TheNikon DK-20C Correction Eyepiece for Rectangular-Style Viewfinder (-5.0)can fine-tune the sharpness of your SLR/DSLR viewfinder to your specific eyesight. The Nikon DK-20C Correction Eyepiece for Rectangular-Style Viewfinder (-5.0) components and a clear (standard) eyepiece in Nikon SLR/DSLR cameras (cameras without eyepiece correction adjustment) form a virtual image of a subject on the focus-screen diopter, the approximate equivalent of a viewing distance of one metre. Minus one is the starting point for the simple arithmetic that many SLR/DSLR users consider the optimal value with the Nikon DK-20C Correction Eyepiece for Rectangular-Style Viewfinder (-5.0).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.CompatibilityD750, D610, D600, D5300, D5200, D5100, D5000, D7100, D7000, D3300, D3200, D3100, D3000, D300, D300S, D200, D100, D90, D80, D70, D60, D50, D40, N80, N75, N70, N65, N60, N55, N50, Pronea, N2-4-5-6000 Series & FM/FE10 Cameras