VIEWFINDER

Besides SLR cameras there are also 'point-and-shoot' cameras. The main difference between those two is the way you get to see what you are shooting.

With a 'point-and-shoot' camera the scene through the viewfinder will not correspond with the image through the lens and on the film. The viewfinder in a 'point-and-shoot' camera is just a simple window through the body of the camera. You only get an idea how your end result will look like.

'point-and-shoot' camera

The scene through the viewfinder of SLR cameras is the same image as the one through your lens and on your film. The mechanism behind this can be compared with a periscope of a submarine. The camera has a slanted mirror positioned between the shutter and the lens, with a piece of translucent glass and a prism positioned above it. The prism makes sure that the image is flipped to it's normal position (side up).

SLR camera

When you press the shutter button, the mirror will quickly change his position. This way, the light beams are now directed to the film. The mirror will only come back when the shutter is back shut. This is why the viewfinder is blacked out during the shot.