Why And How Scope Parallax
By John Barsness
One of the definitions of parallax in my Random House Webster’s Unabridged Dictionary is “the apparent displacement of an observed object due to a change in position of the observer.” As an example, point at something across the room, say the crayon drawing by a kindergarten artist taped to the refrigerator. Point so your forefinger’s tip covers the bottom of the drawing. Now, without moving your pointing hand, tilt your head slightly. The picture will “move” from behind your finger. That’s an example of parallax.
In a riflescope, parallax means the reticle apparently moves across the target when your head moves behind the scope. Obviously, this can affect where the bullet lands, the reason scopes are set to be parallax-free at a certain range, either when manufactured or by some sort of external adjustment.
It’s easiest to see scope parallax with a rifle placed on sandbags on top of a bench. Aim the rifle at a target and, with the rifle held as still as possible, move your head back and forth. If the reticle apparently moves from where it was aimed, then there’s parallax at that range. This occurs because the reticle isn’t precisely in the middle of its focal plane, even if the scope itself is focused and the reticle appears sharp.
Most scopes have some parallax at most ranges, caused by the reticle not being focused for the specific distance to the target. However, most shooters never notice, because the reticle’s sharp, the amount of parallax is tiny, and they never bother to check. The two major considerations are (1) whether the amount of parallax is enough to matter, and (2) if it is, how to correct it. A crude example can also be demonstrated by kitchen finger-pointing. If we point at an object closer than the refrigerator, say the cup of coffee sitting in front of us on the kitchen table, there’s almost no visible parallax when we move our head back and forth.
This is basically why low-magnification scopes have too little parallax to matter, at least in most shooting where we use low-powered scopes. As anybody who’s done much photography knows, wide-angle, low magnification lenses have more “depth of field,” where everything remains in focus no matter how far it is from the camera. The same is true of lower-magnification scopes, and why most scopes up to around 10X—whether fixed or variable—don’t have any external means of correcting parallax, either via an adjustable objective bell (A.O.) or a dial on the left side of the scope (opposite the windage turret). Instead they rely on the fact lower magnification also reduces parallax, so are set at the factory for minimal parallax at around 100 yards. At longer ranges some parallax will be present, but not enough to matter for most shooting.
A buttstock comb too low to provide firm and consistent cheek support (above)
can result in parallax problems, due to slightly inconsistent positioning of
the shooters eye behind the scope from shot-to-shot, which is why many shooters
prefer a higher, adjustable comb (below).
But above 10X focusing becomes more critical, just as it does with telephoto camera lenses, and if we don’t refocus both scope and reticle for a particular range, parallax becomes enough to matter. Shooters normally use higher-magnification scopes when shooting at smaller targets, whether the target is indeed tiny, or long range makes it appear smaller. This is why most scopes above 10X have an external means of correcting parallax, whether an A.O. or side-knob.
However, do not believe the yardage numbers on either. Instead, look through the scope while tweaking the adjustment; you’ll usually find the parallax-free setting doesn’t precisely match the numbers. Also, when shooting only at longer ranges, from 250 to 300 yards out, the parallax adjustment can usually be set on “infinity,” and parallax will be zero or close to it at any longer range.
External parallax adjustments can also be found on lower-magnification scopes meant for closer ranges—or the scopes are set at the factory to be parallax-free at well under 100 yards. Such scopes are usually meant to be mounted on air or rimfire rifles, where even a typical 4X or 2-7X variable set to be parallax-free at 100 yards has too much parallax between 25 to 75 yards for precise shooting at small targets.
Focusing the objective lens has the most impact on parallax, and tweaking the objective is exactly how parallax is “set” at scope factories. But all scopes have adjustable-focus eyepieces, and some lower-magnification scopes can be focused to be parallax-free at closer ranges by twisting the eyepiece.
Objective lens diameter also has an effect. Many shooters believe larger objective lenses are always better, because they allow more light to enter the scope, resulting in a larger exit pupil and hence more light for our aiming eye. But when we move our eye behind a scope to the limits of the field of view, we’re also moving it to the limits of the exit pupil. A larger exit pupil allows us to move our eye farther while still seeing the complete field of view, so results in more parallax. (This is one reason target shooters using aperture sights tend to use the smallest hole possible: The tiny hole limits the possible movement of our eye behind the sight.)
In a scope, smaller objective lenses and higher magnification reduce parallax, because they reduce the size of the exit pupil. But both also result in less light reaching our eye, dimming the view. Magnification in optics always involves compromises.
Many of today’s higher-magnification scopes have a knob
on the left side for eliminating parallax.
One bright spot in all of this, if you’ll pardon the pun, is if our eye remains centered behind the scope when aiming, there’s no parallax at any range. One method for eliminating parallax with scopes lacking an A.O. or side-focus is placing our aiming eye far enough behind the ocular lens for the view to slightly “black out” around the edges. What you’re seeing, of course, is the exit pupil, and if you center it in the ocular lens there is no parallax. (This is one reason many shooters believe in a consistent “cheekweld” for accurate shooting: If your head’s placed firmly on the comb, in exactly the same way for every shot, there’s no variation in parallax.)
Parallax can vary slightly from day to day, even with the same scope focused exactly the same way, due to variations in atmospheric density. In effect, the air acts as yet another lens in the optical system. This is why I try to remember to check every scope for visible parallax at the start of any range tests. Otherwise groups can include bullet holes in surprising places!
Half of John Barsness’s dozen books are on firearms and shooting. Modern Hunting Optics was published by Deep Creek Press in 2014, and is available through www.riflesandrecipes.com, P.O. Box 579, Townsend, MT 59644, (406) 521-0273.
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