Nikon 22mm -2.0 D HP Eyepiece Correction Lens For F100,F90X,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 boxed
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