It protects the eyes from hitting the viewfinder. It also minimises extraneous light and reducing vision fatigue. Rubber EyeCup Eyepiece DK-21 For Nikon D70s/80/D90/D40/D50/D100/D200/D300/D7000. Keep unwanted light out of the way - the eyepiece cuts out stray light so you have optimum light conditions for keeping shots in focus on the viewfinder.
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