Exposure refers to the amount of light being captured by the camera. One of the most basic photographic principles, exposure is directly connected to the brightness and darkness of the image.
An image is said to represent normal exposure when it is similar to what the eye sees. Over exposure happens when too much light is reflected into the camera, rendering an image that is brighter than normal exposure. Under exposure is the opposite: not much light enters the camera, thus creating a dark image.
Creating ideal exposure is often a cumbersome task because some factors are controllable, while others are not. If we are shooting under daylight, obviously we cannot turn off or dim the sun; we must either adjust the camera settings to create the exposure desired or move to a different location. Another possibility is flagging sunlight. If we shoot in a studio, then we have absolute control over lights.
Perfect exposure is subjective because exposure is a creative act. Thus, its properness depends on what is intended by the filmmaker. Underexposure, for instance, is often purposefully used in horror movies to provoke suspense or fright. Subtle overexposure is often used during dream sequences or flashbacks.
Light Meters:
Controlling light is an essential skill for a cinematographer. During usage of film stock, light meters are the only possible way to measure the intensity of light. With that reference, cinematographer controls light and exposure. But today with Digital cinema cameras, usage of light meters has become a rare practice because the cameras offer ‘histogram’ to calculate exposure range.
Histogram:
A histogram is a bar graph that was originally created for statistical analysis. As digital photography was developed, designers incorporated histograms into cameras to provide a graphic representation of the digitally recorded image.
However many cinematographers feel that even today histogram function is not fully understood and the information they provide is often incorrectly used.