think your right there are a lot of simplification in play in photography that break under some circumstances.

If you check that MIT slide deck I linked there is some interesting material on simplifications and covers off both distance shooting and indeed macro shooting where its also busted

I know that there is a slight error in the formula because the circle size is always based on the focus distance error as a fraction of the correct focus distance which is always assumed to be the focal length. That's close enough most of the time but for 1:1 (i.e. true macro) the lens to subject and lens to image distances are both 2xf so the formulae for near and far point [ HD/(H-D) and HD/(H+D) ] don't work properly. I've got a vague memory that you use D+F  instead of D and when F is 50mm and D is 5M the error really doesn't matter.  When D is 100mm ....

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