Rear sight

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Astrolabe with alidade for aiming at the heavenly bodies
Diopter of a competition air rifle
View through the rotating rear sight of a HK MP5

A diopter (from Greek dia “through”, optein “see”) is a sighting device for aiming at targets with the naked eye. In astronomical instruments it is also called Absehe . There are always two simple optical elements in the form of small holes, less than 3-4 mm in size, or slots in metal plates, one behind the other in the line of sight. If they are arranged on a rotating rail, one speaks of an alidade .

The view through a small opening has a sharpening effect like the view through farsighted glasses; the smaller the hole, the sharper, but also the fainter the object appears ; (see also camera obscura ).

The simple sighting with diopter was already used in ancient times for geodetic measurements. In the Middle Ages at the latest , it was used with the astrolabe , the alidade of which is used to locate the heavenly bodies. Other instruments that are used in a comparable way are the diopter ruler , dioptra , sighting disc and mire . On ship compasses attached diopter allow bearings in conjunction with a nautical chart . The same applies to simple compasses for orientation in the country.

Diopter on rifles

The simplest sighting device on the rifle consists of a rear sight and a front sight. The only restriction here is the replacement of the rear sight with a pinhole. The rear sight is the rear sighting device on the weapon, it consists of a height- and side-adjustable plate with a very small viewing opening in the middle, through which the view of the front sighting device is released. This can be a pin ("pen front sight") or a disc with an opening (" ring front sight") . The front sight offers the advantage of not obscuring the target or parts of it. The eye must be very precisely centered behind the hole in the rear sight so that the front sight is visible and can be aimed at the target.

various ring and pin front sights for sport rifles

With a rear sight and a front sight, often arranged in a short tube (“front sight”), a very simple, centrally symmetrical target image is achieved. This type of sight is easier to use than the rear sight and front sight and is usually more accurate. This reduces the angle error when aiming. A variable iris diaphragm is often built into the rear sight instead of a fixed hole . This means that the diopter can be adapted to different lighting conditions.

In most sport shooting disciplines , a magnification in the rear and front sights of competition weapons must not be used (shooters of the age group are often allowed to use a so-called eagle eye in the front sight or a diopter compensation ).


  • Diopter. In: Brockhaus' Kleines Konversations-Lexikon, 5th ed., Volume 1. Leipzig 1911., p. 438. Diopter at .

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