This text is copied from http://www.photo.net/learn/optics/dofdigital/
Depth of Field -
What it is and what it isn't
Let's try to define depth of field. The usual definition runs something like this:
"The region over which objects in an image appear sharp".
While there is some truth in this, there's also some confusion - and some untruth too! Let's try a more accurate definition:
"The depth of field is the range of distances reproduced in a print over which the image is not unacceptably less sharp than the sharpest part of the image".
This definition contains some important points.
First, DOF relates to a print or other reproduction of an image. It's
NOT an intrinsic property of a lens. If you put a lens on an optical bench
you can measure focal length, you can measure aperture, but you can't
measure depth of field. Depth of field depends on some subjective factors
which I'll discuss later.
Depth of field is also NOT directly related to background blur. Depth of field equations tell you over what range of distances objects will appear to be acceptably sharp (or at least not unacceptably unsharp). It tells you nothing about how much blur there will be of objects well outside the depth of field. That's governed by different physical parameters and determined using totally different equations.