ISO is a property of recording photography; it represents sensitivity to capture light. Before the days of digital cameras, all photographic film was categorized with an ISO rating. Digital cameras have an adjustable ISO. Increasing digital ISO is primarily a matter of increasing the gain of the electronics signal. As with most electronics, gain increases not just the signal but also signal noise (stuff you don’t want). However, many digital cameras today have excellent signal-to-noise ratio.
In the days of film cameras, photographers might “push process” film to achieve higher ISO. ISO 400 was commonly pushed to ISO 800; that’s a difference of 1 f-stop.
Today, using digital cameras, 1-stop is child’s play. Digital cameras can provide 6 f-stops or more …. ISO 51,200 or better. The results can be cleaner (less noise) than the old ISO 400 film pushed to 800.
I shot the image here at ISO 5000 … that was ten years ago.
(Intonition Band, at night under a tent at Fraklin County Fair)