Aperture refers to the size of the opening inside the lens through which light enters the camera. It is measured in f-stops such as f/1.4, f/2.8, f/4, f/8, and f/16, where lower numbers correspond to larger openings and therefore more light reaching the sensor. Aperture also has a powerful creative effect on depth of field, the range of distance in a scene that appears acceptably sharp. A wide aperture (low f-number) produces a shallow depth of field, isolating the subject against a blurred background, a look favored in portraits. A narrow aperture (high f-number) produces a deep depth of field, where both foreground and background are sharp, ideal for landscapes.
Shutter speed describes how long the sensor is exposed to light, measured in fractions of a second (such as 1/1000 or 1/60) or in full seconds for long exposures. The shutter governs both exposure and the rendering of motion: fast shutter speeds freeze action, which is typical for sports where 1/1000s or faster is common, while slow shutter speeds introduce blur, whether intentional or as a result of camera shake. As a rule of thumb for handheld shooting, the shutter should be at least as fast as the reciprocal of the focal length, for example 1/100s for a 100mm lens, to keep the camera steady enough to avoid blur.
ISO represents the sensor's sensitivity to light. Modern cameras offer ISO values typically ranging from 100 (low noise) to 25,600 or beyond (with greater digital noise). The sweet spot for image quality usually lies between 100 and 400, though newer bodies can produce clean results at ISO 3200 or higher. Because each of these three settings has trade-offs — aperture affects depth of field, shutter affects motion, and ISO affects noise — the photographer's task is to balance them for the desired exposure and creative effect.