by Tom Gaylord
Writing as B.B. Pelletier

Part 1
Part 2
Part 3

This report covers:

  • What optically centering DOES NOT mean
  • What optically centering really is
  • How to optically center a scope
  • Why do we do it?
  • Field target
  • Counting clicks — mechanical centering
  • Erector tube return spring
  • A better way
  • What about left and right?
  • Why so anal?
  • Pragmatic approach
  • Summary

Today we are going to discuss optically centering a scope. It’s going to be a difficult report for me to write, because the subject does not have much merit for airgunners. So I will compensate by adding some things that do have merit. Let’s go!

What optically centering DOES NOT mean

Let’s start with what optical centering DOESN’T mean. The optical center of the scope is not the place at which there are an equal number of clicks up and down and side to side. I say that and some of you already know it and yet the website “RifleOpticsWorld” has an online article written by “Rifle Optics Team” that says that setting a scope to the optical center is simply returning it to the factory setting. Excuse me????? 

Who in their right mind believes that a rifle scope comes from the factory set to its optical center? The factory assembles each scope as quickly as possible, checks it at certain points for quality and ships it. They don’t spend 45 minutes or more optically centering each scope they make!

This article then proceeds to tell the reader that optically centering is a solution to scope shift! No, it’s not! Optical centering has nothing to do with scope shift and it doesn’t fix it. I will tell you today what really does affect scope shift and how to correct it.

After reading this online article it is obvious to me that it was written by someone (or a team of someones) who was assigned to write it and they made stuff up as they went. If you understand what optically centering is, I invite you to read the article and see how far off the mark it is.

What optically centering really is

The optical center refers to the reticle and the field of view. An optically-centered scope shows zero reticle movement against a distant backdrop when the scope tube is rotated in a complete circle. Theoretically, it’s possible to achieve, but I’ve never seen it. The best I’ve seen is a reticle that moves about a quarter inch against a target 20 yards away when the scope tube is rotated in a complete circle.

How to optically center a scope

There are two ways to optically center a scope. The first way is to set the scope tube in Vee blocks and rotate the scope while watching the reticle against a distant target. I have done this with a quarter-inch dot as the target — set 20 yards from my position. Believe me — it can take a long time to get the scope pointing exactly at that dot — even with two people working at it! The easiest way I have found is to put a large white sheet of paper at 20 yards distance and talk a friend into drawing the dot, while you look through the scope that’s sitting in the Vee blocks and direct him. I initially thought the precision of the Vee blocks mattered, but I’ve since recognized that you can use a cardboard box with two Vee grooves cut in the right place to support the scope tube. It’s not the blocks that give the precision; it’s the fact that, other than rotating on its axis, the scope never moves..

Now, rotate the scope tube in the Vees and adjust the reticles until both lines remain centered on the dot. On a good day with some luck this takes about 45 minutes to get as close as you are going to get and the reticle will still move off the dot in a few places in its rotation, i.e. the intersection will move in and out of the dot as the scope rotates.

The second way to optically center the scope is to stand the objective lens on a mirror in a well-lit room or even outdoors in bright sunlight. Look down through the scope and what do you see? If the reticle appears blurry or doubled, adjust it until all you see is one sharp reticle. In other words, the real reticle is on top of its reflection. This way sounds easier than the first method because it is. But it doesn’t give results that are any better than the first method and maybe not as good. You see, the glass on the mirror is not parallel to the reflective surface on the back of the mirror, and you will always be off by some small amount.

mirror technique
With the scope’s objective resting on a mirror, adjust the horizontal and vertical reticles until the heavy reticles are covering the shadow lines — as best you can.

Why do we do it?

The belief is that once the scope is adjusted to its optical center it can then be mounted in an adjustable scope mount and, without changing the elevation or windage knobs, zero the scope at an ideal distance by adjusting the mount, only. You will have to use a scope mount that adjusts in both directions to do this. Having done this several times I can tell you that it is absolutely impossible to do. But, for the sake of argument, let’s assume it can be.

Once the scope is optically centered and also zeroed, you can then adjust the scope’s elevation reticle for range changes to your target. Because the scope is optically centered, the pellet will stay on the vertical reticle at all practical distances. Let me give an example.

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Field target

You have optically centered your scope and then zeroed it at 20 yards by adjusting the scope mount. You are shooting a .177-caliber pellet at 900 f.p.s. You will be on target from 20 yards to about 24 yards and then your pellet will start to rise above the center of the crosshairs. The pellet is not really rising, of course; it just appears that way to you.

From 24 yards to 28 yards the pellet rises until it is one pellet’s diameter above the center of the crosshairs but exactly in line with the vertical reticle. Starting at 29 yards until 32 yards the pellet will descend on the vertical reticle line but still be on the line, left and right. At 33 yards the pellet will start to descend below the center of the crosshair and by yard 35 it will be one pellet’s diameter below the center of the crosshairs.

dime and sight-in
This is the level of accuracy a field target competitor is seeking.

What you have just done is sight in an air rifle that is on target without any scope adjustment from 20 yards to 35 yards. You will be theoretically able to hold on the center of a kill zone on a field target within that range span and hit the paddle without touching the side of the kill zone — on all targets that have a 3/4-inch kill zone or larger.

All that I have just said is theory and it doesn’t work that way in the real world. In the real world the following is true.

1. It is impossible to optically center any scope. There will always be some slight movement of the reticle against a distant target as the scope is rotated through 360 degrees.

