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GUNS Magazine August 2010 - Page 36
ver since Sir Isaac Newton attached a telescope to a rifle most shooters have been somewhat confused by parallax. It’s especially a problem when we want to put a scope on a rimfire or air rifle. Parallax is defined in my Random House edition of Webster’s Unabridged Dictionary as “the apparent displacement of an observed object due to a change in position of the observer.” An easy demonstration of parallax can even be performed by the average couch potato. Look across the room at a stationary object, say the remote control you left beside the TV, where it isn’t doing you any good. Hold up your hand and, making a circle with the thumb and forefinger, look through the circle at the remote. Now, holding your hand still, turn your head back and forth, as if firmly telling your teenage daughter that no, she can’t go on spring break in Daytona Beach. The TV remote will apparently move back and forth in inside the finger-circle. That’s parallax. In a riflescope, parallax occurs when E John Barsness the image of the target isn’t focused in exactly the same plane as the reticle. Sometimes this will be obvious, because the reticle looks fuzzy, but sometimes the reticle will appear sharp while some parallax exists. To see if your scope is affected, place the rifle on a steady rest with the reticle resting on a target. Then, without touching the rifle move your eye around behind the scope. If the reticle apparently “moves” as your eye moves, then parallax is present—and we might not hit what we’re aiming at, even if we aim steadily and squeeze the trigger very carefully. Once a shooter starts testing scopes for parallax, it becomes obvious there’s at least a little bit of parallax present most of the time. This is because our target isn’t always precisely the same distance away—even if it is, atmospheric conditions change. Atmosphere acts as an additional lens in any optical system, so even if we always shoot at 100 yards, on some days we’ll have to tweak the scope to eliminate parallax. Also, higher magnification makes parallax worse, the reason any riflescope with magnification more than 10X usually has some means of easy parallax correction, whether an adjustable Choosing the correct scope for a rimfire rifle results in more accurate shooting at any range. The CZ .17 HMR (right, top rifle) has a 3-9X Burris Fullfield II designed for centerfire rifles, while the CZ .22 Long Rifle (right, bottom rifle) has a Burris Compact with adjustable objective for shooting at closer ranges. 36 WWW.GUNSMAGAZINE.COM • AUGUST 2010