How to measure ISO

In article , Tony Cooper
wrote:

Not a big problem, but the question is which camera has the correct ISO
setting, of if perhaps both cameras deviate from the true ISO 100.

While you may have a point, it would not affect me to know that the
ISO setting in my camera is 100 off or not. I know what I can expect
from a setting, and set the camera accordingly.

for one camera, perhaps, but that number may not be the same on other
cameras.

Trust you to argue by stating the bloody obvious.

trust you to go on another rant, and i'm not arguing.

If you routinely
use two different cameras, you would soon learn to know what the
result is for a particular setting.

maybe so, but if the iso was correct, that would not be needed.