The term exposure refers to the amount of light the camera’s sensor collects when taking a photo. If the image is exposed for too long (overexposed), the photo will be too bright and washed out. If the image isn’t exposed long enough (underexposed) then it will look too dark. Most digital cameras have built-in meters to measure the light coming into the sensor and automatically determine how long the photo should be exposed for.
However, if you understand how exposures work you can experiment, be more creative, and perhaps get better shots.
The two major aspects of exposure are the aperture and shutter speed. The shutter speed is the period of time light is allowed into the sensor. The aperture is the lens size of the opening that lets the light in.
The speed of the shutter is measured in seconds. A fast speed would be 1/1000 of a second and eight seconds would be considered quite slow. The aperture is measured in f/stops.
Lower the number, more light is let in. For instance, speed of f/2.8 means more light enters the sensor than in f/19.
While both of these features control the amount of light that gets to the sensor they also control other features of the image. For example, a fast shutter speed is able to freeze action while a slow speed is able to blur water.
The aperture also controls an image’s depth-of-field, meaning what is and isn’t in focus in the shot. A wide aperture (a lower f/stop) will blur the background while keeping the subject in focus and a narrow aperture (a higher f/stop) will keep everything in the image in focus.
The sensor’s sensitivity can be changed by the ISO speed. Higher the ISO speed, faster the sensor captures the light. Higher ISO speeds are better for dim light situations.