This adapter fits on many Nikon film and DSLR cameras that have a rectangular eyepiece. It slides on the camera's eyepiece and provides a round ring and retaining lip to hold a Nikon (or 3rd party) DK-19 round rubber eyecup. Note that when used on a film camera, it may interfere with the camera back opening up to load and unload the film cartridge. It can also slightly obscure the top of the rear display on DSLR camera bodies depending on the size of the display. Nikon makes an adapter that takes a smaller diameter corrective lens holder and rubber eyecup but both of those items are discontinued and quite pricey on the online used market (when you can even find them). The DK-19 rubber eyecup this adapter uses is readily available and cheap at any online retailer.