Remember when you used to buy film, and it came in different “speeds” — ISO 200, 400, 800, and so on? Well, those numbers indicate the film’s sensitivity to light; the rating system was devised by the International Standards Organization, thus the ISO abbreviation. The higher the number, the “faster” the film, meaning less light is needed to expose the picture. So a high ISO film lets you use a smaller aperture (higher f-stop setting) or faster shutter speed than if you use a low ISO film. (If that last sentence sounded like gibberish, check out my previous posts, which provide the fundamentals of exposure.)