In a nutshell, 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; this is how the cinematographer paints the film.
An image is said to represent normal exposure when it is similar to what the eye sees. Overexposure happens when too much light is reflected into the camera, rendering an image that is brighter than normal exposure. Underexposure 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 aren’t. If you’re shooting under daylight, obviously you cannot turn off or dim the sun; ergo you must either adjust the camera settings to create the exposure desired or move to a different location. Another possibility is flagging sunlight. If you shoot in a studio, then you have absolute control over lights.
Notice that ideal exposure is subjective because exposure is creative, 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.
Four factors regulate exposure:
- Amount of light. The sun obviously produces more light than any light bulb known to men. If the camera produces an image that looks normal under bright sunlight, it will – with the same settings – produce underexposure in a scene lit by a dim tungsten bulb.
- Aperture. The opening in the camera lens has the capability to block or admit light. The narrower the aperture, less light enters the camera, thus creating dark images.
- Shutter Speed. The plate located between the lens and the camera recording surface rotates at an adjustable speed and determines the time that each frame is exposed to light.
- Recording surface sensitivity. The imaging device is the part of the camera that is hit by light. In a video camera, the recording surface is a CCD chip, where the image is first created. In a film camera, the recording surface is the actual film stock. When the CCD chip or film stock is sensitive to light, then they are more likely to produce bright images with small amount of light.
Controlling these factors is absolutely fundamental to create optimum photographic imagery.