Film ISO, or film speed, is a measure of how sensitive the film is to light. The higher the ISO number, the faster and more light-sensitive the film is.
Current film speeds range between ISO25 and ISO3200. A fast film (Delta 3200) will allow you to shoot in low lighting conditions, or indoors, whereas a slow film (like ISO100 for example) will perform well in bright daylight or when using flash lighting, but will not be light sensitive enough to produce a correctly exposed image in low lighting conditions, unless long exposure is used.
It’s good to keep in mind that film speed also affects the amount of grain visible. Slow films will give you very fine grain images, as the film speed increases so does the grain. High speed films will give you much grainier and grittier images.
35mm film cassettes have a barcode known as a DX code – which stores the information about the film’s ISO and is read by many modern cameras, in which case you won’t need to set it up yourself. Older cameras though will have an ISO dial that you will have to set before shooting.
Many cameras automatically expose the film, or have a built-in light meter to give guidance. If you want to have more control of your exposure a hand-held light meter will be helpful and will tell you exactly what aperture and shutter speed to choose with the film speed you’re using.
As a recommended guide, ISO 100 can be used for outdoor conditions in bright weather and ISO 400 for an overcast day, also good when using flash indoors. If you are shooting at night ISO 800 and above is good to use but don’t forget you may still need a tripod.