In The Depth of the Field

This blog post is about and effect that allows to create a more realistic look to a rendered scene. Truthfully the effect can be used in both photo-realistic graphics and non-photo realistic graphics alike, I guess a better way of putting it is that it helps create a more believable world blurring a certain degree depending on the distance they are from the target object or camera.

Now depth of field can be achieved in two ways one way is the use the distances of the nearest and the farthest parts of an object and feed them into a fragment shader and anything within that range of distance stays on focus but anything outside of it is blurred. The other way is to calculate the blur radius of the camera with some camera properties such as focal length and focal stop. This is the method I am going to explain.

The first thing we do is calculate the how much an object is blurred depending on the distance the object is from the camera. This can be done using the blur disc diameter equation

Focal length(magnification)  distance                                                                              = blur

Focal stop                          distance of object +/- the distance of object from fragment 

Now the pixels/millimeter needs to be calculated. After that we blur, we can do this by separating bur into two passes one for blurring all the pixels horizontally and the other is for all the pixels vertically (aka box blur). When done we combine it all together, the blurred scene we just did and the regularly rendered scene so that the highly blurry parts blends with the really low resolution parts (as defined by the blur-disc equation) and the highly defined parts blend with the high resolution parts (again, as defined by the equation).

Source
http://devmaster.net/posts/3021/shader-effects-depth-of-field