What is the definition of film speed?

E.g., ISO or ASA100 is wonderful for using when there is a good daylight or when you have bright light.

ISO400 is what you would use to photograph when there is poor light, like when you are indoors, or at dusk or twilight.

Then there are films which have ISO1600 or ISO3200, which you would use at night...

Essentially these numbers, mean that a particular film can gather light very fast or very slowly.

ISO100, absorbs light slowly, so it is used in the daytime, or with bright lights or flashes and strobes.

ISO 3200 can gather light very fast, so it is used at night, or with very low lighting levels.

But these have a trade-off, in that, high ISO films give you grainier photos, than low ISOfilms.

You neeed to use a fast shutter speed to freeze any sort of action when you are photographing sports, action or even fast moving wildlife or birds. Usually well over 1/500th of a second. You may need to change your ISO setting to 400 to 800 if you want to use high shutter speeds, on not-so-bright days, that are overcast, cloudy or without sunlight.

First answer by Ronniejacob. Last edit by Ronniejacob. Contributor trust: 32 [recommend contributor]. Question popularity: 1 [recommend question]