SLR macro lenses display a minimum aperture setting of f32, which is twice as much depth of field as with f22. So why not use it for macro? Many photographers do, but some prefer not to. The reason for this is a physical problem called diffraction. When light passes through the aperture of a lens, light waves are affected by the edges of the diaphragm blades. If you use a small f-stop, like f32, the more distinct the effect becomes, and as the light spreads out it begins to soften the image. The effect is unno-ticeable unless the aperture is very small and the lens is being focused at high magnification. Ratios of 1:2 and 1:1 are high magnification, so my advice is to avoid f32 in these circumstances. The balance is to choose an aperture that provides adequate depth of field without causing noticeable diffraction.

This is the theory but it continues to be subject to debate and if the topic of diffraction is of interest to you I would go to a 2008 thread of Alex Mustard. http://wetpixel.com/forums/index.php?showtopic=25360&mode=threaded &pid=181972

I have started to use f32 more and more in the digital era and I see little problems with diffraction.

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