When you take a photograph the subject that you see through the viewfinder is recorded on the film during the brief moment that the shutter opens to let in light through the lens. The film is coated with an emulsion that is chemically affected by light. ‘Fast’ films are more sensitive to the light than ‘slow’ films, so can be used in duller conditions. The speed of the film is indicated on the box and the spool by the ISO number. The higher the number, the faster the speed.
The camera lens concentrates light from the subject of the photograph and projects an inverted image of it onto the film at the back of the camera.
The diaphragm has overlapping leaves that form an aperture which can be made larger or smaller. A big aperture lets more light enter the camera.
A common type of shutter has two blades that open to form a slit that crosses the film. The smaller the slit, the faster the shutter speed.
Picture Credit : Google