The first thing to be clear is what you plan to install the scope on when you buy it. Different firearms have different interfaces for installing accessories, use different ammunition, produce different recoil forces, and have different aiming equipment that can be installed on them. For example, the scope used on a rifle and the scope used on a pistol cannot be the same in terms of eye distance, because the pistol does not stick its face to the butt like a rifle when shooting , but will keep a certain distance from the human eye, so it is necessary to have a long suitable eye distance. For the same reason, the scope used on the scout rifle is usually installed in the front part, so it must also have a longer eye distance. Long eye relief (LER) scopes usually do not have high magnification.
The so-called eye relief (eye relief) is the best distance (between the human eye and the eyepiece) that can provide the human eye with the largest field of view without vignetting, that is, the most suitable for the comfortable observation of the human eye distance. Any distance greater than or less than this optimal distance will produce vignetting and negatively affect the quality of the observation. Vignettes are caused by the fact that the human pupil only allows a limited amount of light to enter the eye, and if the light from the target is partially blocked by the iris, not only will an image partially covered by a dark shadow be projected on the retina, it may also be distorted Interfere with image light that has entered the pupil. At eye distance, the possibility of vignetting is minimal.
In addition to the appropriate eye distance, the exit pupil diameter is also a relatively important optical parameter that is often overlooked and often confused with the appropriate eye distance. The exit pupil diameter is the diameter width of the aperture (exit pupil) projected by the eyepiece on the eye distance, and is a length value perpendicular to the eye distance. In the case of sufficient light (such as sunlight and Hitachi's broad daylight), the diameter of the human pupil is usually about 2 mm. As long as the aperture diameter of the eyepiece at the eye distance is greater than 2 mm, it is basically guaranteed that even the human eye vibrate, there is no vignetting ; but if the brightness of the light drops (under low light conditions), the pupil of the person will dilate, and if the diameter of the eyepiece aperture is smaller than the diameter of the dilated pupil, vignetting will also occur at the eye distance . Therefore, when purchasing a scope, try to consider choosing a slightly larger exit pupil diameter.