Parallax is the perceived change in location of an objective relative to other objects when viewed from various directions. You may not see scope parallax every day, but you do see parallax every day. It is so common that you have learned to ignore it. How does parallax affect how you aim with a riflescope? and what is Rifle Scope Parallax Adjustment
When rifle shooters talk about 'parallax', they typically are referring to scope parallax. Scope parallax is often a convenient excuse for missing a target or shooting disappointing groups without considering other reasons or understanding parallax and how to deal with it. A few rifle shooters mistakenly believe that parallax refers to some optical defect of the riflescope and that parallax is a reason to return the scope for repair, refund, or, replacement. (Disgruntled will be the rifleman who receives a replacement scope that also has 'parallax'.)
Don't fret; your riflescope probably is not defective if you notice parallax. Actually, you may not notice scope parallax unless you know what to look for. You cannot discern parallax unless the rifle is held rock steady on a benchrest, sight-in vise, bipod, or other solid shooting support. Therefore, scope parallax is not the result of the shooter's inability to hold the rifle steady while aiming with a scope. On the contrary, scope parallax is about the shooter having an inconsistent eye position between subsequent shots or not maintaining a stable eye position while aiming with a scope. So what is scope parallax?
Understanding 'scope parallax' is easier if you understand just 'parallax' first. Parallax is the perceived change in location of a stationary object relative to other stationary objects when viewed from various directions.
Still not sure what parallax is? You may not see scope parallax every day, but you see common parallax continuously every day. In fact, parallax is so common you learned to ignore it long ago. Now you must deliberately look for parallax to notice it.
Obviously, some objects are closer to you than others. The farthest stationary object you can see at any moment, whether outside or in a room, appears fixed in one location as you change your location or viewpoint. When you move left and right or up and down while looking at the farthest object, intermediate objects, being stationary, do not follow your line of sight to the farthest object. Instead, intermediate objects appear to move in the opposite direction. Try it outside with tree and a house, for example, or inside with a lamp and a wall. Are the intermediate objects actually moving in the opposite direction or is the farthest object moving in same direction as you? Neither is moving, of course; you, the observer, are moving instead. Intermediate objects appear to move relative to the farthest object and each other, but actually are not moving at all. That apparent movement of stationary objects is parallax. You are aware of parallax because you see it every day. However, parallax is so common that you ignore it.
What does parallax have to do with aiming a rifle with a scope? Scope parallax is the apparent movement of the target relative to the reticle as the shooter's eye position moves relative to the riflescope optical center. Two conditions must exist for scope parallax to occur: the reticle must be own a different optical plane than the reticle and the shooter's eye position relative to the optical center of the scope must vary.
The reticle appears to be superimposed directly on the target when aiming with a scope. Of course, the reticle is not actually located on the target. The reticle is within the scope - between the shooter's eye and the target. And the target is not actually located within the scope. The image of the target only appears to be inside the scope while aiming.
The image plane of the target within the scope coincides with the reticle if the actual target is one certain distance from the objective lens. (The image plane is an optical plane - not a physical object - because the target is not actually located within the scope.) Like a viewing a picture or a calendar hanging on a wall when you move about the room, when the target image and the reticle coincide, the target and the reticle do not appear to move relative to one another whenever you reposition your aiming eye. In other words, there is no scope parallax if the target image plane coincides with the scope reticle no matter how much you move your aiming eye around.
There is only one distance at which the target image will coincide with the reticle. Thus, a typical hunting riflescope has only one range at which the target image and the reticle coincide. Centerfire riflescopes are normally designed and assembled so the reticle and image coincide whenever the target is at 100 yards. Scopes for shotguns and rimfire rifles are set for image and reticle coincidence at 50 yards. Every target, of course, is not 100 yards or 50 yards distance from the objective lens.
If the target, whether a paper sight-in target or a live game animal, is farther or nearer the image and the reticle won't coincide. The target image will still form within the scope, just not at the reticle. For instance, if the actual target is farther than the distance at which the reticle and target image coincide (as set by the scope manufacturer) the target image forms within the scope between the objective lens and the reticle. If the actual target is nearer, the target image forms between the shooter's eye and the reticle.
Don't be too concerned about parallax yet even if the target image does not coincide with the reticle. The target image and reticle being out of coincidence does not by itself result in scope parallax. Another condition must also occur. The second condition is that the shooter's aiming eye wavers about the optical center of the scope. Remember, scope parallax is the apparent relative movement between the reticle and the target whenever the shooter shifts eye position changes. So no eye movement while aiming means there is no parallax even if the target image and reticle do not coincide.
If the target image is closer than the reticle, the target image will appear to follow your eye movement whenever your eye drifts around the optical center of the scope assuming that your rifle is held stable. If the target image is closer than the reticle, the target image will appear to follow eye movement. For example, if you move your eye right, the target image will appear move right. If you move your eye up, the target will appear to move up. The target image will appear to move in the opposite direction of the shooter's eye movement if the target image is farther than the reticle. Neither the reticle nor the target is actually moving, only your viewpoint is changing. Just like with common parallax, the objects - the target and the reticle in this situation - appear to move relative to one another.
Scope parallax is not the reticle wandering around and across the target because of an unstable shooting position. Your aim must be rock steady to discern scope parallax. That does not mean scope parallax is not present if your aim is not steady; it means you cannot distinguish scope parallax from the reticle wavering on the target as a result of an unstable shooting position.
What can you do about scope parallax? Unless your riflescope has a parallax eliminating system such as an adjustable objective lens or side focus, you cannot do much about parallax other than to establish a consistent stockweld before taking each shot.
Should you bother check for parallax if you cannot do much about it anyway? Just be aware that an aiming error induced by parallax is possible and strive to establish a consistent stockweld. Practice aiming with your riflescope to establish a consistent stockweld before your next shot.