33. The higher the number the more sensitive it is and consequently the less light it needs to form an image. The lower the number the less sensitive it is and the more light it will require. Sensitive films are said to be 'fast' and will have a speed of 400 ASA/ISO or above. Films with low sensitivity are said to be 'slow' and will have a speed of less than 100 ASA/ISO. General purpose films suitable for everyday use fall into the 100-400 ASA/ISO range with 100-200 being the most popular