Here are a few pics of the 20D chamber and related mechanisms to help people to understand the DSLR mechanism a bit before they buy one.

An SLR camera enables us to look from the viewfinder right through the actual lens. This is done with a mirror that reflects the image directly upwards and then the image passes through a prism which directs the image back horizontally through the viewfinder. Above you can see a Canon 20D DSLR with the lens removed and the chamber therefore open so you can see the mirror.

When you take the photograph the mirror flips up very quickly (and down also later) in order that the sensor can capture the photo. In a basic sense the exposure is controlled by the shutter curtains which you can see in the above photo. These can open and close in a fraction of a second or several seconds as you wish.

In the above photo you see the shutter curtains opened and sensor exposed. Once the exposure is made the shutter curtains will close quickly and the mirror will come down also. So during the exposure you do not see anything in the viewfinder as the mirror is not available to direct the image to the viewfinder. But this can all happen very quickly if your shutter speed is only a fraction of a second.

Of course this is quite an old system, the SLR film cameras had exactly the same type of system and the film would be where the DSLR sensor is. Normally we would not leave a DSLR with the lens or body cap off for more than a short time because unwanted dust will enter the chamber. Especially also with the sensor exposed because that is exactly where we do not want the dust.

Photos from my old site dslrlab which became defunct