Parallax from Leupold
"Parallax is essentially an optical illusion. It is the apparent movement of the target relative to the reticle when you move your eye away from the center point of the eyepiece. It occurs when the target does not fall on the same optical plane as the reticle.
Maximum parallax occurs when your eye is at the very edge of the exit pupil. (Even in this unlikely event, our 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards.)
At short distances, the parallax effect does not affect accuracy. (Using the same 4x scope at 100 yards, the maximum error is less than 2/10ths of an inch.) It is also good to remember that, as long you are sighting straight through the middle of the scope, or close to it, parallax will have virtually no effect on accuracy in a hunting situation."
Leupold's settings:
Handgun scopes - 100 yards
Shotgun scopes - 75 yards
Rimfire scopes - 60 yards
Hunting scopes - 150 yards
M8 2.5x20mm Compact - 100 yards
Turkey Ranger - 40 yards