What is ISO? Picture ©Al Macphee/MiraclePR
ISO (International Organization for Standardization) is a film sensitivity rating, which was also known as ASA (American Standards Association).
The numbers run from low sensitivity to high. In the days of film, your standard kit bag would have several rolls of film: ASA 100, 200, 400, 800, 1600 and 3200. Sometimes you would push your 1600s to 3200 if you had to. You also had your ASA 50 and 25, but they were for studio stuff.
If your shutter speed was 1/125 with ASA 100 film, but you decided you needed a faster shutter speed, you could swap your roll for an ASA 400 and then increase your shutter speed. Nowadays, with digital cameras, ISO is itself a camera setting. If you’ve got a rainy day, you know you’re going to have less light, so you might need to set your ISO to 400.
To find out how ISO relates to shutter speed and aperture, see my post on The Exposure Triangle.
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