A telescopic sight, commonly called a scope, is a device used to give additional accuracy using a point of aim for firearms, airguns and crossbows. Other sighting systems are iron sights, red dot sights, and laser sights.
This article will concentrate on firearm scopes; the principles described are equally applicable to any device which needs aiming. For the sake of brevity, the term gun will be used to indicate any device aimed by a telescopic sight.
The first experiments directed to give shooters optical aiming aids go back to the early 17th century. For centuries different optical aiming aids and primitive predecessors of telescopic sights were created that had practical or performance limitations. The first practical refractor telescope based telescopic sight was built in 1880 by August Fiedler (Stronsdorf, Austria), forestry commissioner of Prince Reuss.
Telescopic sights are classified in terms of the optical magnification and the objective lens diameter, e.g. 10×50. This would denote 10 times magnification with a 50 mm objective lens. In general terms, larger objective lens diameters are better (collect more light and give a wider field of view). On fixed magnification sights the magnification power and objective diameter should be chosen on the basis of the intended use.
There are also telescopic sights with variable magnification. The magnification can be varied by manually operating a zoom mechanism. Variable sights offer more flexibility regarding shooting at varying ranges and targets and offer a relative wide field of view at lower magnification settings. The syntax for variable sights is the following: minimal magnification – maximum magnification × objective lens, for example, 3–9×40.
Confusingly, some older telescopic sights, mainly of German or other European manufacture, have a different classification where the second part of the designation refers to 'light gathering power', therefore in these cases a 4×81 (4× magnification) sight would be presumed to have a brighter sight picture than a 2.5×70 (2.5× magnification), but the objective lens diameter would not bear any direct relation to picture brightness, as brightness is affected also by the magnification factor. Typically objective lenses on early sights are smaller than modern sights, in these examples the 4×81 would have an objective approximately 32mm diameter and the 2.5×70 might be approximately 25mm.
Telescopic sights come with a variety of different reticles, ranging from the traditional crosshairs to complex reticles designed to allow the shooter to estimate accurately the range to a target, to compensate for the bullet drop, and to compensate for the windage required due to crosswinds. A user can estimate the range to objects of known size, the size of objects at known distances, and even roughly compensate for both bullet drop and wind drifts at known ranges with a reticle-equipped scope.
For example, with a typical Leupold brand duplex 16 MOA reticle (of a type as shown in image B) on a fixed power scope, the distance from post to post (that is, between the heavy lines of the reticle spanning the center of the scope picture) is approximately 32 inches (81.3 cm) at 200 yards (183 m), or, equivalently, approximately 16 inches (40.65 cm) from the center to any post at 200 yards. With a known target of a diameter of 16 inches that fills just half of the total distance post-to-post distance, filling from scope center to post, the distance to target is approximately 200 yards (183 m). With a known target of a diameter of 16 inches that fills the entire sight picture from post to post, the range is approximately 100 yards. Other ranges can be similarly estimated accurately in an analog fashion for known target sizes through proportionality calculations. Holdover, for estimating vertical point of aim offset required for bullet drop compensation on level terrain, and horizontal windage offset (for estimating side to side point of aim offsets required for wind effect corrections) can similarly be compensated for through using approximations based on the wind speed (from observing flags or other objects) by a trained user through using the reticle marks. The less-commonly used holdunder, used for shooting on sloping terrain, can even be estimated by an appropriately-skilled user with a reticle-equipped scope, once the slope of the terrain and the slant range to target are both known.
There are two main types of reticles:
Wire reticles are the oldest type of reticles and are made out of metal wire. They are mounted in an optically appropriate position in the telescopic sight's tube. Etched reticles are images of the desired reticle layout that are etched on an optic element. This optical element (lens) with the etched reticle is then mounted in the telescopic sights tube as an integrated part of the optics chain of the sight. When backlit through the ocular a wire reticle will reflect incoming light and not present a black contrasty reticle. An etched reticle will stay black if backlit. Etched reticles are by most considered to be a more refined solution and offer greater reticle lay out flexibility. Because of this some manufacturers can provide client designed custom reticles on special order. In the more expensive and high end contemporary telescopic sights etched reticles dominate the market. In cheaper telescopic sights wire reticles are still often mounted to avoid a rather specialized and costly production step.
