The Leatherwood ART X-BOW is a 1X to 4X variable power scope that automatically corrects your line of sight from 20 yards to 80 yards. This is accomplished through the use of a unique circular cam that actually raises and lowers the rear of the scope. The effect of this, when aimed at a target, is to correspondingly lower and raise the line of flight of the bolt, and thereby adjust your aim to eliminate the effect of bolt drop due to gravity. The Leatherwood ART X-BOW does not rely on fragile internal ranging devices, complicated and undependable battery-based electronics, or changes to the elevation and windage adjustments. It is actually a mechanical computer that calculates the trajectory correction based on the target being framed in the reticle. By using one trajectory cam that is incrementally adjustable for your bow weight and bolt type, the ART X-BOW is the ONLY scope that is able to mechanically compensate for the curved trajectory of a crossbow.