All film has a speed rating, whether digital or traditional. You may see this number called ASA or ISO (both indicate the film's rated speed). The speed rating describes how quickly the film reacts to light.
Film speed uses stops, just like shutter and aperture
For example, going from ISO 50 to ISO 200 buys you 2 stops more light. Slower films are less sensitive and generally require longer exposures / more light. Faster films react rapidly, and can be used in low light situations.
lower numbers = slower films = need more light = longer exposures
higher numbers = faster films = need less light = shorter exposures
Film Speed vs. Contrast and Grain
The trade-off between slow and fast films is quality. Slow films generally produce sharper, more detailed images, while faster films often have higher contrast and grain.
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