Nikon 22mm +2.0 D HP Eyepiece Correction Lens For Nikon F100F90X,N90S,F801S, N8008S The diopter just screws into the eyepiece and comes in a variety of + & - sizes. There are two main sizes with the earlier being smaller 19mm and fitting the Nikon FE, FM & F3 with STD prism Etc . The later larger 22mm type fited the F801,F90, F100, F3 HP Prism Etc. Used in good condition no box
eBay