Novice rifle shooters are generally familiar with the “focus knob” situated on the side, or sometimes set as a ring on the objective, of the scope. You use it to ensure the target is in focus, right? Maybe not. Many shooters are not familiar with its purpose and importance.
What many shooters consider as the focus of the scope is actually much different. Yes, it gets your target in focus, but the purpose of that knob is much greater than that. Most modern scopes have these adjustments, usually marked in yards. If not adjusted properly the target will often appear out of focus. But is this simply a focusing adjustment, similar to that found on a telescope or set of binoculars? Not quite…
To truly understand the role of this adjustment we need to get down to some of the nitty gritty aspects of the modern optic. We need to understand parallax.
Parallax is, generally speaking, the error that occurs when a slight misalignment of the eye causes the reticle to appear somewhere other than where it is actually aiming. The crosshairs in a modern scope are intended to appear to the shooter to lie directly on top of the target. When they do not, you have what we refer to as a parallax error.
It’s an optical illusion of sorts. Imagine a line from your eye, through the scope and crosshairs to the target. If the crosshairs appear to be anywhere along that line other than the target, you can have a parallax error. That is, a slight movement of the shooter’s head can cause the reticle to appear somewhere other than where it actually is.
Take a look at the image below.
This is a prime example of parallax error. Because of an incorrect adjustment of the parallax knob or ring on the scope, the reticle appears to be between the shooter and the target. If the shooter were to lose cheek weld, or adjust their position in any way, the reticle would move and appear to be in a place other than where the firearm is aimed.
A simple example of parallax error can be found on any iron-sighted gun. Move your head up, down, left, or right, and the front sight will no longer be in the correct position of the rear sight. The same phenomena can occur on a gun equipped with modern optics, but it is significantly more difficult to realize.
Imagine if you will, riding on a train. As you gaze out the window, you notice that the trees on the landscape nearest you seem to fly by at a high rate of speed. The trees farther out seem to move much slower. The mountains on the horizon seem to barely move at all, and the bright sun seems fixed in position. This is a prime example of parallax. The objects in the distance move at a different rate than the objects close to the observer, even though the rate of speed is the same and unchanging. When using a firearm with an optic that has the reticle set to appear at a distance other than the range of the target, any slight movement of the shooter’s eye can cause the apparent zero of the optic to change.
The parallax adjustment, or focus adjustment as some call it, actually changes the position of the reticle along the line depicted above. When you look through a scope and see the target out of focus, it is an encouragement to adjust the parallax knob until the target appears crisp and clear. By adjusting the image back into focus, what you are actually doing is moving the reticle forward or backward along your plane of vision until, when everything appears in focus, it is optically at the same distance as your target.
What you’re doing when you adjust the parallax or focus knob is changing the position that the reticle appears at. It is, as stated above, an optical illusion. In a sense, you are changing the range of the scope. When set properly, the crosshairs appear at the exact same distance the target is at so there is little to no parallax error possible.
It’s a common misconception that holographic and reflex sights are parallax free. This is not quite the case. Red dot and holographic reflex sights usually have the reticle set to infinity. This means they have less error, usually, and it is mostly limited to the sight height. Still, there is some parallax, and it is more pronounced at close range. Like the mountains in our example with the train, the reticle of a reflex scope doesn’t seem to move much, but there is still a bit of movement—even if it is small.
Some scopes come without parallax adjustment because they are intended to be used in a specific range of distances. For example, scopes intended for use on .22 LR rifles for example rarely have a parallax adjustment simply due to the fact they are intended for use solely at ranges from 25 to 100 yards. The focus is set at the factory so that the reticle appears to hover at the ideal distance. Other scopes, such as handgun scopes, are preset in the same manner.
In the end, there are only two ways to completely eliminate parallax errors. The first, keeping your eye precisely in-line with the scope is virtually impossible. The second is to have your range dialed in and your parallax adjustment knob properly set so the crosshairs appear at the same range as your target.
Once you have the reticle literally overlaid onto your target, there is no possible way to have any parallax error. Dial your range in accurately, set your parallax adjustment to match, and then—and only then—let fly with your shot. Once again, know the range to your target, set your scope parallax adjustment accordingly, and you’ll easily be in the X-ring.
Daniel Scott is a long-time firearms enthusiast, hunter, collector, and has worked at various times as a Firearm Expert, Hunting Guide, as well as an Executive Protection Officer (bodyguard). He has been a regular columnist at Western Shooting Journal and has also been published in American Shooting Journal, and GunUp the Magazine as well as numerous places online including AmongTheLeaves.com where he blogs regularly. Daniel makes his home in Fort Worth with his wife and four dogs.