The correct exposure for a photograph is determined by the sensitivity of the film used. Film sensitivity is referred to as 'speed' and is measured as an ISO rating. Faster film requires less exposure and has a higher ISO rating. Exposure is controlled in a camera by shutter speed and lens aperture. Slower shutter speeds and greater lens apertures produce greater exposures.
The Zone System is another method of determining exposure.
Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-in light meter.