The film sensitivity test determines your true film speed for your equipment. Once that is determined, then you expose your film to the 18% gray card, devlope the film, make a print, if it matches the gray card that is your development time (N time). If not, development time needs to be adjusted. Without a densitometer it can be done but it is real time consuming. With a densitometer it's a very simple procedure.(posted 8467 days ago)