thumb|right|View through a 4× telescopic sight
thumb|right|[[Leupold and Stevens Mark 6 scope with variable magnification X3-X18, mounted on an M24 SWS]]
thumb|right|German military sniper rifle with a mounted telescopic sight and dismounted NSV80 clip on optoelectronic [[image intensifier]]A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. Sights are equipped with a referencing pattern (reticle) mounted in a focally appropriate position in its optical system to provide an accurate point of aim. Telescopic sights are classified in terms of the optical magnification (power) and the objective lens diameter.
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. Most early telescopic sights were fixed-power and were in essence specially designed viewing telescopes. Telescopic sights with variable magnifications appeared later, and were varied by manually adjusting a zoom mechanism behind the erector lenses. Other types of scopes include prism sights and low-power variable optics.
Telescopic sights have both advantages and disadvantages relative to iron sights. They are built to various specifications and use a variety of adjustment controls, reticles, features, technologies, and mounting systems. These specifications are often selected based on the intended use of the sight.
Description
thumb|German-made ZF Ajack 4×90 (4×38 in modern terminology) for the [[World War II-pattern Swedish Mauser m/1941 sniper rifle.]]
A telescopic sight is an optical sighting device based on a refracting telescope. The optical components may be combined with optoelectronics to add night vision or smart device features.
Telescopic sights are classified in terms of the optical magnification (i.e., power) and the objective lens diameter. For example, "10×50" would denote a fixed magnification factor of 10×, with a 50 mm objective lens. In general terms, larger objective lens diameters, due to their ability to gather a higher luminous flux, provide a larger exit pupil and hence provide a brighter image at the eyepiece.
History
thumb|A [[American Civil War|Civil War reenactment of the Battle of Bean Station with a Confederate sharpshooter shooting a Whitworth rifle equipped with a Wm. Malcolm long-tube scope.]]
thumb|Russian [[Mosin–Nagant#Russia/USSR|Model 1891/30 sniper rifle with PU 3.5×21 scope]]
thumb|A British soldier testing a [[Zielgerät 1229|ZG 1229 Vampir night scope (circa 1945)]]
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.
In the late 1630s, English amateur astronomer William Gascoigne was experimenting with a Keplerian telescope and left it with the case open. Later he found that a spider had spun its web inside the case, and when he looked through the telescope he found that the web was in focus with distant objects. Gascoigne realised that he could use this principle to make a telescopic sight for use in his astronomical observations.