----- Original Message -----
From: "Matthew Hunt"
Subject: Re: FA 1.4/50mm tested by DPReview
On Thu, Nov 13, 2008 at 9:26 PM, William Robb <[EMAIL PROTECTED]> wrote:
Actually, it depends purely on the magnification ratio and alowable COC.
Focal length doesn't matter, nor does subject distance.
Your statement is only an approximation, useful for close subject
distances. See:
http://en.wikipedia.org/wiki/Depth_of_field#Close-up
"Stated otherwise, for the same subject magnification, all focal
lengths give approximately the same DOF. This statement is true only
when the subject distance is small in comparison with the hyperfocal
distance, however."
Don't try to correct John on this, he's forgotten more than most people
know
on the subject.
I'm not "most people", and he's wrong. John disputes that the
allowable CoC, which you mention above, varies with format. I have
provided numerous sources that state otherwise, and offer clear
explanations.
Of course it varies with format. As an examle, you don't need to magnify an
image from a 4x5 negative as much as you do an image from an APS-C sensor,
so the CoC of the sheet film can be larger.
However, trying to correct John is a fools game, no matter what your opinion
of the facts are.
William Robb
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