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A telescopic sight, or scope, is a small telescope mounted onto a firearm. It uses mirrors and lenses to gather light, brightening and magnifying the view of the person shooting the firearm. Telescopic sights are used to improve shooting accuracy by offering a better view of the intended target. Telescopic sights share features with a conventional telescope, including its external lenses. At the front is the objective lens, transmitting light through the scope; the front of the scope is usually flared to accommodate the objective lens. The lens at the rear of the scope (closest to the shooter's eye) is the ocular lens.
The picture below shows a rifle scope with some of the most important components labeled:
Main components of a rifle scope.
Within a telescopic sight, a series of lenses performs three main roles:
- Magnifying the target—The objective lens captures light reflected from the target and bends the light to create a magnified image of the target. Some scopes use multiple lenses to magnify the target.
- Inverting the magnified image—The image generated by the objective lens is inverted. An erector assembly contains two lenses that flip the image so it is right-side-up. The erector assembly can also contain additional magnifying lenses for a variable magnification scope.
- Focusing the image—The ocular lens focuses the image that has been magnified and flipped right-side-up for the eye. The diameter of the ocular lens also determines the eye relief—the distance between the scope and the tip of the eye.
The main body of the rifle scope is called the tube. The objective lens (farthest from the shooter's eye) housed in the objective bell is larger than the ocular lens (closest to the shooter's eye) positioned in the eyepiece. For reliable use, rifle scope lenses must be waterproof and fog-proof. Scopes operate like telescopes; the objective lens focuses light to the first focal plane close to the location of the elevation adjustment knobs.
The reticle is the aiming point in the shooter's field of view, also referred to as "crosshairs." This may be a simple dot superimposed on the target or more complicated geometric shapes, and it is placed at the first focal plane. Reticles can also be illuminated with an LED that has an adjustable intensity to match the light conditions. The position of the reticle is controlled in the vertical and horizontal positions by two adjustment knobs. The vertical adjustment is referred to as ballistic drop or bullet drop compensation, and the horizontal adjustment is referred to as windage control.
Behind the reticles is a set of erector zoom lenses with two functions:
- Correcting the position of the image created by the objective lens. The objective lens inverts the image at its focal point, and the image needs to be corrected before the eyepiece.
- Controlling the magnification power of the system. By rotating the power ring (or magnification adjustment ring), the distance of the elements of the zoom lenses changes to alter the magnification.
The erector zoom lenses direct the corrected and magnified light to the eyepiece, which creates the final image the shooter sees. Eyepieces can include parallax correction elements; however, it is more common to correct parallax problems in the objective or erector lenses. Parallax errors are introduced when adjusting the magnification, causing the aim on a scope to change if the position of the shooter's eye changes. With the rifle position unchanged, changing the shooter's position will cause the aim to become off target. This becomes a problem at high magnifications.
Magnification defines the apparent size of the target viewed through the ocular lens compared to viewing without the scope. Specifically, the magnification defines the ratio between the focal length of the ocular lens and the focal length of the objective lens. The relationship between the image produced by the telescopic sight and the target is linear and proportional. A target viewed through a 3× magnification scope will appear to be three times closer than its actual distance, and doubling the magnification from 3× to 6× will cause the target’s image to appear half as far away. Also, the height of the target will also increase in proportion to an increase in magnification. This means that a doubling of magnification from 3× to 6× will not only halve the apparent distance to the target but also double its apparent size.
A rifle scope specification includes two numbers. The first number identifies the magnification, and the second number identifies the diameter of the objective lens. Therefore, a specification of 8.5×50 describes a scope with 8.5× magnification and an objective lens that is 50 mm in diameter. Variable magnification scopes have a specification describing the range of magnifications provided by the scope; for example, a scope may quote the magnification as 3.5-21×.
Magnification comes with trade-offs, and there are multiple optical limitations and types of distortion to consider:
- Chromatic aberration or fringing can occur with any lens but worsens as magnification increases. The wavelength of light affects how it interacts with lenses, causing white light to break up into its constituent colors. The result is that an image viewed through a lens without correction for chromatic aberration can appear blurry or fringed with purple.
- Field curvature can cause blurriness at the edges of the image in scopes with low magnification. At low magnification, the sharpest focus for the image is closer to a sphere than a plane. Consequently, a target and reticle that are sharp near the center of the image are blurry at the edges of the image.
- Spherical aberration can become a problem as magnification increases. Spherical aberration happens because light is bent more at the edges of a lens than at the center of the lens. This means that the light from the edges of the lens meets at a slightly different focal point than the light passing through its center. Spherical aberration has the same effect on the image as field curvature—the image may appear blurry at the edges when the center of the image is in focus.
- Field of view can be calculated by dividing the diameter of the objective lens by the magnification. As the magnification increases, the field of view will decrease proportionally since the diameter of the objective lens is a fixed number. This means that less area will be visible as magnification increases.
