Depth of field

Hyperfocal distance (超焦距)
The hyperfocal distance is the nearest focus distance at which the DOF extends to infinity; focusing the camera at the hyperfocal distance results in the largest possible depth of field for a given f-number.  Let f be the lens focal length, N be the lens f-number, and c be the circle of confusion for a given image format. The hyperfocal distance H is given by

Formula for DOF:

This means:

• DOF increases as f-stop increases or aperture decreases
• DOF increases as subject distance (distance at which the camera is focused) increases
• DOF increases as focal length decreases. 

Source: http://en.wikipedia.org/wiki/Depth_of_field