• Spotting Scope: Originally, a telescope used at shooting matches to spot hits made on distant targets. Currently, the common name for small telescopes used by shooters, hunters, bird watchers and other outdoor enthusiasts. • Telescopic sight: A small refracting telescope equipped with a reticle and used as a sighting device for a firearm. Common names include handgun scopes, shotgun scopes and riflescopes, depending on the type of firearm used. Twilight Factor: A numerical guide for comparing the low-light performance of optical instruments, which takes magnification into account. To calculate twilight factor, multiply the diameter of the objective lens by the magnification and extract the square root of the product. Note that twilight factor does not take into account optical qualities, such as light transmission, resolution and image contrast. Variable Optics (zooms): An optical system of variable focal length, used to change the magnifying power within a predetermined ratio, such as three-to-one or four-to-one, an example being a 3-9X riflescope.
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