about focus stacking

Focus stacking (also known as focal plane merging and z-stacking or focus blending) is a digital image processingtechnique which combines multiple images taken at different focusdistances to give a resulting image with a greater depth of field(DOF) than any of the individual source images. 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.

Such as a spider picture to demonstrate

at f/11 DOF is quite limited
320px-Wolf_spider_single_frame

with 8 pictures at f/11 stacked,we can get better result.

320px-Wolf_spider_focus_bracket02

from wiki

Usually,we can process focus stacking picture by Helicon focus,Zerene-Stacker , CombineZP ,Photoshop,etc.

Zerene-Stacker trial 30days  download 
Helicon focus trial 30days  download
CombineZP free software download