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.
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.
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.
Parallax error is a more serious concern for target, benchrest, and varmint shooters, who place a premium on precise shot placement. Riflescopes designed for these demanding applications allow you to adjust the objective lens for exact focus from the horizon to as close as about 40 yards (some airgun scopes focus down to 10 meters).
The focus control may be a calibrated collar on the scope’s objective bell or a rotary control on the turret saddle opposite the windage knob. With either type, take distance scales with a large grain of salt until verified, because they are sometimes surprisingly fanciful, even on expensive instruments.
You can check a scope easily for exact focus and freedom from parallax. With a fixed-focus model, set a target at the distance the manufacturer lists as the factory standard. Immobilize the scope or scoped firearm in a steady rest or sandbag array with the reticle centered on the target.
Without touching the scope or firearm, move your aiming eye slowly from the center of the eyepiece to the edge while observing the reticle’s position on the target. If the reticle seems glued to the target, with no shift in position, the scope is properly focused and there is no parallax error at that distance. An inch or so of parallax error is tolerable in a general-purpose scope. If there’s much more than that, consider having the scope serviced.
With an adjustable-focus scope, check the focusing scale for accuracy using targets at known distances. If it’s slightly off, tweak the focus control until the target looks sharpest and there is no visible parallax error. Repeat the procedure for each relevant target distance. As you go, remark the scale with small dots of paint or nail polish, or stick a marked strip of tape over the factory calibrations. If the scale is way off, send the scope back for a proper fix.
When performing focus tests with a scope that is already mounted on a firearm, do so at a shooting range or other safe venue. Neighbors may find it unsettling to see your rifle poking out of the living room window while you focus on a lamppost that’s a convenient 100 yards down the street.