Quote Originally Posted by Catlady711 View Post

I can't be sure what ISO stands for anymore without googling it ...
From Wikipedia, the free encyclopedia -

ISO film speed scales

International Standard ISO 5800:1987 from the
International Organization for Standardization (ISO) defines both an arithmetic scale
and a logarithmic scale for measuring color-negative film speed. Related standards
ISO 6:1993 and ISO 2240:2003 define scales for speeds
of black-and-white negative film and color reversal film.

In the ISO arithmetic scale, which corresponds to the older ASA scale, doubling
the speed of a film (that is, halving the amount of light that is necessary to expose the film)
implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale,
which corresponds to the older DIN scale, doubling the speed of a film implies
adding 3° to the numeric value that designates the film speed.
For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.

To continue with a really *serious* technical discussion - go to >>>
http://en.wikipedia.org/wiki/Film_speed

/s/ Phred