Definition
ISO is the camera setting that controls how much the sensor's signal is amplified. Doubling it, from ISO 100 to 200 or 800 to 1600, brightens the image by one stop without changing aperture or shutter speed. The cost is noise: higher ISO values bring more grain and less fine detail, especially in the shadows.
Indoors at f/2.8 and 1/125s, the meter asks for ISO 3200. Dropping to ISO 800 is two stops darker (3200 to 1600 to 800), so to keep the same brightness the shutter has to slow to 1/30s, which is slow enough for a moving person to blur. That is why ISO usually rises before the shutter drops.
Calculate it for your own shot with the Exposure Calculator.
Also used in: Lens & Settings Recommender, EXIF Viewer
ISO is the setting a photographer reaches for once aperture and shutter speed are already spoken for. At a wedding reception, the aperture is at f/2 for light and the shutter cannot fall below 1/100s without the first dance going soft, so ISO is the only lever left and it may sit at 6400 all evening. Knowing your camera's usable ceiling before that night matters.
On many recent full-frame bodies, files stay clean to ISO 6400 and hold up to 12800, then come apart above that as shadow noise turns to coloured blotches. Deliberately underexposing at a high ISO to protect highlights, then lifting later, usually looks worse than exposing correctly in camera, because noise lives in the shadows and raising them amplifies it. A photographer who knows that limit sets it on purpose, rather than finding it in a folder of muddy files the next morning.
The same 50mm f/1.8 lens shoots a portrait outdoors at ISO 100, 1/500s, f/2.8, and the file is clean enough to print at any size. That evening, indoors under a single lamp, holding f/2.8 and a safe 1/125s needs ISO 6400. At normal viewing size the shot still looks fine, but the shadows under the chin and in the hair show fine luminance grain and a slight loss of colour depth.
Push the same room to ISO 25600 to try for 1/500s instead, and the noise turns obvious across the whole frame, with dark areas breaking into red and green speckle. Noise reduction can smooth that, but only by also softening eyelashes and fabric weave, so the real cost of the high ISO is paid in fine detail rather than brightness.
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