On 28/10/2015 08:07, Alfred Molon wrote:
I have two cameras. At the same ISO, aperture and exposure time one
takes brighter images than the other one.

How to measure the real ISO values at which the cameras took the
pictures?

Sensorgen has some measured vs claimed ISO data.

Some examples at set ISO 3200, calculated ISO is:

EOS 7DII 2458
EOS 5DIII 2518
EOS 1DX 2423

D5300 2483
D750 2332
D810 2367

OMDEM5 1489
OMDEM10 1663

DMCGH2 2989
DMCGH4 1860

Phase One IQ180 990

A7R 2240
A7S 2627

The general trend seems to be that most overstate ISO by about 1/3 stop.
That's probably sensible (blown highlight protection).

Some are well over that 1/3 stop. Of course there's suspicion it's also
done to "cheat" - to make the cameras look better than they really are
in side by side high ISO comparisons.