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 36 mm diameter and the 2.5×70 should be approximately 21 mm (relative luminosity is the square of the exit pupil as measured in mm; a 36 mm objective lens diameter divided by the 4× magnification gives an exit pupil of 9 mm; (9×9=81) Prismatic telescopic sight A relatively new type of telescopic sight, called prismatic telescopic sight, prismatic sight or "prism scope", replaces the image-erecting relay lenses of a traditional telescope with a roof prism design commonly found in compact binoculars, monoculars and spotting scopes. The reticle is etched onto one of the prism's internal reflection surfaces, which allows an easy way to illuminate the reticle (from the back side of the prism) even when active illumination is turned off. Being optical telescopes, prism sights can focally compensate for a user's astigmatism. Prismatic sights are lighter and more compact than conventional telescopic sights, but are mostly fixed-powered in the low magnification ranges (usually 2×, 2.5×, 3× or more commonly 4×, occasionally 1× or 5× or more), suitable for shooting at short/medium distances. One of the best known examples is the battle-proven Trijicon ACOG used by the USMC, US Army, and USSOCOM, although variable-magnification prism sights do also exist, such as the ELCAN Specter DR/TR series used by the Canadian Army. Low-power variable optic Variable-zoom telescopic sights in the low magnification range (1–4×, 1–6×, 1–8×, or even 1–10×) are known as low-power variable optics or LPVOs. These telescopic sights are often equipped with built-in reticle illumination, and can be dialed down to 1× magnification. As low magnifications are mostly used in close- and medium ranges, LPVOs typically have no parallax compensation (though a few rare models do) and have a completely cylindrical shape ahead of the eyepiece, since the image illuminance is often sufficient without needing an enlarged objective bell to enhance light-gathering. Most LPVOs have reticles mounted at the second focal plane, but recently first-focal plane LPVOs have become popular, especially those with high zoom ratios above 6×. LPVOs are also informally referred to as "AR scopes" or "carbine scopes", due to the recently increasing popularity on modern sporting