Optional Nikon eyepiece diopters allow you to optimize your SLR viewfinder's 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 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. Each Nikon eyepiece diopter incorporates a numerical value, represented by a minus value (nearsighted) or a plus value (farsighted) number, enabling you to select the appropriate diopter for your vision and your camera model.