Understanding ISO Ratings and Film Speed
When choosing a film, you'll always see a corresponding ISO number – but what does this mean? The ISO number is an indication of the film speed, which tells you just how sensitive the film is to light. The lower the number, the less sensitive your film is to light and the finer the grain and vice versa. So what does that mean? Well, if you're shooting somewhere with a lot of light e.g. outside on a sunny day, you'll need a film with a lower ISO number (100 ISO is perfect for sunny days!).
However, if you're shooting in low light scenarios, it's much better to go for a higher number; although keep in mind the higher the ISO number is, the higher the chance is that the image could become grainy. The more you shoot, the better you'll be at choosing the right speed!
Learn more about the ISO in Lomography’s Little Lessons!