I is for ISO
ISO is a measure the sensitivity of film to light, or “film speed.” Higher ISO films are more sensitive to light and therefore better for taking photos in low-light conditions. (However, as noted in an earlier post, more sensitive films tend to have larger grain.)
ISO stands for the International Organization for Standardization, located in Geneva, Switzerland. While many different classifications for film speed have been used (e.g. DIN and ASA), in 1974 the universal ISO system became the standard.
Typical film speeds are 100, 200, and 400. ISO 100 is best used under sunny conditions. ISO 400 (or the even faster ISO 1600) are best used in low-light conditions like indoors or outdoors at night. ISO 200 is a general purpose film that does well in sunlight and in the dark.