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Re: [OM] Extension rings and focus - newbie Q

Subject: Re: [OM] Extension rings and focus - newbie Q
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Sat, 16 Sep 2000 07:15:21 +0000
At 13:50 9/15/00 , Lex wrote:
>And if I remember correctly from something I read in the distant (not to say 
>close up) past, DOF calculations go wacko where macro photography is 
>concerned.  There's a formula to calculate it, but I don't remember what it 
>is.
>
>Hope I got that right.
>
>Lex

Yes and no, depending on what you meant by "wacko."  If you're using the
proper equations for it *and* using the correct numbers, it works for all
focus distances.  However, the proper equations use a "subject distance"
which is defined as the distance from the subject to the front lens node,
*not* the film plane, which is what most lens focus markings show.  For
anything beyond a few feet (about a meter), the relative difference between
the distance of subject to front lens node and subject to film plane is not
enough to worry about.  At very short macro focus distances, the relative
difference does get great enough to throw computations off if you aren't
using subject to front lens node distance.

-- John

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