Photophys.com: The Science of Photography

Avoiding diffraction problems in macro-photography:  We all know about the loss of resolution that occurs with small apertures (large f#’s), but it is easy to forget that the effective f# is not equal to the one shown in the camera.  The f# we need to use is: f#(effective) = f# times (1 + m).  Here m is the magnification.  My Sigma 150mm macro lens will focus to 1X magnification.  At that point all the f#’s shown should be multiplified by two in order to estimate the DoF and the effect of diffraction.  It gets worse with extension tubes.  I add 68mm of additional extension to get 1.68X and the f#  shown needs to be multiplied by 2.68!  When I forgot this factor and set the camera for f# = 22, I got unsatisfactory images.  As an example take a look at this figure:

Sundew plant (100% crop of 1X macro photos) (635 kb, jpg)

The setting of f/4.5 is probably the best and f/8 is satisfactory, but f/22 is fuzzy.  You can find useful information in John Shaw's book, "Closeups in Nature."