With one eye closed, point your finger at a small object located several feet away. Holding your finger still, move your head from side to side and notice how your finger appears to move laterally relative to the object. This optical phenomenon, called "parallax," also appears in improperly focused telescopic sights as an apparent movement of the reticle relative to the target image when the eye is moved from side to side in the scope's exit pupil.The illustrations below show two ways parallax can occur. The first shows the image of the target, formed by the objective lense, falling in front of the reticle. The second shows the image falling behind the reticle. And the third shows a parallax-free image. The viewing distance determines the target-image position within the scope. The good news is that scopes with adjustable objectives can eliminate parallax for any particular distance by focusing the objective so that the target image falls precisely on the reticle.Scopes with fixed-focus objectives are permanently adjusted by the manufacturer to be parallax free at a specific distance: either 100 or 150 yards/meters for centerfire riflescopes and somewhere between 50 and 75 yards/meters for rimfire, shotgun and muzzleloader scopes. Unless defective, such scopes won't have more than about 1 inch of parallax at any reasonable shooting distance.