nospam
it's misuse of terminology.

Sandman
Nope.

nospam
it definitely is.

iso and f/stops are a logarithmic scale, which you confirm below.

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.

<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°"

<https://books.google.se/books? id=3VEdAAAAQBAJ&pg=PA305&lpg=PA305&dq=iso+logarithmic+arithmetic&source=bl&ot s=BVxb3MFc7k&sig=NuaNPm14QivoSkXtfJuZO66WThU&hl=en&sa=X&ved

AhWCJXIKHZX-DDcQ6AEIOTAD#v=onepage&q=iso%20logarithmic%20arithmetic&fúlse>

<https://books.google.se/books? id¶NJcpMFJzAC&pg=PA69&lpg=PA69&dq=iso+logarithmic+arithmetic&source=bl&ots>6Bid-4-PRm&sig=3lfbCQxzcKLi8DC- 2LIPTksYhhY&hl=en&sa=X&ved

DDcQ6AEIPjAE#v=onepage&q=iso%20logarithmic%20arithmetic&fúlse>

<https://books.google.se/books? idåmC5TXlBw8C&pg=PA114&lpg=PA114&dq=iso+logarithmic+arithmetic&source=bl&ot s=txe_hlIFCP&sig=L4Tp_UGNLOcTLNPnvs8DFelvEOE&hl=en&sa=X&ved

AhWCJXIKHZX-DDcQ6AEISDAH#v=onepage&q=iso%20logarithmic%20arithmetic&fúlse>

<http://dpanswers.com/content/tech_iso.php> "The sensitivity scale defined by ISO actually defines two parallel scales, one linear (arithmetic) scale and one logarithmic scale. This is because the ISO sensitivity scale was created in 1987 by merging two older scales known as ?ASA? and ?DIN?. The ISO linear scale corresponds to the older ?ASA? scale, and the ISO logarithmic scale corresponds to the older ?DIN? scale."

<http://camerapedia.wikia.com/wiki/Film_speed> "With an arithmetic scale, the exposure decreases in direct proportion to an increased speed - so a doubling in film speed requires half the exposure. For example if an exposure of 1/250s at f.8 is required for a 100 ISO film, a 200 ISO film would require either 1/500s at f.8, (or 1/250s at f.11) and 400 ISO film would need 1/1000s at f.8 for the same scene.

A logarithmic scale increases a fixed amount for a doubling of speed - 24 DIN is twice as fast as 21 DIN, and 27 DIN is four times as fast as 21."