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°"

That part is rarely used these days, however.

nospam
it's misuse of terminology.

Sandman
Nope.

nospam
f/stops are also a logarithmic scale, with each step 1.4x the previous one (f/1.4, 2, 2.8, 4, 5.6...), versus 2x for iso (100, 200, 400, 800, 1600...).

if you're going to call iso an arithmetic scale, then you must also call f/stops an arithmetic scale.

Sandman
http://www.merriam-webster.com/dictionary/arithmetic%20scale "a scale on which the value of a point corresponds to the number of graduations the point is from the scale's zero"

I.e, a doubling of the value (ISO 100 ->200 ->400) is related to a doubling of the scale (for instance).

http://www.merriam-webster.com/dictionary/logarithmic+scale "a scale on which the actual distance of a point from the scale's zero is proportional to the logarithm of the corresponding scale number rather than to the number itself"

I.e. a step in the value (DIN 1 ->2 ->3) corresponds to a percentage of the scale.

And yes, f-stops are logarithmic and adheres to this, where each step (f1.4 -> f2 ->f2.8) corresponds to a percentage of the scale.