The Nikon DK-17C (-3) Eyepiece Correction Lens enables you to optimise the sharpness of your SLR viewfinder in conjunction with your own eyesight. Cameras without in-built adjustable eyepiece correction have base diopter value of -1, which is thought to be the optimal starting point for SLR users with good vision. The components of the cameras viewfinder combined with a clear (standard) eyepiece form a virtual image of the subject that is the approximate equivalent of a viewing distance of one meter. The numerical value of an eyepiece is represented by either a minus value (nearsighted) or a plus value (farsighted). The base diopter value does not affect the end value, thus if you require a diopter of -3, this eyepiece is the right choice for you.