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01-17-2011, 09:37 AM   #31
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QuoteOriginally posted by joelovotti Quote
.....
Any thoughts to proper aperture for best depth of field without losing sharpness?
.....
Here's what theory has to say (but I don' t see much effect in practice.)

Say your lens/camera combination becomes diffraction limited for normal photography at f-number Ndiff.

Then for macro work the maximum f-number to use is about:

Ndiff/(1+m)

Because that's where diffraction will interfere with macro resolution.

The maximum macro DOF one might expect before diffraction interferes with resolution is close to:

DOFdiff ~ (Ndiff/20m)^2 in millimeters, m is magnification

Say your lens is diffraction limited at f:20 under normal conditions. For maximum depth of field in a 1:1 macro with that lens you should set the f-stop to 20/(1+1)=10. You can expect a DOF of about (20/20*1)^2 = 1mm.

Say you must have a DOF of 1/4 mm. What is the maximum magnification you can use to meet this requirement? Solve the DOF equation for magnification:

m = (Ndiff/20)/√DOF

m = (20/20) / √(1/4) = 2

Greater magnification may increase apparent DOF but it will also soften the "in focus" portion of the image. The "in-focus" portion will in fact be out of focus.

Dave

PS The above assumes that diffraction is limiting when the diffraction spot size equals the DOF's "Circle of Confusion."


Last edited by newarts; 01-18-2011 at 05:47 AM.
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