Understanding Digital Photography

Exposure

Exposure is the total amount of light captured by the camera while taking a single picture. Two variables affect the exposure in an image. The shutter speed determines the length of time the sensor is activated to capture light, and the aperture determines how much light passes through the lens while the sensor is activated. While there are many combinations of shutter speeds and aperture settings that result in a correct exposure, ideally, there is only one combination for each shutter speed. If there's too much light, the image will appear washed-out and overexposed. If there's not enough light, the image will appear dark and underexposed.
These photos were taken with the camera set for "Manual" mode at the same shutter speed to show the effect of using different aperture openings. It's important to remember that the ISO setting ultimately determines the sensitivity of the camera's sensor. That translates to how much light is needed to make a correct exposure.