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.
A larger image sensor has larger pixels with better light signal to noise (S/N) ratio than a small sensor has, and produces “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.
In theory, two same-sized sensors should exhibit the same noise performance. However, if one of them has less resolution and its pixels are larger, then the sensor with the less resolution and larger pixels should theoretically perform better in low light situations and give better noise performance. However, in real practical terms, that difference might not always be apparent to your eyes when viewing an image on a large screen or printed in large format.
Trivia
ISO is derived from the Greek isos, meaning “equal“.
The International Organization for Standardization chose this short all-purpose name instead of using its acronym “IOS” so that in 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.