Film speed is a number that represents the film’s sensitively to light. The higher the number the more sensitive to light, in that the less light is needed to take a well exposed photo. The number is also an indicator of the detail you will receive from the negative. The higher the number the more likely that you’ll see a graininess to the print when enlarged. Film speed goes from 25 to 1600 speed film.
25 to 200 Best for still life and portrait work, in studio conditions where the lighting is controlled. This is not the film for family shots indoors even with a camera mounted flash. You’d really need a complete lighting set up to use this film effectively. 200 speed film is very good for outdoor sunny conditions when you’re trying to get a shot of a beautiful landscape. It offers excellent detail and color saturation.
400 Considered the all purpose film. Most films touted as all subject or general purpose are really 400 speed film. When in doubt use 400 speed film. Though you may still be using your camera mounted flash in room lighting conditions. Also good for outdoor conditions, will give you some flexibility in darker conditions and where you are trying to capture a moving subject.
800 to 1200 Made for capturing fast moving subjects in all types of lighting situations. People running, playing ball, etc. This is the film you want if you want to freeze frame the action of a baseball game. This film speed can be used for capturing fast moving wildlife, like birds, but you will see less detail if you enlarge above a 16 by 20 size.
1600 This film is for super high speed shots. Unless you shooting a car or boat race you probably won’t need this film. Don’t use this for nature and landscape images the lack of detail will be obvious in enlargements.
Most of the time you’ll only need a 400 speed film for basic snapshots. But it doesn’t hurt to use the other speeds for special occasions, you’ll notice a difference.