September 23, 2010
Parallax can be a pesky problem or a convenient excuse.
The parallax adjustment knob for many AO scopes has moved from the objective bell to the left side of the scope.
The fellow looked disapprovingly at his target. "Scope's got parallax," he said, as if it had just been diagnosed with an infectious disease. Actually, parallax is a congenital condition, present in every conventional riflescope, and it's merely a change in apparent target position when you change viewing angle.
In other words, when you move your eye above or below the scope's optical axis, or to the right or left, a target that you had right in the middle of the scope field will shift position relative to the reticle. When your eye is on the scope's optical axis--in the center of the exit pupil--there is no parallax. The same is true when the target image falls on the plane of the reticle.
Because it's a function of target distance, parallax cannot be corrected by compound lenses. The images formed by targets at various distances fall at different points between objective and erector lenses--and therefore at different points between the erector and ocular lenses.
As the scope's reticle does not move along the tube's axis to compensate, it meets a focused image only when the target is a certain distance from the rifle. That distance can be programmed into the scope.
Every scope is set at the factory for "zero parallax" at a specified distance. You might liken this setting to a zero setting when you sight in your rifle. Typically, scopes for centerfire rifles are "zeroed out" for parallax at 100 or 150 yards. In Europe, the standard range is 100 meters. Scopes for rimfire rifles, and shotgun/muzzleloader scopes, get a 75-yard zero.
This setting has nothing to do with the windage and elevation adjustments you use to sight in your rifle. It is simply a focus setting made at the factory.
Some scopes allow you to adjust the parallax-free distance. The traditional device is an adjustable objective, a front-end collar that, when rotated by hand, moves the objective lens in and out. A recent option is the turret-mounted dial, installed on the scope's left side.
When either is turned, it moves an auxiliary lens inside the scope. Of course, this lens too is in front of the reticle. Though I've fired many prone matches with a Redfield 3200 that has the front collar, I prefer the turret dial because it takes less effort and does not pull me out of shooting position. Also, the dial enables me to look through the scope easily while making the adjustment.
By the way, the numbers on collar or turret are guides; don't assume they or the witness marks are exact settings for any given range. You may find sharpest focus with collar or dial slightly either side of the appropriate mark.
I explain parallax in detail in my Gun Digest Book of Sporting Optics, and for this column I've also asked two experts for their take on parallax.
John Barsness, a Montanan with many credits in hunting and shooting magazines, plus an optics book, shares his take on parallax this way:
"Make a circle of your forefinger and thumb, then hold your hand at arm's length and look through this circle across the room at, say, a mounted deer head. If you move your head a little to the side, the deer mount will move out of the circle. That's parallax.
"In a riflescope, parallax means that the observed object will apparently move behind the reticle, if your head moves behind the scope. Unless defective, all scopes are set to be parallax-free at a certain range, either when manufactured or with some sort of external adjustment. At other ranges they'll have some parallax. In lower-powered scopes this isn't enough to matter, but at around 10X or above parallax becomes a problem, the reason most scopes over 10X have some means of quick parallax adjustment."
Bill McRae, another Montanan, is an acknowledged optics guru, and I called him up for his take on the phenomenon. "You won't miss an elk at 200 yards because of parallax," Bill says. "But you can certainly miss a gopher at that distance if you let your eye wander from the scope's optical axis. With a scope set at high power, you may catch yourself because the exit pupil is relatively small, and you'll see a black half-moon suddenly encroaching on your field of view. An unforgiving exit pupil thus helps prevent parallax error."
He adds that low-power scopes actually permit greater parallax error "because the exit pupil is huge, and you can see a full field with your eye well off the optical axis." But adjustable objectives don't make sense on such scopes because they add weight, bulk and expense, and because few shooters incur so much parallax as to noticeably affect point of impact on big game at normal hunting ranges.
Remember, parallax is no problem at all if your eye is on the scope's axis, regardless of range, magnification or exit pupil size.
Focus matters at high magnification because depth of field, or depth of acceptable focus, shrinks as you crank up the power. A 6.5-20X variable set at the top end shows soft edges not noticeable at 6X. That's why the adjustable objective and its turret-mounted counterpart appear only on powerful scopes (typically 10X or higher, though a few sights for specific target games offer the adjustable-objective option at lower power).
Wait a minute. You thought those adjustments were to zero out parallax. Well, they are. But at the same time they correct for focus. Remember that parallax is nil regardless of eye position only when the target image is sharply focused on the reticle plane. The objective collar and, these days, the turret dial take care of both issues at once.
McRae emphasizes that it's important to focus the reticle first, so you see it sharply before you fool around with a front-end lens. If you adjust a front lens before you adjust the eyepiece, neither will be in sharp focus when you aim quickly. And if you subsequently adjust the eyepiece, you must start over with the front lens.
A sharp reticle makes sense whether or not you have a parallax adjustment up front. And parallax has no effect on your shooting if you keep your eye centered in the scope. Good things to keep in mind next time your target disappoints.