What is the difference between film speeds?

I have a film assignment for school and we can use any speed film (100, 400, 800, etc.) what is the difference between film speeds?|||The higher the speed the more light-sensitive the film is, but also the higher likelihood that the picture will be grainy. ASA 1600 film is used for astronomy, for instance. You can compensate for a low speed film by leaving the shutter open for longer (on cameras that have an adjustable shutter speed), but then anything that moves in the image will get blurred. With a faster film / shutter speed, you can take action pictures, but they really only work well in strong light such as daylight.





ASA 400 is a good "general" film that can be used both outside and inside (with a flash). It takes crisp, clear pictures and doesn't lose a lot of detail.|||The lower the ISO, the less sensitive the film is to light. So if conditions are bright %26amp; sunny, then a 100 ISO film will be good as you'll have enough light to expose it with a decent shutter speed and the film grain will be 'fine'. In dimmer conditions, a higher ISO film is more often used, like ISO 400. This is more sensitive to light, so a faster shutter speed can be used than if you were using ISO100 and that helps to prevent blur from using too slow a shutter speed.





Each 'stop' between ISO film speeds (100, 200, 400, 800, 1600) represents a doubling or halving of the light, exactly the same as fir shutter speed or aperture 'stops'.|||Each film speed differs from the next one in your chain of them by a stop.





So if you are shooting 100, to shoot 200 you need to close your aperture 1 stop or increase your shutter speed one notch, say from 1/125 to 1/250. 200 to 400 is another stop and from 400 to 800 is another stop, etc.





there are some films that don't sit right on even numbers, like 125, 320 or 360, but you probably will not be seeing anything other than the 125 is that grouping