The parallax is the difference in the direction (angle) of the line of sight when observing the same target from two points in different positions, and this difference will be obvious when compared to other background objects. To put it simply, objects that are closer to the observation point will have greater visual position changes when the observation position changes than objects farther away (closer to the background). The main reason for the parallax phenomenon is that the vertical distance between the target object and the reference object to the observation point position change track is different, and the two are not in the same visual plane.

Applied to the scope, the above-mentioned target object is the target image (target image) produced by the visible light from the observation target after passing through the objective lens. The reference object is the marking line used for aiming, and the observation point is the human eye. The sight picture finally observed by the human eye is the overlap of the target image and the marking image. If the focus of the target image does not coincide with the position of the marking line in the scope, the images produced by the two will be on different focal planes, not only the definition of the target image will be different from the marking line, but also Will make the slight changes in the position of the human eye will cause the marking line to appear to float around on the target, which is called parallax shift (parallax shift). In this case, unless the shooter can ensure that the eyes are kept at the same position directly behind the center of the aiming pattern (nearly impossible) every time the shooter is aiming, the accuracy must be greatly affected, because the visual reference is not consistent every time.

In modern scope design, many internal optical components are movable and can be manually adjusted through the external dial to change the line of sight ("elevation" represents the vertical axis, "wind deviation" represents the horizontal axis), magnification, and The focus of the marking and the target image.

Because the position where the marking line is installed in the scope is usually the same, if you want to adjust the parallax, you need to change the focus of the target image to cater to the marking line. Low-end scopes usually only have the function of adjusting the line of sight, magnification and the focus of the markings. They cannot move the focus of the target image to cater to the markings to neutralize the parallax. Therefore, these scopes are usually designed with a parallax. A distance of zero (the most common is 100 yards) is used as the best shooting distance.

Some high-end scopes have the function of adjusting the parallax. There are usually two ways to change the focal length of the target image:
·Adjust the focus of the target image by changing the front and back positions of the objective lens assembly, which is called adjustable objective (AO).

·Adjust the focus of the target image by changing the front and rear positions of the erector lens assembly, because the adjustment wheel is usually installed on the left side of the lens body, so it is called side focus (SF) .

In addition to the above two, there is also a very rare rear focus (RF) design, which can only be seen on a fixed magnification scope, because the fixed magnification scope does not have a magnifying wheel, which happens to Leave a place to install the parallax adjustment dial. The detailed information on the principle of this design is hard to find, but I guess that the marking line can be moved behind to cater to the target image (unlike the other two, the target image is moved to cater to the fixed marking line). The rear focus type design is not as convenient as the side focus type, but it is stronger and more reliable than the adjustable objective lens type.

The principle of mobilizing the focus of the target image and the marking line to make the two overlap to eliminate the parallax effect is complicated, but it is actually very simple-imagine if you can virtually pull the target directly over the marking line (or Project the marking line directly to the target), so that the visual distance difference between the marking line and the target becomes zero, and the two become one, just as if you directly aim the crosshair on the target, then it will not appear problem of parallax drift.

The closer you shoot, the greater the corresponding change in the angle of the human eye when the position of the human eye changes, and the greater the impact of the resulting parallax on the aiming consistency. Therefore, when choosing a scope for guns with short range applications, such as air guns and flanged primer rifles, the ability to adjust the parallax is a functional setting that needs to be prioritized. Basically shooting within 100 meters, unless you don’t care about accuracy at all (the shooting target is just a big hit, it doesn’t matter where you hit it), the parallax adjustment function is necessary.