I realise that this may be unnecessarily technical, but
frustrated by the lack of definitions of ISO useful to a cinematographer, I
went and worked one out myself:
The ISO scale is calibrated so that at 25fps and T2.8, a
luminance of 14 foot-lamberts (equivalent to a grey card under approximately 84
foot-candles) will be rendered by ISO 100 as Zone V, and ISO changes are
arithmetic, so that a doubling of the number corresponds to an increase of
one stop and a halving of the number corresponds to a decrease of one stop.
We all know what foot-candles and foot-lamberts are, and we have a box that can measure them by making use of photoresistance. Easy. But then that box tells us what ISO, T-stop and shutter speed to use. Whaaaat? Isn't it refreshing to try and make the jump that would otherwise be forever hidden in the depths of the black box?
So bust out your lightmeters and give me some feedback.
