by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1
Part 2

This series was started for blog reader David Enoch; but after reading the comments many of you have written, I have to think it’s for most of you. Today, we’ll look at optically centering a scope — what’s involved and why you’d want to do it.

What is optical centering?
Optical centering means adjusting the scope until the center of the crosshairs is actually in the center of the field of view. This is difficult to understand; because when you look though a scope, the crosshairs always look like they’re centered. That’s because they’re permanently fixed in the center of a tube called the erector tube. It’s this tube that gets moved when the scope knobs are adjusted.

Not all scopes work this way, I’m aware, but the majority of modern scopes do; so let’s not get into discussions of German scopes and Russian scopes whose reticles actually do move. They’re sufficiently uncommon that there’s no need to confuse the average shooter with their differences.

An optically centered scope is one whose crosshairs remain fixed on a target as the scope is rotated 360 degrees on its axis. I’ve never seen a scope that was perfectly centered, and I doubt one exists. The closest I’ve seen was a scope whose reticle moved about one-eighth inch when rotated 360 degrees while focused on a target 20 yards away. Most scopes can get only to within three-eighths of an inch under those circumstances.

crosshair shift when scope is rotated
When the scope tube is rotated, the intersection of the crosshairs moves against a distant target. The object of optical centering is to get the movement as small as possible.

Why optically center your scope?
This practice started and died with field target. Shooters discovered that if their scopes were not parallel with the axis of the rifle’s bore, not only would the pellet impact rise and fall as the elevation knob was adjusted for different targets — it would also move from one side to the other — typically from right to left, though not always. That’s because the scope was right on at the sight-in distance, but off to one side when the scope was adjusted closer and to the other side when it was adjusted farther.

scope alignment problems This drawing of a top-down view of a scoped air rifle is greatly exaggerated, but it shows how a scope may not be aligned with the axis of the bore.

 

group movement over distance
When the scope isn’t aligned with the bore, this is how the rifle can shoot. You can adjust the vertical reticle for elevation to get all the groups level with the target, but they’ll still land to either side if the scope isn’t aligned.

I was writing The Airgun Letter (1994-2002) when I competed in field target. Although I started out using springers and the holdover method of sighting, I switched to PCPs, which gave me a better chance to compete. I also started adjusting the scope’s elevation for every change in distance. That was when I discovered optical centering.

The way to optically center a scope in those days was to put it on a solid rest that did not move but allowed the scope to rotate around its axis (in this case that means the scope tube) 360 degrees. Then sight at a target at some distance and watch the center of the reticle move against the target.

optical centering fixture
I started with actual machined Vee blocks, until I realized that precision isn’t required to optically center a scope. A cardboard box with 2 Vee notches works just as well.

As you rotated the scope tube, you adjusted both the vertical and horizontal reticles until the center of the crosshairs appeared to move as little as possible against the target. I used graph paper with a quarter-inch grid and a tiny black dot aim point that was about half the size of a pencil eraser.

Adjusting the reticle was not straightforward. If often took longer than an hour to get the reticle moving as little as possible against the aim point. And you never got it perfect. There was always some perceptible movement as the scope tube rotated.

Avoid this trap!
Some people would read about optical centering, then go to the range with thoughts of performing it at twice the distance to get even greater precision. It never worked because at 40 yards you can’t see the movement of the crosshairs shifting by one-sixteenth of an inch against a target.

Others were simply never satisfied with the results they got from optical centering. They knew their scopes were not perfect, and they couldn’t live with that. So, they kept swapping scopes and returning to the range again and again, searching in vain for the scope whose crosshairs could be adjusted to remain centered when the tube was rotated.

In the end, those who’d been proponents of optical centering realized they were chasing their tails. Perfection was impossible and there were other easier things that could be done that would deliver the same results. Mounting the scope in line with the bore is just as successful as optically centering it.

Why did optical centering die?
Many shooters are still not aware that they don’t need to optically center their scopes, so it hasn’t really died…but most field target competitors — at least the ones that win — don’t do it anymore. Instead, they take great pains to align the scope with the axis of the bore so centering becomes a non-issue.

If your scope is not optically centered but the scope is aligned, you can correct any misalignment of the reticle during the sight-in. You aren’t fighting the angles of the line of sight and axis of the bore. So, extra time spent mounting the scope pays off in not needing to go through this cumbersome procedure. The results are the same either way. As you adjust the vertical reticle, the shot group remains centered at all practical distances.

Big Shot of the Month
Pyramyd Air’s Big Shot of the Month on Facebook is Roberto Martinez. He’ll receive a $100 gift card. Congratulations!

Pyramyd Air Big Shot of the Month

Roberto Martinez is the Big Shot of the Month on Pyramyd Air’s airgun facebook page.