A little more in depth... Parallax is a condition that occurs when the target image is not focused precisely on the reticle plane. It appears as movement between the reticle and the target when the shooter moves his head. Manufacturers usually pre-set most optics to be parallax free at certain yardages. Many shotgun / ML scopes are typically factory-set for either 50, 75 or now, even 100 yards. Most rimfire scopes are set for 50 (usually). Centerfire scopes under 12X or so are typically set for 100 yards. Higher powered variables often come with Adjustable Objectives (A.O.) that the shooter can change depending upon conditions and yardages. For most typical big game hunting situations, 100 yard parallax free settings are fine. Many "experts" advise against A.O. scopes for regular hunting. They seem best suited for target shooting or varmints where adjusting time is less critical.