In traditional film photography I.S.O. (or ASA) refers to the sensitivity of a given film’s emulsion to light. In Digital Photography I.S.O. refers to a given range of sensitivity of the image sensor to light.I.S.O. is measured in numbers that double as each grade of film increases in it’s sensitivity or as power is increased to the sensor:

50, 100, 200, 400, 800, 1600, 3200



The size of the Silver Halide Granules in the emulsion dictates the Film Speed and it’s resulting Resolution. The lower the number, the lower the sensitivity of the film, and the finer the grain of the final negative. The Higher the number, the coarser the grain resulting in a lower resolution image but increasing the range of acceptable low light.

Digital I.S.O. variation is acheived by adjusting signal gain to the Image
sensor. By amplifying the electric signal to the sensor an increase in light
sensitivity is obtained. The tradeoff for this increased sensitivity is an increase in the signal to noise ratio.

 Image Sensor Dissection

This Image was shot using a slow, Fine Grain 50 I.S.O. film, allowing a very High Resolution that shows a wide range of value and details in the image, but requires a lot of light.

Example of a Fast Film (1600 I.S.O.) 

Robert Capa 1936 “Death of a loyalist Soldier”

Example of a Fine Grain Film (50 I.S.O.) 

Ansel Adams “Moon and Half Dome ” 1960

This image was shot with a Fast, Coarse Grain 1600 I.S.O. film, notice the lack of resolution and value. Fast film does not require much light.