The other morning before I got up, I thought I would enjoy some light reading, and was browsing through my camera's manual, and came across a section talking about an optional addion of an optical view finder cover, for use to stop light entering in through the view finder when you don't have your eye up to the view finder.

So my question is, how much of a problem is light entering in through the view finder? (eg, when you have the camera on a tripod, or using live view)

and does anyone ever make use of the view finder cover?