As an image enters our scope, each lens focuses it onto the face of the lens behind it. The image gets squished, flipped upside down a couple times and then spit out the back end for us to admire. Parallax occurs during the journey through the scope when the image gets projected onto the lens containing our reticle. If the focused image is just a fraction of a millimeter in front of or behind the lens that contains our reticle, we'll have some parallax.
The tried-and-true method of detecting parallax includes stabilizing the rifle and then moving our head up and down or side to side without disturbing the rifle. If our reticle moves around on the target while we move our head, we have parallax.
The thing to remember about parallax is that it won't shift our point of impact on most hunting scopes more than a couple inches at 300 yards, a distance beyond what most of us are comfortable shooting game. The reason it won't do any more damage than that is that most scopes that lack a parallax or side-focus knob have parallax fixed at 100 yards. The factors that exacerbate parallax are high magnification and increasing the distance to the target.
Let's assume we're using an inexpensive 3-9X on our deer-hunting rifle. It has no side-focus knob, so parallax is fixed internally at 100 yards. That means the scope was designed and built around a target at 100 yards being precisely focused onto the lens containing the reticle, probably in the second focal plane. As the distance to the target increases, the amount of parallax that creeps into the scope increases as well.
The reason our parallax error remains minimal at 300 yards is because we're only using 9X, at maximum. If we're really worried about parallax, we can dial down our magnification, thus reducing the amount of parallax, as the light is manipulated less aggressively inside the scope.
The more magnification we use, the more our lenses have to bend the light that passes through them. The more light bends, the less it wants to focus into a single plane. That inability to get our image focused into the plane where our reticle is located is the source of parallax. Dial down the magnification, and the parallax diminishes.
Another purveyor of parallax is the distance to the target. With our scope's parallax set at 100 yards, anything past that is going to challenge our scope's ability to get the focused image where it needs to be. We know we can dial down magnification if we detect a problem, but even then parallax will still become an issue if the distance increases enough.
To demonstrate this recently, I used a 5-25X optic and set the parallax at 100 yards on 25X. I dialed the scope to 5X and put the crosshairs on targets at 300, 400 and 550 yards and then moved my head side to side and up and down to see how far I could get the crosshairs to move on the target with no parallax correction. At 5X, I could get the crosshairs to move 3 inches at 300 yards, 6 inches at 400 yards and 10 inches at 550 yards. At 10X, I had 4 inches at 300 yards, 8 inches at 400 yards and 10 inches at 550 yards.
This test confirmed that magnification and distance do increase the effects of parallax. Most telling was that I had to work hard to get 3 to 4 inches of parallax at 300 yards with 5-1 OX magnification. In order to get that much crosshair shift, I had to move my head to uncomfortable positions where it was obvious I wasn't where I'd normally be on the rifle. With my head in anywhere near a normal position, that figure would be half.
The reason we had 10 inches of crosshair shift at 550 yards with both 5X and 10X had nothing to do with parallax but with the smaller exit pupil found at 10X. The higher we go up in magnification, the less we can move our head around before our image blacks out because we move outside the exit pupil. At 10X, the image in the scope blacked out before the crosshairs stopped moving. All I could measure was 10 inches of shift, but there was certainly more on the way.
The next time you're shopping for a scope with a maximum magnification of 10X or less, don't worry about paying for the side focus/parallax knob unless you plan on shooting past 400 or 500 yards. The only caveat is if you shoot a lot at 100 to 200 yards and want the smallest groups possible. In that case, the parallax knob is a good buy. While it won't make a huge impact at that close of a distance, it can be the difference between a half-MOA group and a 1 MOA-group.
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|Title Annotation:||RIFLES & GLASS|
|Publication:||Guns & Ammo|
|Date:||Nov 1, 2015|
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