Nikon eyecups are rubber or silicone eye pieces that attach to the viewfinder of a Nikon camera. They serve several purposes: they block out extraneous light, preventing distractions and enhancing image visibility, especially in bright outdoor conditions.

Additionally, eyecups create a comfortable seal around the eye, reducing eye strain during extended shooting sessions. They also help to keep the viewfinder clean and protected from accidental damage. By using a Nikon eyecup, photographers can improve their viewing experience and capture more precise and focused images.