Cameras produce best results when subjects are within focus and properly lit. Though they are able to capture images within a fraction of a second, low lighting conditions require additional time. Increasing a camera's ISO (sometimes pronounced "eye-so"), automatically or manually, increases the noise-based speed within a range of acceptable image quality in relation to the number of allowable errant pixels.
Digital camera ISO settings typically advance through four or more of 80, 100, 200, 400, 800, and 1600. Higher ISO values shorten exposure time. With ideal lighting, 80 or 100 ISO provide long enough image exposure for best results. ISO 200 is often used on overcast days, dusk or somewhat fast motion in good lighting. When action speeds up or lighting decreases, a photographer may switch to 400 ISO.
Less expensive cameras employ small image sensors that greatly accentuate granularity, errant pixels, or noise at 400 ISO or more. Telephoto lenses amplify camera instability and reduce the amount of light reaching the sensor, requiring a high ISO. Professional digital cameras include a much larger image sensor and better lenses to sustain image fidelity at 800 ISO or higher.
Noise can be minimized post production (after captured) with professional image editing software.