What is Parallax?
By Earl Hines, Best of the West Master Gunsmith
When we talk about the accuracy of a shooting system, there is one element that most people know little about, don't understand, don't know how to apply, or don't recognize. Many people have indicated that they understand it but when asked, they give an inaccurate response. What I am referring to is that thing called parallax. What is it? How does it work? How do I adjust it? Is it important? How do I know when it's correct? And where do I start? All important questions. I will try to answer all these and more.
First, what is parallax? Parallax is having both the cross hairs and the target in the same focal plane so that no matter where your eye looks through the rear lens, the cross hairs do not change position on the target. How can that be accomplished? Well, modern scope manufacturers provide us with an adjustment mechanism on the opposite side of the right and left windage adjuster. Older scopes without this adjustable feature were parallaxed at a given distance. Depending on the manufacturer, they would parallax their scope between approximately 75 yards to 150 yards. At normal hunting ranges of the time, maximum of 3 or 4 hundred yards, the small amount of parallax was minimal (but still there).
Let’s look at understanding the concept of parallax. Look at the simple drawing in figure 1. In the center of the scope we have a cross, representing the reticle or cross hair inside your scope. On the left we have the part of the scope you look through, and on the right is our target. This being a simple drawing, we have no internal lenses or way to adjust for anything. If my position on the stock causes me to be absolute centered, then I would see along line “A” through the scope. If, for some reason my position causes me to look through the bottom portion of the scope I would see my cross hair on the upper part of the target, line “B”. That would be exactly opposite if I looked through the top of the scope, my cross hair would appear to be on the bottom of the target, line “C”.
So as you can see, by looking through the scope in different positions the cross hair would appear in different locations on the target. Now to apply this to shooting this simple system on a rifle, you can also see that if you were exactly in the center for 5 shots with a perfect rifle, your group would be acceptable and in the center of the target. Look at figure 2. Same rifle but this time your eye is at the bottom line “B”, and you fired 5 shots, again your group would be acceptable but at the bottom of the target because you would see the cross hair at the top of the target and compensate by moving your rifle cross hairs into the center of the target. Same for position “C”. But if you are not in the same position or on the same line every time, as illustrated in figure 2, your group would be much larger and you would be wondering where your accuracy was. That is why modern scopes have parallax adjustment capabilities.
When correctly adjusted, the internal workings of the scope will be able to focus both the target and cross hairs at the same time (on the same focal plane) without movement no matter where your eye is located. This would be correct parallax. Look at figure 3. Most scopes have this adjustable knob on the left side or your scope. There are numbers, varying anywhere from 25 ft. to the lazy 8, indicating infinity. These are “not” and I repeat “not” exact ranges that you can rely on to be accurate. They are approximations. So now you must focus the rear ocular (the lens that you look through to see the target) so that these numbers are close to the distance you want the parallax to be “free”. The term used when your scope is adjusted free of error or movement.
Start with your brand new scope. Turn the parallax knob to the lazy 8 position. Now go outside, with a clear blue sky preferably, point your scope towards the sky and turn your rear eye piece clockwise until it stops. Next, while looking through your scope, turn the ring counter clockwise. Turn the ring until the cross hair becomes crystal clear. Go slightly past this point and then back to insure that it is at the clearest point. At that point your scope is now adjusted to you and only you. The cross hair, being in focus, has been adjusted for your vision requirements to be correct for you. Now you can mount your scope on your highly accurate rifle and head to your favorite shooting spot and finish adjusting things.
If you are sighting in at 200 yards you need to secure your rifle on a bench using sand bags or a very steady bench rest and adjust your parallax knob to 200 yard setting. Position your rifle so that it is pointing at the center of the target when you let go of it. Now without touching your rifle, position your eye at a point so that you are looking through the very center part of the scope. This would be line “A” in figure 1. Now move your eye, not the rifle, up and down, side to side and 45 degrees in all directions. If the cross hairs move in the opposite direction on the target from your eye position, you are “out” of adjustment. To correct or to verify that you are in adjustment, turn the parallax knob either clockwise or counterclockwise a little. This does not take a great amount of adjustment. The cross hair will stop moving if you were out of parallax, or have greater movement if you were already in correct adjustment. The thing that you are looking for is no movement of the cross hair on the target.
If you have a rifle capable of shooting 1 inch groups at 200 yards, even with the most expensive scope in the world, your group size and consistency cannot be realized until your scope is adjusted for you and you understand parallax and adjust for it. Is parallax important? You bet ya. Your bullets will not go where you want them to until you have perfect parallax.
It is all very simple once you have seen improperly adjusted parallax and a parallax free scope setting. Remember, your rifle can shoot no more accurately than your parallax capabilities.
Just a thought for you. How many rifles have been taken to a gunsmith because the rifle “would not group”? You can bet many rebarrels, bedding jobs and such have been sold or charged for that only need parallax adjustment.