optics ambient can be collected and directed to an illuminated daytime reticle. Fiber-optics reticles automatically interact with the ambient light level that dictates the brightness of the reticle. Trijicon uses fiber optics combined with other low-light conditions illumination methods in their AccuPoint telescopic sights and some of their ACOG sights models. Additional features Bullet drop compensation Bullet drop compensation (BDC, sometimes referred alternatively as ballistic elevation) is a feature available on some telescopic sights, usually those used by more tactically-oriented semi-automatic and assault rifles. The feature provides pre-determined reference markings for various distances (referred to as "bullet drops") on the reticle or (much less commonly) on the elevation turret, which gives reasonably accurate estimations of potential gravitational deviation upon the bullet in flat-firing scenarios, so the shooter can proactively adjust their aim to compensate without needing to trial with missed shots or dealing with complex ballistic calculation. The BDC feature is usually tuned only for the ballistic trajectory of a particular gun-cartridge combination with a predefined projectile weight/type, muzzle velocity and air density. Military sights featuring BDC reticles (e.g. the ACOG) or elevation turrets with range markings (e.g. PSO-1) are fairly common, though commercial manufacturers also offer the option to install a BDC reticle or elevation turret as long as the customer supplies the necessary ballistic data. Since the usage of standardized ammunition is an important prerequisite to match the BDC feature to the external ballistic behaviour of the employed projectiles, telescopic sights with BDC are generally intended to assist with field-shooting at targets within varying medium to longer ranges rather than precise long range shooting. With increasing range, inevitable BDC-induced errors will occur when the environmental and meteorological circumstances deviate from the predefined circumstances for which the BDC was calibrated. Marksmen can be trained to understand the main forces acting on the projectile and their effect on their particular gun and ammunition and the effects of external factors at longer ranges to counter these errors. Parallax compensation Parallax problems result from the target image projected from the objective not being coplanar with the reticle. If the target and the reticle are not coplanar (i.e. focal plane of the target is either in front of or behind the reticle), when the shooter's pupil position changes (often due to small alterations in head alignment) behind the eyepiece, the target will produce a different parallax to the reticle image. This parallax difference