To have a better Film Speed system, ISO (International Standards Organization) began the setting of a new system by stages. Basing on Sensitometric theory, the Characteristic Curve created by Hurter and Driffield:

From the Speed Point, extending horizontally by 1.3, the Log Exposure value. The corresponding Density of this value must be 0.80 above Speed Point, with a variation not over plus or minus 0.05.

This is the requirement of ISO, if any film failed to meet, it either required a modification or the developer has to change, until meeting the rule. Then follows the ISO listing of film speed, in a format like: ISO100/21, for general public it looks almost the same as ASA100/DIN21.

For photographing, it has nothing to change, just take the ISO100/21 as ASA 100. This ISO standard replacing ASA and DIN has a tremendous effect, but it is for manufacturing only. It does not changed ASA and DIN, instead ISO combines the best part of two, making it a true international standard.

Basing on our finding, after the ISO standard, Kodak Professional Films have no difference between the expected Film Speed and their test result, later there are no such printing at all. In reality, photos taken by most people show that a better exposure result is existed in general. This is the factual outcome of the ISO.