Engineered specially for crossbow hunters, the 512-XBOW™ uses the unique properties of holography to create optimal range-scaling data in a heads-up display. A pre-calculated ranging scale (in yards) is projected on the target and measures the back to the belly of a typical mature whitetail deer (18"). Simply position the base of the scale on the belly and read from where the back intersects the scale — it's that easy. No guesswork. No extra movement.