Well speaking historically (1800's), Photographers would have a hat with a hole cut in the top. They would hang the hat on the lens, open the shutter, pull the slide,wait till everything settled- then move the hole up to the lens to do the exposure. After the exposure, they would move the hat back down, put in the slide, close the shutter. Of course, many an Photo-artist was accused of have a hole in the hat as an insult.