Yestarday i found some time to finally do some tests iīve been wanting to perform since the red one was out. And iīm talking about ISO, dynamic range, zone system and exposure.
Now, in short, the RED ONE has an ISO setting (well, it says ASA, but lets say ISO to get an international agreetment rather than just American, allthoug they are the same). This ISO setting does work like if you were increasing the sensitivity of the sensor, meaning, being able to see in darker places. However this is not really what is happening in camera and definately we are not recording a brighter image at all.
In order to understand ISO setting we must have in mind what a zone system is. Thatīs all. Now according to my tests the cameraīs dynamic range is something like this (at build12):
This means that at 320ISO at camera we are getting up to +4 stops above 18% grey and -6 stops under. I can see a bit more detail below -6 but it is so full of noise that we would never want to use it, so we will set our black (0,0,0) at that point.
The most important thing here is to understand that changing the ISO will change the position of the 18% grey in the system zone. It does not affect the image itself. It doesnīt make the image brighter, it just looks brighter (call it LUT if you want). By setting the 18% grey one position to the right of our system zone we are seeing previously darker areas brighter now. And also we are affecting somehow the dynamic range inside the image (more precisely the stops over and under the mid gray), since we are trading highlights vs shadows.
So at 640ISO we should have +5 stops obove mid gray and -5 stops under it.
Now, what is the trade off? Well, it seems that the less light that hits the sensor the more noise those areas will have. Meaning that dark greys in our zone system are going to have some noise. IF we leave those dark greys just as "dark grey", we get a pretty clean image (250 or 320ISO). But if we bring those pixels up to a higher zone (ie: from I to III) by changing ISO (and therefore affecting the position of 18% grey), the noise is enhanced and very visible (the noise was there since the beggining but we were not able to see it).
So does the ISO setting affect the exposure of the image? No. It just plays within the dynamic range of the image to make darker areas brighter. Ok, ok, ok, this is not new for many people. Let me go on to the really interesting part.
A couple months ago, Stu Maschwitz (prolost.com) made a very interesting description of what was the best way of exposing in cinema digital cameras in order to get the best dynamic range out of them. It was all academic since i think he had not time to play with a RED ONE as much as he would have liked.