DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
falconeyes Senior Member • Posts: 1,452
Re: So, your conclusion is that 'equivalency = false'?

awaldram wrote:

Since M = b/v = f/(v–f), it is readily verified that the condition CN/f « M corresponds to v « H. Hence, at close focus the depth of field depends only on the image magnification, the F-number, and the pupil magnification, regardless of the focal length.

I am not reading this thread nor do I want to join your party, just trying to help.

So, here we go. It is generally difficult enough to settle discussions around the principle of equivalence at large focus distances. Which is why I typically ignore aspects related to close focus in my discussions.

However, if one really needs full arguments, then it can be shown that all formulas of equivalence hold true at close focus distances too IFF the crop factor c is replaced by a magnification-dependent "effective" crop factor c(M). Where c(0)=c and c(M) approaching 1 for very close focus. I don't remember where I published this result, it isn't in any of the LumoLabs papers. But because an effective crop factor exists, equivalence-based arguments need not be altered for close focus scenarios while actual formulas do.

Hope this helps clarifying things a bit in this discussion.

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