[newarts]

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Any thoughts to proper aperture for best depth of field without losing sharpness?
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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."

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