Exposure refers to the total amount of light that reaches the sensor, and it determines whether an image is too bright, too dark, or properly balanced. The three fundamental elements that control exposure are aperture, shutter speed, and ISO, often called the exposure triangle. These three settings work together, so changing one generally requires adjusting another to maintain the same overall brightness.
Aperture is the adjustable opening inside the lens that controls how much light enters the camera. Its size is measured in f-stops, where smaller numbers such as f/2.8 represent wider openings that admit more light, while larger numbers such as f/16 represent narrower openings. Beyond brightness, aperture also controls depth of field: wide apertures produce shallow depth of field with blurred backgrounds, while narrow apertures keep more of the scene in focus.
Shutter speed is the length of time the shutter remains open and is measured in seconds or fractions of a second. Fast shutter speeds, such as 1/1000 s, freeze fast motion, while slow shutter speeds, such as 1/15 s or longer, allow moving subjects to blur. ISO measures the sensor's sensitivity to light. Low values like ISO 100 produce clean images in bright conditions, while high values like ISO 3200 allow shooting in dim light at the cost of increased noise, which appears as grainy artifacts.
Because aperture, shutter speed, and ISO each affect exposure differently, they must be balanced to achieve a properly exposed image. Widening the aperture, slowing the shutter, or raising the ISO all let in more light, but each also affects other qualities: aperture changes depth of field, shutter speed affects motion blur, and ISO changes image noise. The concept of a stop is central to these adjustments, since one stop represents a doubling or halving of light. Moving from f/4 to f/2.8, from 1/100 s to 1/200 s, or from ISO 200 to ISO 400 is each a one-stop change. When exposure is too high, an image is overexposed with bright highlights that may blow out to pure white; when exposure is too low, an image is underexposed with shadows that lose detail.