The manufacturer's rated film speed is based on lab conditions for scenes of normal brightness. It is when we deviate from these conditions that we need to change the effective film speed - i.e. we increase or decrease exposure and match development accordingly. In addition, we are also trying to match the negative to the end result - the paper we shall be using. The kind of paper we will be using also determines effective film speed. More variables, more confusion. Test, test, test! Eventually you will find yourself saying "One Scene, One Shot".