Q: I have been shopping around for a new scope for my Browning X-Bolt .270 and have been offered several models that have parallax adjustment. At high school I must have slept through the lesson on optics. Can you bring me up to date on the subject? How do you recognize Parallax if it is present in a scope?
A: Parallax can be detected by getting the scope on a steady rest, placing its crosswires on the centre of a target at say
100 yards, and while looking through the scope, moving your head from side to side. If parallax exists, the reticle will appear to move from side to side on the target. This presents no real problem as long as the eye is perfectly aligned with the centre of the scope, but if the eye is not perfectly centred for each shot, points of impacts of bullets fired into a group will be
displaced by the amount of parallax (or eye misalignment).The eyepiece (ocular lens) is adjusted to focus the scope for the individual shooter’s vision,, but parallax is adjusted to focus the scope at various ranges, either by having an adjustable objective or a third turret. Being able to completely eliminate parallax in a scope is extremely important if the scope is to be used for benchrest competition or other types of long-range target shooting, or on a high-magnification varmint scope. Hunting scopes with fixed objectives are usually adjusted at the factory to be free of parallax at 100 or 150 yards, and don’t need to be adjusted for parallax.
To see it graphically see the video below.