In photography, an ISO number is an indication of the sensitivity of the image sensor, where a higher number indicates higher sensitivity.
This is usually expressed as a range, e.g. ISO 100 – 1600.
A higher sensitivity allows us to take pictures in low light without using flash. However, this gain usually comes at a price: as we amplify the light signal, we also amplify the noise signal, and high ISO images are usally more “noisy” than low ISO images.
Noise reduction software can smooth out the noise but it comes at the expense of losing fine detail.
Larger image sensors have larger pixels with better light signal to noise (S/N) ratio, and produce “cleaner” high ISO images.
In the collage above, the picture on the left is taken at ISO 100 and the one on the right at ISO 6400. We say that the picture on the left (ISO 100) is cleaner than the picture on the right (ISO 6400), and conversely that the picture on the right (ISO 6400) is noisier than the picture on the left (ISO 100). In fact, we can barely see any noise in the picture taken at ISO 100 while the picture taken at ISO 6400 is not only noisy, but fine detail has been obliterated.
Derived from the Greek isos, meaning “equal“.
International Organization for Standardization chose this short all-purpose name instead of using its acronym “IOS” so that whatever the country and language, the short form of the organization’s name is always “ISO” (pronounced “eye-so”, though I and many others happily mispronounce it as “eye-s-oh”).
Read our What Is… ISO tutorial.