Modern military and law enforcement reticles are generally designed for (stadiametric) rangefinding purposes. Perhaps the most flexible ranging reticle is the "Mil-dot" reticle, which consists of duplex crosshairs with small dots at milliradian (Mil) intervals in the field of view. A milliradian equates to 3.43774677078493 MOA, that is, approximately 21.6 inches at 600 yards; each MOA equates to 1.0471975511966 inch at 100 yards, often rounded to 1 inch at 100 yards for fast mental calculations.
Users who use the metric system are better off with a Mil-dot reticle, since they do not have to hassle with the unnecessary complications of a non metric system of measurement during mental calculations. Also the Mil-dot measurements and ranging calculations are always exact in the metric system.
A trained user can relatively accurately measure the range to objects of known size, the size of objects at known distances, and compensate for both bullet drop and wind drifts at known ranges with a Mil-dot reticle-equipped scope.
This is what a Netherlands Army sniper sees through his Schmidt & Bender 3-12x50 PM II telescopic sight. The Mil-dots can be seen on the cross hairs. By means of a mathematical formula - (size of target / number of mil of dots) x 1000 = distance - the user can measure the range to a target. An object of 1 meter tall or wide is exactly 1 Mil tall or wide at 1000 meters distance. If the user sees an object of 1.8 m tall for example as three mil dots tall through the riflescope the object is at 600 m distance - (1.8 / 3) * 1000 = 600.
The four horizontal bars over the horizontal line are also intended for (quick) ranging purposes.
Variable power telescopic sights with front focal plane reticles have no problems with point of impact shifts. Variable power telescopic sights with rear focal plane reticles can have slight point of impact shifts through their magnification range caused by the positioning of the reticle in the mechanical zoom mechanism in the rear part of the telescopic sight. Normally these impact shifts are insignificant but make accuracy oriented users, that wish to use their telescopic sight trouble-free at several magnification levels, often opt for front focal plane reticles. Around the year 2005 Zeiss was the first high end European telescopic sight manufacturer who brought out variable magnification military grade telescopic sight models with rear focal plane mounted reticles. They get around impermissible impact shifts for these sights by laboriously hand adjusting every military grade telescopic sight. The American high end telescopic sight manufacturer U.S. Optics Inc. also offers variable magnification military grade telescopic sight models with rear focal plane mounted reticles.
Illumination is usually provided by a battery powered LED, though other electric light sources can be used. The light is projected forward through the scope, and reflects off the back surface of the reticle. Red is the most common colour used, as it least impedes the shooter's night vision.
Radioactive isotopes can also be used as a light source, to provide an illuminated reticule for low-light condition aiming. In sights like the SUSAT or Elcan C79 Optical Sight tritium-illuminated reticles are used for low-light condition aiming. Trijicon Corporation uses tritium in their combat and hunting-grade firearm optics, including the ACOG. The (radioactive) tritium light source has to be replaced every 8-12 years, since it gradually loses its brightness due to radioactive decay.
To eliminate parallax induced aiming errors, telescopic sights can be equipped with a parallax compensation mechanism which basically consists of a movable optical element that enables the optical system to project the picture of objects at varying distances and the reticle crosshairs pictures together in exactly the same optical plane. There are two main methods to achieve this.
Most telescopic sights lack parallax compensation because they can perform very acceptably without this refinement. Telescopic sights manufacturers adjust these scopes at a distance that best suits their intended usage. Typical standard factory parallax adjustment distances for hunting telescopic sights are 100 yd or 100 m to make them suited for hunting shots that rarely exceed 300 yd/m. Some target and military style telescopic sights without parallax compensation may be adjusted to be parallax free at ranges up to 300 yd/m to make them better suited for aiming at longer ranges. Scopes for rimfires, shotguns, and muzzleloaders will have shorter parallax settings, commonly 50 yd/m for rimfire scopes and 100 yd/m for shotguns and muzzleloaders. Scopes for airguns are very often found with adjustable parallax, usually in the form of an adjustable objective, or AO. These may adjust down as far as 3 yards (2.74 m).
