Nikon Correction Diopter (-4) optimises the sharpness of your film camera’s viewfinder to your eyesight. Cameras without built-in adjustable eyepiece correction have a base diopter of -1 which is considered the optimal starting point for photographers with good vision. The viewfinder’s components in combination with a clear, standard eyepiece form a virtual image of the subject that is the approximate equivalent of a viewing distance of one metre.
Numerical values are minus for nearsighted users and plus for farsighted users. The base diopter value does not affect the end value, so if you require a diopter of -4, this is the correct eyepiece for you to use.