The Nikon DK-17C -3.0 Eyepiece comes with the strength of -3 as its diopter value. Designated with a negative value of minus three, it indicates that this eyepiece will help in correcting the shortsightedness of your vision. While some Nikon SLR Cameras have built-in adjustable eyepiece correction, the Nikon SLR Cameras without built-in adjustable eyepiece correction have a base diopter value of minus one. Easy to calculate, the value of the eyepiece is a combination of the viewfinder’s diopter value of minus one. This eyepiece can be easily attached to the viewfinder of your camera. Moreover, it is compatible with Nikon D-SLR cameras that feature a circular-eyepiece.