Choose from 18 neutral and 14 corrective apertures in three diopters… Features A sight is the “opening” to the aiming tunnel which takes us to the scope. It is measured in tenths of a millimeter, defines the sharpness and the amount of light you want to obtain when aiming. How to choose your diopter? Sharpness : Neutral: clear scope and bubble, blurred target face. Corrective: the stronger the correction, the sharper the target, and the more blurred and/or distorted the scope and bubble. Wide aperture: the target face is bright, but blurry. Small aperture: the target face is dark, but sharp.