The reason why scopes intended for short range use are often equipped with parallax compensation is that at short range (and at high magnification) parallax errors become more noticeable. A typical scope objective has a focal length of 100 mm. An optical ideal 10x scope in this example has been perfectly parallax corrected at 1000 m and functions flawlessly at that distance. If the same scope is used at 100 m the target-picture would be projected (1000 m / 100 m) / 100 mm = 0.1 mm behind the reticle plain. At 10x magnification the error would be 10 * 0.1 mm = 1 mm at the ocular. If the same scope was used at 10 m the target-picture would be (1000 m / 10 m) / 100 mm = 1 mm projected behind the reticle plain. When 10x magnified the error would be 10 mm at the ocular.
A telescopic sight can have several adjustment controls.
Most contemporary telescopic sights offer the first three adjustment controls. The other three are found on telescopic sights that offer a variable magnification, an illuminated reticle and/or parallax compensation. A rather common problem with the elevation and windage adjustment controls is that once smooth working adjustment turrets ‘get stuck’ over the years. This is generally caused by long time lack of movement in the lubricated turret mechanisms.
Older telescopic sights often did not offer windage and elevation adjustments in the scope, but rather used adjustable mounts to provide adjustment. Some modern mounts also allow for adjustment, but it is generally intended to supplement the scope adjustments. For example, some situations require fairly extreme elevation adjustments, such as very short range shooting common with airguns, or very long range shooting, where the bullet drop becomes very significant. In this case, rather than adjusting the scope to the extremes of its elevation adjustment, the scope mount can be adjusted. This allows the scope to operate near the center of its adjustment range. Some companies offer adjustable bases, while others offer bases with a given amount of elevation built in. The adjustable bases are more flexible, but the fixed bases are more durable, as adjustable bases may loosen and shift under recoil.
The SAM (Shooter-supporting Attachment Module) measures and provides aiming and ballistic relevant data and displays this to the user in the ocular of the Zeiss 6-24x72 telescopic sight it is developed for. If the SAM module is still under development or currently (2008) available in the Zeiss defense optics range remains unclear.
As very few firearms come with built-in telescopic sights (military bullpup designs such as the Steyr AUG being the primary exception) mounting a scope to a firearm requires additional equipment. Equipment is available to mount scopes on most production firearms. A typical scope mounting system consists of two parts, the scope base and the scope rings. By picking the appropriate combination of scope base to fit the firearm and scope rings to fit the scope, a wide range of scopes may be mounted to most firearms. With the appropriate combination of adjustable scope bases and scope rings it is also possible to mount several telescopic sights on the same gun to make the gun more versatile. However, it is important to take into consideration whether or not a gun is particularly hard to mount. If it is or if a gun is intended for long-range shooting, it could be that the amount of vertical adjustment range is smaller than required. This can be solved with the help of a vertically canted base or canted rings. Typical cant angles offered by mounting components manufacturers are 20 and 30 MOA. It is always wise to buy telescopic sights that provide a decent adjustment range, preferably at least 60 MOA or more .
European telescopic sight manufacturers often offer the option to have mounting rails underneath the riflescope to provide for mounting solutions that do not use scope rings or a single scope ring around the objective of the scope. These rails are an integral part of the scope body and can not be removed. The mounting rail permits the riflescope to be securely and tension-free mounted at the preferred height and correct distance from the shooter's eye and on different guns.
There are several mounting rail systems offered:
The traditional standard prism mounting rail system requires to have the scope rail drilled from the side for fixture screws. The more recent propriety systems mainly offer aesthetic advantages for people who have problems with redundant drill holes in sight in case the riflescope is used on different guns. To avoid drilling the scope rail, the propriety rail mounting systems have special shape connections machined in the inside of the rail. These shape connections prevent ever showing any exterior damage from mounting work on the rifle scope. The propriety rail systems use matching slide-in mount fasteners to connect the riflescope to the gun. Some propriety rails also offer the possibility to tilt the scope up to 1° to the left or right.
Scopes on heavy recoiling firearms and spring piston airguns (which have a heavy "reverse recoil" caused by the piston reaching the end of its travel) suffer from a condition called scope creep, where the inertia of the scope holds it still as the firearm recoils under it. Because of this, scope rings must be precisely fitted to the scope, and tightened very consistently to provide maximum hold without putting uneven stress on the body of the scope. Rings that are out of round, misaligned in the bases, or tightened unevenly can warp or crush the body of the scope.
