The lens causes the scenery to be inverted and focused on the film. In order to make the object of a different position a clear image, in addition to the lens itself need correction
As if the aberration, it should also make the material distance, elephant distance to maintain conjugate relations. For this reason, the lens should be able to move around the focus, so the camera should generally have a focusing mechanism.
Different brands of SLR cameras usually use different cards, the following table lists some common lens cards.
Cameras should be fitted with a viewfinder to determine the scope of the scene and facilitate the shooting composition. The viewfinder of modern cameras also has ranging and focusing.
Shutter and aperture
The body that controls exposure--shutter and aperture
In order to adapt to the light and dark different shooting objects, in order to obtain the correct amount of photographic film, must control the length of exposure time and the strength of the lens ray. The camera must set the shutter to control the length of the exposure time and set the aperture to control the amount of light by adjusting the light hole size.