2. It is impossible to zero a scope with an adjustable scope mount. You can get close, but never exactly on.

Before we continue, let me define what I mean by exactly on. I mean the pellet is striking the point where the reticle lines intersect, with an equal amount of the pellet on either side of each line. This sounds anal, but the sport of field target makes shooters anal pretty quick. You see, there are kill zones that are smaller than 3/4-inches — some as small as 15 mm. When I competed in the late ’90s they were even smaller than that — down to 3/8-inch (9.44 mm).

caliper and dime
This is the size (15 mm) of the smallest field target kill zone today.

If your pellet touches the side of the kill zone while passing through it can push the target “face” backwards hard enough to lock it upright, even though the paddle has been hit. The target won’t fall and you won’t get a point. This is the reason field target shooters are so concerned with accuracy.

Once when I was competing, one of the shooters brought his friend to the match to try it. He was a SWAT sniper and was confident he would do well. I think he thought he would teach us all a lesson in how to shoot. He finished in the middle of the pack of about 20 shooters and when it was over he told us that he was trained to shoot someone in the middle of their head. He reckoned that field target shooters would aim for one particular hair on the head — which is just about the case.

Counting clicks — mechanical centering

I hope I have made my point why optically centering a scope is impossible. Later on I’m going to tell you a very practical way to get the result that people desire from centering, but right now let’s discuss mechanical centering.

When you center a scope’s adjustments mechanically you are finding the spot in both adjustment where there are an equal number of clicks in all directions. If there are 123 clicks down there have to be 123 clicks up. Same for left and right, though they may not be the same number of clicks left and right as up and down. But don’t worry — it doesn’t matter, and here’s why.

Erector tube return spring

The reticle lives in the erector tube and the lines never appear to move when adjusted. That’s because they don’t. The entire tube moves while the lines remain stationary. Yes, there are European and Russian scopes whose reticle line actually do move, but they are an exception and not a part of this discussion.

There is a spring that’s mounted on a 45 degree angle to the erector tube and across from bothe adjustment knobs. It pushes back against both the vertical and right adjustments to keep the erector tube in whatever position the adjustments have put it. To the shooter it looks like nothing has moved.

At some point this spring gets relaxed and doesn’t push as hard. Then the tube can move without being adjusted — as in when it is jostled or bumped. That is when the scope starts to lose its zero and shifts randomly. So, centering the reticle (erector tube) mechanically doesn’t make much sense. Yes there may be 123 clicks on upward adjustment but the last 63 of them may be with the spring relaxed, so they are worthless. You don’t want to adjust the elevation there. Now that the scope is mechanically centered you have a lot of good downward adjustment that is useless (because you never adjust the scope that way) with very little upward adjustment before you start experiencing scope shift.

erector tube return spring
When the erector tube return spring relaxes, the erector tube starts moving on its own from vibration. Goobye zero!

A better way

Now I will tell you what really works and what top shooters around the world have discovered. Forget optical centering. Forget mechanical centering. Instead, adjust your scope until there is very little downward adjustment left. Once you zero the scope (with the adjustable scope mount) you will never use any downward adjustment. But you will use the upward adjustment, and this procedure has left a lot of it in the scope.

Everything I just said also applies to left and right adjustment, though it is not as critical. Gravity pulls pellets down; it doesn’t move them left and right.

What about left and right?

Okay, you understand how up and down works. What about left and right? Let’s assume that when you adjust the scope up the pellet stays glued to the vertical reticle. It never moves off the vertical line. It never does, but let’s pretend for a moment that it does. If you haven’t optically centered your scope, what happens when the farther out you shoot the farther the pellet strays to the left?

Let’s also assume you are using a scope level for every shot, because none of this works if you aren’t. You notice that at 35 yards the pellet is half a diameter off to the left and at 45 yards it’s more than a full diameter off. What do you do?

What you do is check your zero at every 5-yard distance from 10 yards to 55 yards, because that is the distance at which you compete. Yes, I am aware the rules have changed and those distances are now stated in decimal fractions, but let’s keep this simple. And I said you check your zero every 5 yards and keep making small adjustments, but champions will then refine that to every yard — from 10 to 55 yards, or every meter from 10 to 50 meters.

What you do is adjust the left-right setting on your scope to get it as close to the centerline as possible at all distances. You never will get it perfect, but let’s say with careful work you get it to the place where your pellet is one diameter off to the left at 51 to 55 yards and one pellet diameter off to the right at 10 to 14 yards. It’s off by a lesser amount at the intermediate distances. Most field target competitors would be thrilled to have a scope that was that dialed-in.

Why so anal?

Do you really have to do any of this? Of course not. I don’t. You can go right on with your life, just as before. Nothing has changed. I wrote today’s report for those readers who were asking about optical centering. 

Pragmatic approach

The steps I have just given you (the last ones — the ones that really work) can take DAYS to complete! If you always want to see the bullseye get hit, watch a movie. This level of commitment to perfection is why some scopes cost more than $3,000 and some mounts cost over $500. You would be fooling yourself to think that serious competition can be done without a serious investment. Yes, you can follow these steps and do quite well on a budget, but remember — nobody races real cars in NASCAR.

My way is hard work. All the theory is out the window. As Jedi master Yoda told us, “Do, or do not. There is no try.”

Summary

In this article I have given you lots of things to do. But at their core there is one piece of invaluable advice. That is — it is impossible to do any of these things exactly. That applies to all of the procedures and desired results I have presented. You may think you are a perfectionist, but also recognize that for human beings there is no such thing.

You still should do your best to get as close as you can — so close that your hard work becomes a humorous anecdote that you can tell for many years to come, as I have just done for you!