What is Back/Front Focus when referring to camera lenses?
Back focusing and front focusing refer to the tendency of a lens – or sometimes the camera body – to focus slightly behind (back focus) or slightly in front of (front focus) the intended subject when using autofocus. In some cases, it can be eliminated by adjusting the lens at a repair center, and Canon has recently added a feature to the 1D Mark III, 50D, 5D mark II, 7D that allows the individual user to make adjustments on the camera.
It’s VERY annoying to shoot your pic of the day of your wife/husband/child/friend and then find out that their face is out of focus but their ears are tack sharp. Not nice.
There are many things that can cause this problem and almost all of them can be chalked up to operator error. Occasionally though, the camera and/or lens is to blame.
DOWNLOAD HERE :Focus test chart by Tim Jackson