* Optional Nikon eyepiece diopters allow you to optimize your SLR viewfinders sharpness in conjunction with your individual eyesight. Nikon SLR 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 users with normal vision. The combination of the cameras viewfinder components & a clear (standard) eyepiece form a virtual image of a subject on the cameras focusing screen that is the approximate equivalent of a viewing distance of one meter.