Parallax is an interesting phenomenon we experience continuously while viewing the world around us. It helps us navigate the three-dimensional mazes of daily life. In practical terms, parallax is the apparent change in spatial relationships between elements in a three-dimensional scene that occurs when we alter our viewing position. Move slightly, and stationary objects located at different distances in the scene seem to shift their positions relative to each other. Parallax may become troublesome, however, when we encounter it in optical sights.

In this simulation of the view through a riflescope, the image of the red pointed-post reticle is sharp. The target representing the target image is slightly blurred because it is several inches beyond the plane of the reticle. There is no parallax error, however, because the camera lens lines up precisely with the tip of the reticle and the desired impact area of the target, like an eye centered with respect to the eyepiece of an actual riflescope.

When we look through a scope and see the reticle neatly planted against the target, we are likely to interpret what we see as a single “picture,” something akin to a photograph, when what we actually see are two pictures that are superimposed. The forward, or objective, lens system of the scope forms an image of the target area, and the rear, or ocular, lens system forms an image of the scope’s reticle. Ideally the two images coincide at the same plane within the scope as though they were printed on an invisible plate. The result is that you cannot make the reticle image shift relative to the target image, even if you change your eye position from dead center to the far edge of the eyepiece.

We do not live in an ideal world. Most general-purpose scopes are focused at the factory for a particular target distance, typically about 100 to 150 yards. A target at the prefocused distance will look clear and sharp through the scope, and its image will fall on the same plane as the image of the reticle. There will be no visible parallax discrepancy regardless of your eye position, and you may reasonably expect your bullet to strike the target where you set the reticle.

If the target is significantly nearer or farther than the scope’s optimal distance, its image will be less sharp, although not always obviously so, and will be formed within the scope a bit ahead of or behind the image of the reticle.

The images will be separated in depth. If your eye is well centered with respect to the scope’s eyepiece, you will view along the scope’s optical axis and the separation in depth of the reticle and target images will have no negative effect on the outcome of the shot.

Moving the camera lens slightly from its original position introduces parallax error that makes the tip of the reticle appear to shift from its original on-target position. Neither reticle nor target has actually moved.

If your eye drifts off center, though, a slight apparent shift will occur between the relative positions of the reticle and the target images. You are now likely to move the firearm to place the image of the reticle where you first established it on the target. You have parallax error in aiming, and your shot will impact slightly away from where you expect it to land.

Fortunately, parallax error is rarely great enough to spoil an otherwise well-executed shot in the field. Furthermore, the human eye has a proclivity for centering itself well with respect to apertures through which it is viewing, such as a scope eyepiece, so extra care when mounting the firearm will help, too.