by Tom Gaylord, a.k.a. B.B. Pelletier
Parallax is an optical term describing how the point of view affects what the viewer sees. The driver of a car may see his speedometer needle at 60 m.p.h, while a passenger to his right may see it hovering just above 57 m.p.h. In the UK, the passenger is on the driver’s left and the speedo needle will appear to be over the 63 mp.h. mark. The needle hasn’t moved in either case, but the observer’s viewpoint has moved.
And so it is with a scope. You look through it and see the crosshairs exactly in the center of the bullseye; but if you move your head on the stock, the crosshairs will also appear to move slightly. So, where you hold your head relative to the scope determines where the scope appears to be “looking.”
Many scopes today have a parallax adjustment. Some scope manufacturers call this an adjustable objective, or simply AO; but that just stems from the fact that it is the objective bell that’s turned to correct for parallax. On other scopes, this adjustment is a knob on the left side of the adjustment turret.
The adjustable objective is an objective bell that turns to remove as much parallax as possible. The scale on the bell indicates the yards to the target that have been focused.
On scopes with a sidewheel parallax adjustment, a knob on the left side of the turret adjusts for parallax. This is much easier to reach than the objective bell when holding a rifle.
The parallax adjustment adjusts the scope lenses so the least amount of parallax exists at that distance (the distance to the target). To the shooter, it looks like the scope is focusing on the target. But here’s the important point: No amount of parallax correction is ever enough. There will always be some parallax in the scope, no matter how well it’s been adjusted. Where you hold your head on the stock is very important, whether or not your scope has a parallax adjustment.
Range of scope adjustability
Every modern scope comes with vertical and horizontal adjustments, so the crosshairs may be adjusted to the point of pellet impact. Open sights often have these same adjustments. But there’s one big difference between an open sight’s adjustments and those of a scope. As an open sight is adjusted, the sight is moved mechanically. Usually just the rear sight moves, but sometimes the front sight moves, as well.
When a modern scope is adjusted, you cannot detect any movement. It’s inside the scope but can’t be seen from the outside.
What gets moved isn’t the scope tube, but a smaller tube inside the outer tube. This inner tube is called the erector tube, and it contains the reticle and other things. The wires or lines of most reticles do not move when adjustments are made. Instead, the entire erector tube moves, carrying the reticle lines with it. Since the lines are fixed, they always appear to be centered when you look through the scope.
There are some scopes that do have moving reticle lines. These are older technology scopes and are often from Germany or Russia. But these are so infrequently encountered on the modern market that they aren’t worth discussing.
I’ve mentioned the term modern in relation to the scope of today. Fifty years ago, there were scopes that did not contain erector tubes. Instead, these scopes were adjusted from the outside, so their entire tubes moved whenever adjustment were made. The adjustments were in the scope rings.
This vintage Unertl scope is held in spring-loaded rings that adjust the whole scope like the erector tube in a modern scope. The spring is inside the button at the 4 o’clock position and works for both the vertical and horizontal adjustments.
The section of the modern scope that contains the adjustment knobs is called the turret.
By looking at vintage adjustable scope rings, we can see how the erector tubes inside modern scopes move when they’re adjusted. And this is what is important. The adjustments work against a coiled steel return spring that pushes the scope back when the adjustment screws are backed off.
When the scope is adjusted higher or to the right, the erector tube springs expand and lose their tension. At some point, which differs from scope to scope, these springs relax. That’s when the scope no longer holds a zero and won’t adjust in that direction anymore.
This scope’s elevation knob is adjusted as high as it will go. The erector tube spring is fully relaxed, and the scope will be free to shift. It would be better not to adjust this scope above the horizontal No. 3 line.
Quality scopes have more adjustment range than cheaper scopes, but all scopes have difficulty adjusting out to their upper and right limits. As a general rule, I tell shooters they should never adjust beyond the three-quarter point in the high or right directions.
People sometimes ask if there’s a problem when adjusting the other way (down or to the left), and I tell them there isn’t. All you do when you adjust all the way down or to the left is compress a coil spring until it’s coil-bound. That doesn’t damage the scope, nor does it affect accuracy in any way.
Scope sights are wonderful tools that can enhance your shooting experience. They’re not magic, however. They operate by rules, just like anything else. Learn how they work, and they’ll do their part to make your shooting more successful.