Parallax adjustments are extremely important in long range scopes, but it often gets confused with focus. When a target is out of focus, you need to adjust the reticle distance to the back of the shooter’s retina, which would be an ocular adjustment. Parallax is adjusting the target to the reticle. The very simple test for parallax scope shifting is to clamp your rifle in position with the reticle on target, look through it, then move your head left to right. If your reticle shifts one way or the other, your parallax is off. Here’s a simple diagram of the effect:
As you can see, when the objective and reticle lenses are coplanar, parallax is eliminated. It is critical, however, that you maintain the same eye relief (known as cheek weld) in order to keep parallax scope shift from reoccurring. This is part of what makes long-range shooting challenging, but also fun. It’s like golf, but for men. 😉
It’s important to adjust for parallax at each distance you are calibrating for. It’s much easier to adjust for parallax at 6X magnification than it is at 22X.
Nightforce, a manufacturer of some of the finest (and most expensive) long range scopes on the market, has an excellent video on parallax.
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