A prism sight or prismatic sight, sometimes also called prism scope or prismatic scope, is a type of

telescopic sight A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a ''reticle'' – mounted in a focally appropriate p ...

which uses a

reflective prism An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled—elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...

for its image-erecting system, instead of the series of

relay lens In optics, a relay lens is a lens or a group of lenses that receives the image from the objective lens and relays it to the eyepiece. Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically manipulate the ligh ...

es found in traditional telescopic sights. The use of prisms makes it possible to construct a shorter and lighter sight, or with an offset between the

eyepiece An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as Optical telescope, telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks thro ...

and

objective Objective may refer to: * Objectivity, the quality of being confirmed independently of a mind. * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pron ...

axes Axes, plural of ''axe'' and of ''axis'', may refer to * ''Axes'' (album), a 2005 rock album by the British band Electrelane * a possibly still empty plot (graphics) See also * Axis (disambiguation) An axis (: axes) may refer to: Mathematics ...

, although restricting the achievable range of

magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...

Description

Prism sights are a type of

with similar features and limitations. Most prism sights use

roof prism A roof prism, also called a Dachkanten prism or Dach prism (from German: ''Dachkante'', lit. "roof edge"), is a reflective prism containing a section where two faces meet at a 90° angle, resembling the roof of a building and thus the name. Re ...

s commonly seen in compact

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

and

spotting scope A spotting scope is a compact lightweight portable telescope optimized for detailed observation of distant objects. They are used as tripod mounted optical enhancement devices for various outdoor activities such as birdwatching, skygazing and ...

s. The

reticle A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...

is etched onto one of the internal reflective surface of the prism, making it easy to illuminate the reticle from the back of the prism even when active illumination is turned off.

Magnification

Prism sights often have fixed low-power

such as 1× (parity magnification or "non-magnifying"), 2×, 3× or 4x magnifications, sometimes 5× or more. Although the low magnification range means not the most visual details when aiming and observing targets, it allows larger

fields of view The field of view (FOV) is the angular extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to electromagnetic radiation. It ...

, making them best suited for shooting at short or medium engagement distances. There are prism sight designs with variable magnification, such as the ELCAN Specter DR/TR series.

Reticle

Like telescopic sights, prism sights have an etched

which makes them suitable for shooters with

astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors ...

, unlike

reflex sight A reflector sight or reflex sight is an optical sight that allows the user to look through a partially reflecting glass element and see an aiming point or some image (helping to aim the device, to which the sight is attached, on the target) sup ...

s or

holographic sight A holographic weapon sight or holographic diffraction sight is a non- magnifying gunsight that allows the user to look through a glass optical window and see a holographic reticle image superimposed at a distance on the field of view. The hologram ...

s which may be near useless for these shooters depending on the severity of the astigmatism. The reticle is often illuminated, and is visible without illumination. Etched reticles make for the possibility to have more advanced reticles. Some prism sights are available with ballistic reticles which can be used for holdover for shooting at varying distances without turning the adjustment knobs.

Diopter

Prism sights usually have

s with adjustable

diopters A dioptre ( British spelling) or (American spelling), symbol dpt or D, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, . It is normally used to express the optical power of a lens or curved mi ...

so that the image can be adjusted for

nearsightedness Myopia, also known as near-sightedness and short-sightedness, is an eye condition where light from distant objects focuses in front of, instead of on, the retina. As a result, distant objects appear blurry, while close objects appear normal. ...

farsightedness Far-sightedness, also known as long-sightedness, hypermetropia, and hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blur is due to incoming light being focused behind, instead o ...

Parallax

Prism sights have a somewhat larger parallax error than red dot sights. Usually, they do not have an adjustable

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

like some telescopic sights.

Eye relief

Another disadvantage is eye distance. Like telescopic sights, the eye must be within a certain distance from the scope.

History

Prisms have been used in

since the 1890s. During

World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

, the

US Army The United States Army (USA) is the primary land service branch of the United States Department of Defense. It is designated as the Army of the United States in the United States Constitution.Article II, section 2, clause 1 of the United Stat ...

chose to equip the

M1903 Springfield The M1903 Springfield, officially the U.S. Rifle, Caliber .30, M1903, is an American five-round magazine-fed, bolt-action service repeating rifle, used primarily during the first half of the 20th century. The M1903 was first used in combat ...

repeating rifle A repeating rifle is a single-barreled rifle capable of repeated discharges between each ammunition reload. This is typically achieved by having multiple cartridges stored in a magazine (within or attached to the rifle) and then fed individually ...

for the

sniper rifle A sniper rifle is a high-precision, long range shooting, long-range rifle. Requirements include high accuracy, reliability, mobility, concealment, and optics, for anti-personnel weapon, anti-personnel, anti-materiel rifle, anti-materiel and sur ...

role by mounting a sight similar to half a binocular, a prismatic sight developed by the

Warner & Swasey Company The Warner & Swasey Company was an American manufacturer of machine tools, Measuring instrument, instruments, and special machinery. It operated as an independent business firm, based in Cleveland, from its founding in 1880 until its acquisiti ...

. It was a short and compact sight, and the prisms allowed the objective to be angled to the side so that the 1903 rifle could be loaded with a clip from above. It was still possible to use the iron sights with the scope mounted. The M1908 version of the scope had 6-times magnification, while the later M1913 version had slightly less magnification at 5.2×. The background for choosing a lower magnification was a larger

field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...

and improved light transmission. The sight was constructed of steel and brass, was painted black inside for optical performance, and had a relatively high mass of . The

eye relief {{unref, date=June 2023 The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece within which the user's eye can obtain the full viewing angle. If a viewer's eye ...

was only , so the sight was equipped with a rubber

shield to prevent the scope from hitting the face during

recoil Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, for according to Newton's third law the force requ ...

. The

was a thin crosshair, as was common for this time period. These sights were fitted to the

Hotchkiss M1909 Benét–Mercié machine gun Hotchkiss may refer to: Places Canada * Hotchkiss, Alberta * Hotchkiss, Calgary United States * Hotchkiss, Colorado * Hotchkiss, Virginia * Hotchkiss, West Virginia Business and industry * Automobiles Hotchkiss, a French automobile manufact ...

. Canadian Forces ordered 500 samples of the 5.2× variant. Moisture on the inside of the lenses was one of several problems in the field, and the scope was known to loosen. In total, approximately 8,000 units of these prismatic sights were produced. The scope sight had the greatest magnification of the First World War, but in return only had a 4.5 degree field of view. A well-known later example is the fixed-magnification Trijicon ACOG which has been used in combat by the

US Marine Corps The United States Marine Corps (USMC), also referred to as the United States Marines or simply the Marines, is the Marines, maritime land force service branch of the United States Department of Defense. It is responsible for conducting expedi ...

and

USSOCOM The United States Special Operations Command (USSOCOM or SOCOM) is the unified combatant command charged with overseeing the various special operations component commands of the United States Army Special Operations Command, Army, United Stat ...

.Low Power Variable Optic vs. Prism Scope for Your Budget AR-15
/ref> In the 2010s, several manufacturers including Trijicon, and Vortex began offering prism sights on the consumer market. In October 2019, the Chinese

QBZ-191 The QBZ-191 Automatic Rifle () is a Chinese assault rifle chambered for the 5.8×42mm intermediate cartridge, designed and manufactured by Chongqing Jianshe as a new-generation service rifle for the People's Liberation Army (PLA) and People's A ...

automatic rifle was revealed at the 70th Anniversary National Day military parade. It has a 3× daylight prismatic sight called QMK152 and QMK-171A.

References

{{Firearm accessories mounting standards Firearm sights