Sandman
That said, ISO *has* a logarithmic part, inherited from DIN, where each step is one third of a double in sensitivity. Hence the ISO 100/21, ISO 200/24 etc.

nospam
iso is a logarithmic scale, with each step double the previous, or log base 2. din is an arithmetic scale, incrementing by 1 each step. it's 7th grade math. with din, each step represents a doubling of sensitivity,

Sandman
<https://en.wikipedia.org/wiki/Film_speed#Current_system:_ISO>

"The ISO system defines both an arithmetic and a logarithmic scale. The arithmetic ISO scale corresponds to the arithmetic ASA system, where a doubling of film sensitivity is represented by a doubling of the numerical film speed value. In the logarithmic ISO scale, which corresponds to the DIN scale, adding 3° to the numerical value constitutes a doubling of sensitivity. For example, a film rated ISO 200/24° is twice as sensitive as one rated ISO 100/21°"