Vortex Optics Viper PST Second Focal Plane Riflescopes CRS Zero Stop - Customizable Rotational Stop (CRS) prevents dialing more than one rotation below your zero so you don't lose the zero when dialing large amounts of elevation corrections. Patent pending. Tall Uncapped Turrets - Make range calculation and shooting adjustments fast and accurate with subtension marks that match the reticle Single Spring Construction - Fully constrains (without over-constraining) the optical system for predictable, improved accuracy throughout the range of elevation adjustment. Glass-etched Reticle - Protected between two layers of glass (not made from wire) for optimum durability and reliability. Illumination Control - Can be turned on or off with a single click between each intensity levelallowing access to a preferred illumination setting without having to progress through each intensity level. Positioned at 45 degrees off the left side of the eyepiece for easy adjustment without blocking view of the turret setting or magnification.