When you buy a new camera, you might find an oddly-shaped piece of plastic that comes in a small plastic package. Sometimes it’s attached to your strap. What is it? Probably a camera eyepiece cover. Some topline pro cameras have this feature built into the camera itself. Others provide this piece of plastic to cover the viewfinder when you’re shooting in a manner that takes your eye away from the camera.
Any time you take a picture without looking through the camera’s optical viewfinder, the eyepiece should be covered. If you don’t cover the viewfinder, your meter could be tricked by the light entering the back of the camera. This can throw off your exposure. While most photographers know light can enter your camera through the lens, many don’t realize it can enter through the rear of the camera. So if you’re shooting on a timer and you move your eye away from the camera viewfinder, use the cover. Otherwise you might just end up with an underexposed photo.