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).

This Leupold 35X Competition riflescope features a rotary control on the left of the turret saddle for focusing the objective to achieve maximum image sharpness and to eliminate parallax. Other adjustable-focus riflescopes may accomplish the same ends with a focusing collar on the objective bell.

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.