The foundation of photographic exposure rests on three interconnected variables: aperture, shutter speed, and ISO. Aperture is the size of the opening in the lens, measured by f-numbers such as f/1.4, f/2.8, or f/16. A smaller f-number corresponds to a larger physical opening, which lets in more light and produces a shallower depth of field. The f-number itself expresses the ratio of the lens's focal length to the diameter of its entrance pupil, so f/2.8 on a 50mm lens corresponds to a physical aperture of roughly 17.9 mm. Cinema lenses instead use T-stops, which account for the actual light lost passing through the glass and ensure matched exposure between lenses.
Shutter speed controls how long the sensor is exposed, with each full stop halving or doubling the time: 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 second, and so on. Bulb mode extends exposure beyond the camera's built-in limit of around 30 seconds, holding the shutter open as long as the release is pressed. ISO controls the sensor's sensitivity, and doubling ISO doubles the signal amplification; ISO 6400 amplifies 64 times more than ISO 100, which is why higher settings produce visible luminance and chroma noise. The "native" ISO of a sensor is the setting where the analog signal is read directly without software amplification, typically ISO 100 or 200.
The three variables are linked by the principle of reciprocity: halving the light with one setting can be compensated by doubling it with another. A classic shortcut is the "sunny 16" rule, which says that on a bright day you can set the aperture to f/16 and the shutter speed to 1/ISO for a correct exposure. For the moon, the "looney 11" rule suggests f/11 at 1/ISO, assuming a clear night and a full or near-full moon. One of the most common working rules is the 1/focal length guideline: to handhold a 200mm lens sharply without stabilization, use at least 1/200 second. Modern in-body image stabilization can extend this by three to five stops, compensating for angular handshake but never for subject motion itself.
Although stopping down the aperture increases depth of field, it does not always sharpen the image. Past a lens's "sweet spot"—often around f/5.6 to f/8 for fast primes—diffraction begins to soften the picture as light bends around the aperture blades. On a 24-megapixel APS-C sensor this softening becomes visible near f/11 to f/13, making f/8 a practical limit. Dynamic range, measured in stops between the deepest shadow with detail and the brightest highlight without clipping, reaches 14 to 15 stops on modern full-frame sensors, while many cameras allow adjustments in one-third-stop increments for fine-tuning.