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

Focus stacking (also known as focal plane merging and z-stacking[1] or focus blending) is a digital image processing technique which combines multiple images taken at different focus distances to give a resulting image with a greater depth of field (DOF) than any of the individual source images.[2][3] Focus stacking can be used in any situation where individual images have a very shallow depth of field; macro photography and optical microscopy are two typical examples.