Comparing ISO on different cameras

[Samuel H]

ISO values don't mean the same on all cameras. Case in point: ISO 12800 on the Canon C300 looks actually darker than ISO 6400 (+24db) on an FS100. So I downloaded the very excellent Philip Bloom's Christmas 2011 shotout, loaded the "low light tests" into the editor, and looked at the waveform monitor. Here you can see a vertical crop of such waveform (corresponding to the area where the guy's face is) for each camera and sensitivity setting:
http://www.similaar.com/foto/iso/pbtest.html

 

[joe 1008]

Your proposition to push all the footage to match the brightest one sounds promising. That way maybe an objective ISO comparision would be possible.