Film Speed definition

The measurement of the sensitivity of light needed for the necessary exposure of a particular film stock.  It is mainly the result of the size of the silver halides within the emulsion.  The standard rule is that the bigger the grain, the less light becomes necessary for exposure.  Most of the time film stocks are referred to as being slow or fast.  A slow film contains tinier grains and it needs more light.  And fast film contains bigger grains and requires less light.