Subtleties of optics

Parallax is a phenomenon found when observing the surrounding space, which consists in a visible change in the position of some fixed objects relative to others located at different distances from each other, when the observer's eye moves. We meet the parallax phenomenon at every step. For example, looking out of the window of a moving train carriage, we notice that the landscape, as it were, revolves around a distant center in the direction opposite to the movement of the train. Close objects leave the field of view faster than distant ones, and therefore the impression of a rotation of the landscape is created. If the objects lie in the same plane, then the parallax will disappear, there will be no different movements of objects relative to each other when the eye moves. Parallax in sights is the misalignment of the plane of the target image formed by the lens with the plane of the reticle of the sight.


Tilting the reticle causes parallax at the edges of the field of view. This is called oblique parallax. The absence in the sight of a flat image of the target over the entire field of view, due to poor manufacturing of lenses and assembly of the sight, or with significant aberrations of the optical system, causes "irreparable parallax".


Typically, the sight is made in such a way that the image of a target at a distance of 100-200 m is projected by the lens into the plane where the reticle is located. In this case, the parallax range is, as it were, halved between far and near targets. When the target approaches the shooter, its image also shifts closer to the shooter (in the optical system, the target and its image move in the same direction). Thus, in the general case, the sight is characterized by a discrepancy between the image of the target and the reticle. When the eye is displaced perpendicular to the axis of the sight, the target image moves in most cases in the same direction relative to the center of the reticle. The target, as it were, "slides away" from the aiming point, when tilting, shaking the head, "rushes" around the aiming point. In addition, the reticle and the target are not sharply visible at the same time, which impairs the comfort of aiming and minimizes the main advantage of a telescopic sight over a conventional one. Because of this, a sight without focusing on the shooting distance (without a parallax elimination device) allows a high-precision shot to be carried out only at one specific distance.


A high-quality sight with a magnification greater than 4x must have a device to eliminate parallax. Without this, it is quite difficult to find and keep the eye in the desired position, on the line connecting the reticle and the point on the target, the reticle is generally not in the center of the field of view. A slight movement of the reticle along with the target image can be detected when shaking the head, especially when the eye is displaced from the calculated position of the exit pupil, which is explained by the presence of distortion in the sight eyepiece. This can be eliminated only in sights with a parabolic lens in the eyepiece.


Focusing the sight is the operation of setting the image given by the lens to a given plane - the plane of the reticle. By calculation, the relationship between the longitudinal shift of the focusing lens and the value of the image shift is determined. Typically, scopes move either the entire lens or its internal component located near the reticle. A scale is applied to the scope of the objective lens, indicating the focusing distance in meters. By moving the lens to the desired division (shooting distance), you eliminate parallax. A sight containing a focusing device is undoubtedly a more upscale and complex product, since a moving lens must maintain its position in space relative to its own axis, that is, keep the line of sight unchanged. This centering of the focusing component of the lens relative to the geometric axis of the lens tube is achieved by maintaining tight tolerances in the manufacture of the focusing component.

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