ISO
0x8827Also: ISOSpeedRatings, PhotographicSensitivity
How sensitive the sensor was to light.
ISO measures the camera sensor's sensitivity to light. A low ISO like 100 needs more light but gives the cleanest image; a high ISO like 6400 lets you shoot in the dark but adds visible noise (grain).
In EXIF, ISO is stored as a plain number. Doubling it (100 → 200 → 400) doubles the sensitivity, brightening the exposure by one stop.
The number is a rating, not a measurement. Extended settings such as ISO 50 or ISO 102400 are pushed in software, so two cameras both reporting ISO 6400 can differ by a stop of real sensor gain. Phones that stack several frames report the sensitivity of one frame, not of the picture you end up with.
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