ISO RATINGS AND FILM SPEED
When selecting a film you will always see a corresponding ISO number. This ISO number is an indication of the sensitivity of the film to light (film speed). The higher the number then the more sensitive your film is to light and the thicker the grain and vice versa.
What does that mean?
If you are shooting somewhere outside on a sunny day, you will need a film with a lower ISO rating (iso 100 is perfect for pictures at the beach).
If you are shooting in a low light scenario, it’s a good idea to go for a higher ISO number, but keep in mind that the higher the ISO is, the more grainy the picture will become.