Exposure is the amount of light reaching the camera sensor. The term “exposure” can also refer to a single image capture. The exposure of a photo is determined by the amount of light from the scene, as well as aperture, shutter speed, and ISO settings.
Exposure value (often abbreviated to EV) is a measurement of exposure that is relative to the aperture and shutter speed settings. For example, a photo with a long shutter speed and low aperture can have a similar EV as a photo with a shorter shutter speed and higher aperture.
When using a camera that shows an EV measurement, one can adjust shutter speed and aperture to obtain a desired effect while maintaining exposure.
Photos that are overexposed will have areas that appear white and have reduced detail. Conversely, an underexposed photo will have areas that appear black and lacking detail. A photograph that is “properly” exposed with have a minimum of overexposed and underexposed areas.
Photos can be intentionally over exposed or under exposed purposely to achieve a specific effect. For example, overexposing a photo to wash out whites, or underexposing to reduce shadow detail. See Composition.
Digital cameras come equipped with built-in light metering. A light meter adjusts the camera for proper exposure, and usually has options for different methods and locations. The most common metering method is “spot metering”, which adjusts the exposure based on a single spot in the frame, though most cameras include multiple options for metering.