- Dimness is caused by thicker or more lenses being used. This causes more light to be reflected and absorbed rather than transmitted. Even coated lenses will transmit less than 100 percent of the light incident on the lens. As a result, higher magnification scopes will produce dimmer images than lower magnification scopes.
Eye relief is the distance between the lens of the scope and the shooter's eye. Viewing a scene through any lens with incorrect eye relief distance causes the picture to become distorted, either with a fuzzy image or with a black ring around the field of view. Using a short eye relief distance with a powerful rifle can cause it to strike the shooter's eye due to recoil.
There are two general types of eye relief:
- Standard eye relief—3.5 to 4 inches, for long-range shooting in open, flat areas using medium-caliber rounds. Standard eye relief allows shooters to use higher magnification levels while reducing field of view.
- Long eye relief—above 4.5 inches, for shorter range shooting using higher-caliber rifles. Long eye relief allows for a greater field of view while sacrificing high magnification capability.
Diopter is a measurement expressing the refractive power of a lens. A lens with a focal length of 1 meter has a refractive power of one diopter. Lenses with shorter focal lengths have greater power in diopters. Accordingly, a lens with a focal length of 0.25 meters has a power of four diopters. The refractive power of converging (convex) lenses is positive; the refractive power of a diverging (concave) lens is negative.
Measured in millimeters, exit pupil is the diameter of the beam of light that leaves the eyepiece of a riflescope. The larger the exit pupil, the brighter the image will be, which is advantageous in low-light conditions. The human eye can dilate to sizes between 5mm – 7.5 mm after it adapts to the dark. The closer the exit pupil is to the eye dilation, the brighter the images will appear. To calculate the exit pupil, divide the size of the objective lens by the magnification of the riflescope.
Although the telescope was invented in 1608, it wasn't until the late seventeenth century that rifles had enough range for people to experiment with telescopic sights.
The first recorded use of a telescopic sight occurred on January 11th, 1776, when Charles Wilson Peale shot his rifle in the state house yard in Philadelphia (now known as Independence Hall). Peale was a prominent portrait painter of the period, best known for portraits of George Washington and other revolutionary leaders. Peale worked on the telescopic sight with David Rittenhouse, a well-known American astronomer. Their experimentation with the telescopic sight is detailed in Peale's brief diary notations during early 1776:
January 1, 1776: attended Mr. Rittenhouse all Day about a Riffle with a Tellescope to it. January 2: Ditto. January 3: Bought a Gun Lock 22/6 I found it faulty & offered the Man 2/6 to take it back but he would not. I bought another at 40/. January 5: a set of Loop to hang up a Gun 6/6, spent in attending & working about my riffle. Threatened to complain to the committee [this would be the Committee of Safety] of Henry ——– who had taken an extortionate price for the Gun Lock on Wednesday last. January 6: attending the man stockg sd. Gun. January 8: still attendg about my Gun. January 9: pd for stockg my riffle 22/6 to Mr. Williss. January 10: Attnd Mr. Palmer & Mr. Rittenhouse about sd G-n. January 11: pd. Mr. Palmer for a Riffle Barrel 3=10.0 Bullit moulds 3/9 a screw wiper [attachment for ramrod to hold cleaning patch] 1/3 – finished the Riffle this morng: Shot her afternoon in the Stadt House [State House] yard, not quite sighted.
The diary continues detailing Peale experiments with rifles and telescopic sights until March 2nd, 1776. Peale biographer Charles Coleman Sellers stated:
…he made a telescopic sight… the weapon had also a breach box hollowed in its stock to carry bullets and wiper, a bayonet, worn in a scabbard at the side, and a steel ramrod. The two men tested ammunition and different loadings of powder. Rittenhouse accounted for the inaccuracy of a shot fired with a too heavy charge as due to the compression of air against the bullet.
A biographer of Rittenhouse, Brooke Hindle, wrote Rittenhouse:
worked with Peale on two rifle improvements. One was a telescopic sight, and the other an idea of Peale’s for building into the stock a box large enough to carry bullets and wipers. Rittenhouse set one of his journeymen to work on this invention, and, when it was ready, Peale and Rittenhouse took it out in the fields to test it. Neither inventor was disheartened when the device opened accidentally, spilling all its bullets; neither was at his best in pursuits related to soldiering.
The first telescopic sights finding wider use were invented between 1835-1840 by Morgan James of Utica, New York. James's new rifle sights were described in the book The Improved American Rifle, published in 1844 by British engineer John Ratcliffe Chapman. The two men later worked together to produce the Chapman-James sight featuring simple lenses. Although basic by modern standards, these optics stayed relatively true even after a number of shots, becoming the first practical telescopic sights in use.
In 1832, British soldier David Davidson (then a lieutenant) first applied a telescopic sight to a firearm acquiring a single-barrelled, flint action rifle made by Samuel Staudenmeyer (Swiss-born gunmaker) with a simple draw telescope attached to the barrel. While stationed in India, Davidson spent significant time practicing with his telescopic sight, hunting large game, and developing improvements. In 1839, he wrote to famous gunmaker James Purdey to commission a heavy-barrelled “small-bore” percussion rifle and fit it with a telescopic sight. The rifle and telescopic scope were not to Davidson's satisfaction, and he had it modified and improved.
