Exposure

The amount of exposure is a matter of the amount of light that strikes the film and the amount of time the light is allowed to strike the film. Therefore the shutter speed and aperture mechanisms work together to control the degree of exposure of the film.

When the shutter speed is reduced the aperture must then be increased in order to compensate for the reduction in light passing through the lens. For example, if the correct exposure was 1/125th second and ‘f’ stop 8 in a given light, the following combinations of shutter speeds and apertures will give the same exposure for the same light level.

The exposure is either measured by an in-built or hand held light meter. These meters will provide an exposure value. These values are not always the ‘correct’ exposure and depending on the lighting conditions may require exposure compensation. If a scene is very bright, the light meter may provide an exposure value that will lead to an image being underexposed and in those circumstances one would have to over expose the film by one ‘stop’. Similarly on overcast days, one may have to under expose the film by one ‘stop’ to compensate for the light meter reading.