This page illustrates the loss of sharpness induced by a small aperture.

Both images were taken on a Canon 5D MKII, 100 ISO, 100mm Macro lens, mirror lockup, 10 seconds delay. They are both 1:1 crops of a RAW file, converted with all defaults in Lightroom 2.2, and saved as 100% quality jpeg.

The first image was taken at F11, the second image was taken at F22