On Davidson's return to Britain in 1848, he consulted with gunmakers James Purdey and Charles Lancaster about developing more accurate, longer-ranged hunting rifles. Davidson further developed his ideas on telescopic scopes with Scottish gunmakers John Dickson & Son and Alexander Adie & Sons and corresponded with Professor Charles Piazzi Smyth, the Astronomer-Royal for Scotland. David Davidson showed the range of his telescopic-sighted weapons for the first time in public at the Great Exhibition of 1851 in London's Hyde Park.
In 1855, William Malcolm of Syracuse, New York, introduced achromatic lenses into rifle optics. Malcolm had an extensive background working with telescopes, and his new achromatic lens design allowed for a better definition of targets and a flatter field of view. Malcolm's scope also incorporated windage and elevation adjustments with magnification thought to be between x3 and x20.
Several years later, Davidson's ideas were adopted through the efforts of engineer Joseph Whitworth in Manchester. Whitworth developed rifles incorporating a four-power telescopic sight designed by Davidson; the sight was fitted in an adjustable mount on the gun's left side. The scope could easily detach to prevent interference with the iron sights. Whitworth's rifles (some with Davidson's telescopic sight design) were supplied to the Confederate armies during the US Civil War.
Whitworth rifle with a mounted telescopic scope (designed by David Davidson) used by the Confederate States of America during the Civil War.
The late nineteenth and early twentieth centuries saw significant advances in rifle optics technology, including the development of refractor scopes that allow light to pass directly into the eye of the shooter, improving their ability to see targets in low light conditions. Although refractor lenses had been available, it took until 1880 for them to become small and rugged enough for hunting and military use. In 1880, August Fiedler from Stronsforf in Austria developed a new telescopic sight design using a refractor telescope to gather more light and increase visibility. This development led to one of the world's major factories manufacturing telescopic sights, the KAHLES company.
During the world wars, major powers of the world made significant advances in military-based technologies, including telescopic sights. The development of scopes with extra-long eye relief further enhanced the shooter's capability in low-light conditions. During World War I, the German Army seized the initiative when it came to rifle optics. Germany had long been preeminent when it came to optical design and manufacture. Trench warfare provided plenty of hiding places and large cleared areas, perfect for snipers. Germany deployed 25,000 then-advanced scoped rifles. In response, the US Army mounted a 6x magnification Warner and Swasey scope on top of the .30-06 M1903 Springfield rifle. Although the weapon was difficult to carry in combat, it proved to be effective.
World War II saw the rapid development of rifle optic technology. The move towards smaller caliber and more powerful rounds increased the effective range of rifles significantly, and scope magnification followed suit. The most advanced optical systems used by US soldiers in this period were the 2.5x magnification Lyman Alaskan, fitted to the M1 rifle, and the x10 magnification Unertl scope. 1940 saw the introduction of variable power rifle scopes that allowed users to control the magnification level. Until this point, all scopes had a fixed power. However, the new variable power rifle scopes had multiple issues, including moisture getting inside and forming fog, rendering them unusable (it wasn't until 1960 that a waterproof variable power rifle scope was developed).
World War II also saw an early example of a night vision device. Developed by Germany during the later stages of the war, the ZG 1229 Vampir was an active infrared device for use with the StG 44 assault rifle. Rather than developing the Vampir for a longer-range weapon (which the German Army felt would be ineffective at night), it was intended to provide close-quarter visual capability for infantry soldiers. It was introduced to the battlefield in 1944, but its use was limited.
Post World War II, telescopic sights developed for the US military began finding civilian use, with the first large-scale civilian adoption of scoped rifles. As hunters got access to more powerful rifles, they also acquired scopes to improve accuracy at greater ranges. Scopes with magnification up 20x became available to civilians. Due to the larger range of rifles, scopes began to incorporate windage and bullet drop reticles for the first time.
The Vietnam war saw widespread use of long-range scopes. During Vietnam, scout sniper Carlos Hathcock mounted a basic telescopic scope onto an M2 .50 caliber heavy machine gun. Not designed as a long-range weapon, when fired in single-shot mode, the M2 delivered both huge range and power. Firing .50 caliber rounds from an unmounted weapon generates a huge amount of recoil; therefore, operating the M2 with a scope required a well-trained soldier. Hathcock used this weapon for the then-record longest kill for an infantry soldier, 2,286 meters. The US army initially viewed using rifle optics with .50 caliber weapons skeptically. However, in 1990 they purchased a consignment of .50 cal BMG M82 Barrett for use as a sniper rifle. This proved effective and was later standardized as the M107.
Modern scopes incorporate electronic technology in their design, including the use of lasers to identify ranges of targets.