Another problem is mounting a scope on a rifle, such as some lever action designs, where the shell is ejected out the top of the rifle. Usually this results in the scope being offset to one side (to the left for right-handed people, right for left-handed) to allow the shell to clear the scope. Alternately a scout rifle type mount can be used, which places a long eye relief scope forward of the action.
A firearm may not always be able to fit all aiming optics solutions, so it is wise to have a preferred aiming optics solution first reviewed by a professional.
Until the 1990s, military use of telescopic sights was restricted to snipers because of the fragility and expense of optical components, though they had been used as early as the American Civil War on rifles, and even earlier for other jobs. The glass lenses are prone to breakage, and environmental conditions such as condensation, precipitation, dirt, and mud obscure external lenses. The scope tube also adds significant bulk to the rifle. Snipers generally used moderate to high magnification scopes with special reticles that allow them to estimate range to the target.
Telescopic sights provide some tactical disadvantages. Snipers rely on stealth and concealment to get close to their target. A telescopic sight can hinder this because sunlight may reflect from the lens and a sniper raising his head to use a telescopic sight might reveal his position. The famous Finnish sniper Simo Häyhä preferred to use iron sights rather than telescopic sights to present less of a target. Harsh climate can also cause problems for telescopic sights as they are less rugged than iron sights. Many Finnish snipers in WWII used iron sights heavily because telescopic sights did not cope with very cold Finnish winters.
The market for military telescopic sights intended for military long-range shooting is highly competitive. Several high end optics manufacturers are constantly adapting and improving their telescopic sights to fulfill specific demands of military organizations. Two European companies that are active this field are Schmidt & Bender and Zeiss/Hensoldt. American companies that are also very active in this field are Nightforce and U.S. Optics Inc. These high end sighting components generally cost € 1500 / $ 2000 or more. Typical options for military telescopic sights are reticle illumination for use under adverse light circumstances and the presentation of scope settings or ballistic relevant environmental measurements data to the operator through the sights ocular. Military organizations also are a main driving force behind the development of ever more versatile mil-dot reticles, like the Generation II mil-dot reticle from Premier Reticles the US Marine Corps specified for their 7000 USMC M8541 Premier/Schmidt & Bender 3-12x50 PM II LP telescopic sights. Other range finding reticle variations like Schmidt & Bender's P4-fine reticule, which uses mil-hash marks instead of mil-dots for ranging purposes, also were developed on request of active snipers and other long-range field shooters.
The former Warsaw Pact members produce military telescopic sights for their designated marksmen and developed a range finding reticle based on the height of an average human. The reticle used in the Romanian I.O.R. LPS 4x6° TIP2 4x24 rifle scope is calibrated for ranging a 1.7 m tall target from 200 m to 1000 m. This Romanian scope shares the basic design and stadiametric rangefinder found in the reticle of the original Russian PSO-1 and POSP scope series. The target base has to be lined up on the horizontal line of the range-finding scale and the target top point has to touch the upper (dotted) line of the scale without clearance. The digit under which this line up occurs determines the distance to the target.
The Israeli military began widespread use of telescopic sights by ordinary infantrymen to increase hit probability (especially in dim light) and extend effective range of standard issue infantry rifles. Palestinian militants in the al Aqsa Intifada likewise found that adding an inexpensive scope to an AK-47 increased its effectiveness.
Today, several militaries issue telescopic sights to their infantry, usually compact, low-magnification sights suitable for snap-shooting, like red dot sights. American GIs in Iraq and Afghanistan frequently purchase their own combat optics and carry them from home. The British army fields the SA80 rifle with the SUSAT 4× optical sight as standard issue. The Canadian Forces standard C7 rifle has a 3.4 Elcan C79 optical sight. Both Austria and Australia field variants of the Austrian Steyr AUG which has built an integral 1.5x optical sight since its deployment in the late 1970s. The German Army G36 assault rifles have a more or less built in dual combat sighting system consisting of a ZF 3x4° telescopic sight combined with an unmagnified electronic reflex red dot sight. The dual combat sighting system weighs 30 gr (1 oz) due to a housing made out of glass fibre reinforced polyamide. All rifles are adapted to use the Hensoldt NSA 80 II third-generation night sight, which clamps into the G36 carry handle adapter in front of the optical sight housing and mates with the rifle's standard dual combat sighting system.