ISO 50 film is only half as sensitive as ISO 100 film. Generally, the size of the grain in the film affects how quickly light is absorbed. The higher speed films usually have larger films grains, so they can absorb light quicker, and we call that a faster film. These ISO numbers, which used to be referred to as ASA, are just a relative rating system for film stock light sensitivity. So now that you've set the meter properly and are ready to shoot, you might be wondering which meter is best for what, and how to use them.
Incident light is light coming from the source before it strikes a subject or surface.
All other shadow and highlight values that fall within the same main light area also will be accurately reproduced. It's as if there were a hypothetical gray scale in the meter. If you do what the meter tells you to do, any middle gray tone in the scene will be exposed as middle gray, white will be white, and black will expose properly as black. The incident meter is used primarily to meter subject areas. Whether you're shooting film or tape, this meter can be useful to check consistency on a set, such as check the light levels at each chair on a news set or talk show.