35mm Camera and Lenses:

35mm cameras are divided into two broad categories depending on the mechanism used for viewing the subject through the camera: Point and shoot (or compact) and Single Lens Reflex (SLR) cameras. 

In compact cameras, the subject is viewed through a device  instead of the lens (placed above the lens).

Compact Camera

In SLR cameras, the subject is viewed through the lens with the help of a mirror and pentaprism.

Single Lens Reflex Camera

Rays of light from the subject gets reflected in the mirror and then in the surfaces of the prism as shown above and comes out through the  eyepiece. At the time of taking the picture, the mirror moves up, shutter opens and hence light rays can fall on the film.

The device used for viewing the subject through the camera is called the viewfinder. As described above different kinds of viewfinders are used in compact and SLR cameras.

A lens is used to gather and focus reflected rays from a subject on a recording medium known as film. The amount of light/reflected rays passing through the lens can be controlled by controlling the size of the aperture.

           Aperture - Closed                                  Aperture - Little bit open

The aperture is placed in the middle of the glass elements of a lens. There is another mechanism known as shutter which is used for controlling the amount of light/reflected rays passing through the lens and also the precise moment at which the picture is to be taken. In SLR cameras the shutters are in the focal plane whereas in compact cameras, it is in the middle of the lens or near the lens.