When purchasing a scope, it is necessary to first clarify which firearm it will be installed on. Different firearms have differences in mounting accessories, ammunition use, recoil, and the types of sighting devices that can be installed. For example, the eye relief of scopes for rifles and handguns cannot be the same, because handguns are not held with the face pressed against the stock like rifles, but are held at a certain distance from the eye, so they require a longer eye relief. For the same reason, scopes for scout rifles are usually mounted at the front and must also have a longer eye relief. Long eye relief (LER) scopes usually do not have high magnification.
Eye relief refers to the optimal distance between the eye and the eyepiece that provides the maximum field of view without dark edges. Any distance greater or less than this optimal distance will result in dark edges and negatively affect the quality of observation. Dark edges are caused by the pupil only allowing a limited amount of light to enter the eye. If the light from the target is partially obstructed by the iris, not only will a partially shadowed image be projected onto the retina, but it may also interfere with the imaging light entering the pupil. At the eye relief distance, the possibility of dark edges is minimized.
In addition to the appropriate eye relief, the exit pupil diameter is also a relatively important but often overlooked and easily confused optical parameter. The exit pupil is the diameter of the aperture (exit pupil) projected by the eyepiece at the eye relief distance, and is a length value perpendicular to the eye relief. In well-lit conditions (such as sunlight and daylight), the human pupil diameter is usually about 2mm. As long as the aperture diameter of the eyepiece at the eye relief distance is greater than 2mm, dark edges will basically not occur even if the eye has tremors; but if the light becomes dim (low light conditions), the pupil will dilate, and if the eyepiece aperture is smaller than the dilated pupil diameter, dark edges will also appear at the eye relief distance. Therefore, when purchasing a scope, it is best to choose one with a slightly larger exit pupil diameter.
Another factor to consider is the impact of the firearm’s recoil on the scope. Scopes with different design specifications have different abilities to withstand severe vibration. Scopes specifically designed for rimfire rifles can usually only withstand low-intensity recoil. If used on a large-caliber rifle, the vibration may cause the internal components to loosen or even become misaligned – fortunately, most products on the market can at least withstand the recoil of a .308. In fact, the most stringent test of a scope’s manufacturing quality is air rifles, because the springs in air rifles produce two opposing recoil forces instantaneously during firing. If the scope used does not meet the specifications for air rifles, even very expensive high-end products will have their internal components damaged and become unusable after a few uses.
At the same time, the different ammunition specifications used in the firearm will also affect the choice of scope.