Teach me about macro lenses

I beg to differ. Diffraction is a consequence of the interaction of light with the edges of an aperture and is described by the Fourier transform of the aperture:

http://en.wikipedia.org/wiki/Airy_Disk

The intensity described by this transform has only angular dependence. So, for any given aperture, the amount of diffraction is directly proportional to the distance from the aperture. This goes as the effective f/number.

So let's say your lens is set to f/8. From the Rayleigh criterion of 1.22 (wavelength)*(f/number), you'll get about 5 microns of diffraction blur. But if you use the lens at unit magnification, it will be twice the distance from the sensor. So it will have twice the blur, or 10 microns. This correlates exactly with the effective aperture (f/16 in the above situation).

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Leonard Migliore