at various ranges will only function as intended if the telescopic sight is mounted without cant. "Zero-stop" features prevent inadvertently dialing the adjustment knob "below" the primary zero (usually 100 meters/yards for long-range sights), or at least prevent dialing more than a couple adjustment clicks below zero. This feature is also useful on long-range sights because it allows the shooter to physically verify the elevation knob is dialed all the way down, avoiding confusion regarding the elevation status on two- or multi-revolution elevation knobs. Windage control – meant to adjust (or "track") the horizontal deviation of the optical axis. Magnification control – meant to change the magnification factor by turning a coaxial wheel that is generally marked with corresponding magnification numbers. Reticle illumination control – meant to regulate the brightness level of assisted illumination on the reticle. Parallax compensation control – mean to neutralize the focal difference between the target image and the reticle. All telescopic sights have the first three (diopter, elevation, windage) adjustment controls, and the fourth (magnification) control is offered on variable-power sights. The remaining two adjustments are optional and typically only found on higher-end models with additional features. The windage and elevation adjustment knobs (colloquially called "tracking turrets") often have internal ball detents to help accurately index their rotation, which provide a crisp tactile feedback corresponding to each graduation of turn, often accompanied by a soft but audible clicking sound. Each indexing increment is thus colloquially called a "click", and the corresponding angular adjustment of the optical axis is known as the click value. The most commonly seen click values are MOA (often expressed in approximations as " inch at 100 yards") and 0.1 mil (often expressed as "10 mm at 100 meters"), although other click values such as MOA, MOA or MOA and other mil increments are also present on the commercial and military and law enforcement sights. Older telescopic sights often did not offer internal windage and/or elevation adjustments in the telescopic sight. In case the telescopic sight lacked internal adjustment mechanisms adjustable mounts are used (on the scope rings, or the mounting rail itself) for sighting-in. Reticles Telescopic sights come with a variety of different reticles, ranging from the simple crosshairs to complex reticles designed to allow the shooter to range a target, to compensate for the bullet drop and windage required due to crosswinds. A user can estimate the range to objects