How does an SLR camera work?

Answer 1

The single-lens reflex (SLR) is a type of camera that uses a movable mirror placed between the lens and the film to project the image seen through the lens to a matte focusing screen. The shutter in almost all contemporary SLRs sits just in front of the focal plane. If it does not, some other mechanism is required to ensure that no light reaches the film between exposures.
In SLR camera, you see the actual real image that the film will see. If you take the lens off of an SLR camera and look inside, you'll see how this works. 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. This configuration works like a periscope -- the real image bounces off the lower mirror on to the translucent glass, which serves as a projection screen. The prism's job is to flip the image on the screen, so it appears right side up again, and redirect it on to the viewfinder window.
When you click the shutter button, the camera quickly switches the mirror out of the way, so the image is directed at the exposed film. The mirror is connected to the shutter timer system, so it stays open as long as the shutter is open. This is why the viewfinder is suddenly blacked out when you take a picture. The mirror in an SLR camera directs the real image to the viewfinder. When you hit the shutter button, the mirror flips up so the real image is projected onto the film

Answer 2

There's a mirror between the lens and the shutter, that sits at a 45 degree angle. With the mirror down the light coming through the lens is reflected up into the viewfinder, so you can see exactly what's coming through the lens.

When you push the shutter release, several things happen. First, the lens "stops down" to wherever it's set, so you get the correct exposure. Second, the mirror flips up out of the way. Last but not least, the shutter opens, and the film is exposed.

The shutter in most SLRs consists of a pair of cloth panels. The first one slides open, exposing the film, and the second then moves in the same direction, but this one covers the film again. For very fast exposures, the second curtain starts moving right after the first one, so you essentially have a slit that moves across the frame.