The DK-20C -4.0 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.
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.
The eyepiece's diopter value is, when combined with the viewfinders minus one value, the end-result value. No combination of the camera's base value of minus one and a given accessory diopter's value is required to arrive at the final desired value. In other words, if you require a diopter of plus 3, simply add a plus 3 diopter to your camera.