Film speed ASA/ISO
Sensitivity to light affects how quickly film exposes and image quality
Film speed refers to the sensitivity of film to light. In general, films with a higher sensitivity (larger number, fast) have coarser grain and do not register detail or reproduce enlargements as well as films with lower sensitivity (lower number, slow). Slower film speeds equate to better detail and sharpness in your photographs, but need to be exposed for longer than faster film speeds.
Film sensitivity is measured by a set of standards established by the American Standards Association (ASA) and the International Standards Organization (ISO). For all practical purposes, ASA and ISO numbers are interchangeable. A typical number series for film speed, from slow to fast, is:
25 50 100 200 400 800
Each stop on this scale is twice as sensitive to the number on the left, and half as sensitive as the number to the right. N.B. Remember to set the camera for the same number that is on the film to get the correct exposure readings from your light meter.