A telescopic sight, (Buy EOTech Vudu 2.5 Gun Scope-Buy Gun Optics) commonly called a scope, is an optical sighting device that is based on a refracting telescope. They are equipped with some form of graphic image pattern (a reticle) mounted in an optically appropriate position in their optical system to give an accurate aiming point. Telescopic sights are used with all types of systems that require accurate aiming but are most commonly found on firearms, particularly rifles. Other types of sights are iron sights, reflector (reflex) sights, and laser sights. The optical components may be combined with optoelectronics to form a night scope.
Telescopic sights(Buy EOTech Vudu 2.5 Gun Scope-Buy Gun Optics) are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters. Those parameters are:
Magnification — The ratio of the focal length of the eyepiece divided into the focal length of the objective gives the linear magnifying power of telescopes. A magnification of factor 10, for example, produces an image as if one were 10 times closer to the object. The amount of magnification depends upon the application the telescopic sight is designed for. Lower magnifications lead to less susceptibility to shaking. A larger magnification leads to a smaller field of view.
Objective lens diameter – The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters.
Field of view — The field of view of a telescopic sight is determined by its optical design. It is usually notated in a linear value, such as how many meters (feet) in width will be seen at 100 m (110 yd), or in an angular value of how many degrees can be viewed.
Exit pupil — Telescopic sights concentrate the light gathered by the objective into a beam, the exit pupil, whose diameter is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye — about 7 mm, reducing with age. If the cone of light streaming out of the eyepiece is larger than the pupil it is going into, any light larger than the pupil is wasted in terms of providing information to the eye.
However, a larger exit pupil makes it easier to put the eye where it can receive the light: anywhere in the large exit pupil cone of light will do. This ease of placement helps avoid vignetting, which is a darkened or obscured view that occurs when the light path is partially blocked. And, it means that the image can be quickly found which is important when aiming at game animals that move rapidly. A narrow exit pupil telescopic sight may also be fatiguing because the instrument must be held exactly in place in front of the eyes to provide a useful image. Finally, many people in Europe use their telescopic sights at dusk, dawn and at night, when their pupils are larger. Thus the daytime exit pupil of about 3 to 4 mm is not a universally desirable standard. For comfort, ease of use, and flexibility in applications, larger telescopic sights with larger exit pupils are satisfying choices even if their capability is not fully used by day.
Eye relief — Eye relief is the distance from the rear eyepiece lens to the exit pupil or eye point. It is the distance the observer must position his or her eye behind the eyepiece in order to see an unvignetted image. The longer the focal length of the eyepiece, the greater the eye relief. Typical telescopic sights may have eye relief ranging from 25 mm (0.98 in) to over 100 mm (3.9 in), but telescopic sights intended for scout rifles or handguns need much longer eye relief to present an unvignetted image. Telescopic sights with relatively long eye relief are favourable to avoid recoil induced facial and eye injuries and use in instances where it is difficult to hold the eyepiece steady. Eye relief can be particularly important for eyeglass wearers. The eye of an eyeglass wearer is typically further from the eye piece which necessitates a longer eye relief in order to still see the entire field of view.
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