Nowadays when we refer to the index of the photographic sensitivity of a film, photosensitive surface or of a sensor, we speak of the ISO. Not everyone will know that ISO means International Standard Office, but even less, especially those who have little time in this photography and if they have only shot digitally they will know that ISO is something new.
In the past, the ISO sensitivity values were known as DIN (Deutsche Industrie Normen), and later it was renamed ASA (American Standard Association). The ASA and ISO values are identical, just changed the name, but being handled in DIN, things were different, because when the sensitivity is doubled, the DIN value increases by three units, while in the ASA and ISO values it is multiplied by two.
Then you have the equivalences between ISO-ASA and DIN.
and so on
As a curiosity to say that in the Soviet bloc used a different scale of sensitivities, called GOST (Gosudarstvenny Standart meaning state standard) that remained until 1987. The level ISO-ASA / GOST is this:
and so on
I hope you have found it curious this brief review that we have given to one of the most critical pillars of photography.