>Recent threads got me puzzled over Depth of Field (DoF) measurements,
>particularly as one changes either focal length or lens to subject distance
>(read magnification here <g>). In researching the web, I came across
>http://www.syspac.com/~gillettm/DOF.html. Mr Michael C. Gillett provides
an
>outstanding little DoF calculator page where you enter any focal length,
>subject distance and aperature, and it does the rest. The formulas on
which
>the calculations are based are described on
>http://www.syspac.com/~gillettm/DOFormula.html.
>
>A few observations:
>For any given focal length, DoF goes to near zero at a lens to subject
>distance of about focal length*4. As you get in closer, either because the
>lens can (as in the case of the 50mm MAC, or with extension (bellows,
>auto-tube or extension tubes) DoF essentially becomes zero regardless of
the
>aperature setting. Interestingly, it is at that same lens to subject
>distance ratio that exposure factors begin to become significant for
errors
>less than 1/3 stop. Hhmmmmm.....
>
>As in "normal" shooting ranges, the longer the lens, the smaller the DoF
for
>a given len to subject distance. At distances less than one meter, these
>differences are quite small.
>
>I'm still looking for the correct expression(s) to show how increased DoF
>thru ever smaller aperatures becomes meaningless, even detrimental, but
have
>yet to locate them. They're here somewhere....... But I think Gary summed
>it up best - shoot a set of slides, look at them carefully and see what
>works best for you, and the heck with the math.
>
>John P
>
>
>
>
>
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