The DK-20C +3.0 Correction Eyepiece for Rectangular-Style Viewfinder from Nikon allows you to optimize your SLR/DSLR viewfinder's sharpness in conjunction with your individual eyesight. Nikon SLR/DSLR cameras (without built-in adjustable eyepiece correction) have a base diopter value of minus one. Minus one is considered the optimum starting point for the majority of SLR/DSLR users with normal vision. The combination of the camera's viewfinder components and a clear (standard) eyepiece form a virtual image of a subject on the camera's focusing screen that is the approximate equivalent of a viewing distance of one meter.