Monday, July 13, 2009

Rifle scope parallax problems can bite you in the posterior if you’re not careful, putting your carefully aimed shots off course.

To get a quick idea of what a parallax problem is, let’s do an experiment. Hold your left hand up in front of you, fingers pointing at the ceiling, palm facing you. Now, hold your right hand up in front of your left with the fingers in a fist and the thumb pointing straight up. Now position your right thumb so that it appears to be over the middle of your left palm.

All set? Good. Now, without moving either your left or right hand, move your head slowly to the left a couple of inches then to the right a couple of inches. Does your right thumb appear to move in relation to your left palm? It should, and it’s a perfect demonstration of parallax error. Even though your right thumb has not moved relative to your left palm, it appears to slide from side to side.

A similar thing can happen in a rifle scope. In a perfect universe, what a rifle scope would like to “see” is the image of the target focused in the same plane as the crosshairs. If the image of the target is not “coplanar” with the crosshairs, as you move your head, the crosshairs will appear to move on the target per the illustration below.

When you have a parallax problem, the crosshairs appear to move on the target.

If the image is properly focused, the crosshairs will stay on the same spot on the target, regardless of how you move your head . . . per the illustration below. You can check it out for yourself. Put your rifle on a rest at a known distance, say ten yards. Then, using the highest power, set the distance on the bell your scope at a distance that you know is wrong . . . perhaps 25 yards. Look through the scope, wiggle your head, and notice how the crosshairs move on the target. Now properly focus the scope and watch those crosshairs settle down.

When the target image is focused in the same plane as the crosshairs, the crosshairs stay put, not matter how you move your head, and you don't have a parallax problem.

On low power scopes, where everything tends to seem in focus no matter what distance the scope is focused at, you’ll have to focus the scope, move your head to check for parallax, and repeat until you have removed parallax error at a particular distance. Otherwise, parallax error can throw off your shots at surprisingly short distances.

While we’re on the subject, make sure your scope’s ocular is properly adjusted. Drape a tissue over the objective (the end of the scope toward the target) so that you are not distracted by the view through the scope. Now, unscrew the ocular (moving it away from the body of the scope) until the crosshairs look fuzzy. Next, screw the ocular back in until the crosshairs are just in focus and then just a tiny bit more. More your head in and out toward the ocular to make sure that the crosshairs are sharp.

There, now you know about parallax.

Til next time, aim true and shoot straight.

– Jock Elliott

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