One thing that’s becoming very clear with the on-going FS5 discussions is that the use of ISO with video camera is confusing the hell out of people. ISO is an almost meaningless sensitivity measure for a video camera. Especially with a video camera that uses different types of gamma curves. The ISO rating is there so that when you use an external light meter you will get the correct exposure, but it doesn’t necessarily tell you a great deal about the cameras actual sensitivity.
Take a Sony S-log camera. If you set the camera to 0dB gain and expose white card or piece of paper using standard gamma correctly (white at 90%), then still at 0dB gain switch to S-log does the piece of paper get brighter? The answer is NO it does not. White will drop to about 70% depending on the log curve chosen. That should be telling you that the actual sensitivity of the camera is NOT changing. If you set the camera to ISO, the ISO indication will change from 1000 ISO to 3200 ISO when you switch between standard gamma and log on an FS5 for example, but these are both 0dB gain and the picture brightness does not significantly change when you switch between standard gamma and log (the picture does become flatter, don’t confuse this for “brighter”. Use a waveform or histogram and measure the levels of a fully exposed scene).
If in both cases, S-Log and Standard Gamma, the gain is 0dB, even thought the ISO rating is changing, the actual sensitivity of the camera is not changing, the only thing that’s changing is the shape of the gamma curve.
The ISO rating changes because for normal gammas you would expose white at 90% while for log it is exposed darker at 60%. So if using an external light meter you want the light meter to tell you to close the aperture relative to standard gammas so that white ends up at 60% (the lower white level makes space for more dynamic range above white).
When you change your external light meter from 1000 ISO to 3200 ISO the light meter will tell you to close the aperture by a stop and a half and that will give you that correct and now darker log exposure.
This is why I hate the use of ISO in video cameras because it is miss-leading people. The FS5 (and the same applies to the FS7, F5, F55 etc) isn’t actually any more sensitive to light in log than in standard gammas (video camera sensitivity is governed by the sensors efficiency at converting light into electrons and we cannot change the sensor). Neither does it have any more gain at 3200 ISO in log compared to 1000 ISO in standard gammas. Both are 0dB. That’s why when you shoot at 3200 ISO in Log you don’t get any more noise and you don’t get any additional NR artefacts…. because the camera isn’t actually becoming any more sensitive and it’s not adding any extra gain, it’s just a rating change to make sure an external light meter would give you the correct log exposure. If people actually went back and used the correct and appropriate terms for video cameras: ie: dB we wouldn’t have all this stupid confusion, it would be obvious that 0dB = 0dB. The gain and thus sensitivity in standard gammas with 0dB is actually the same as in Log at 0dB. If people actually took the time to look at their monitors it would also be pretty obvious that the sensitivity does not change.
Sadly all this is resulting in some pretty ridiculous statements like: I use log in low light because it’s the only way I can get 3200 ISO without adding extra noise. The reality is that the only thing that’s really changing is the little number on the viewfinder overlay, in standard gamma it says 1000, in log it says 3200. But the sensitivity and noise levels are not changing. S-Log3 does give raised shadows, but those raised shadows will show more noise due to the raised shadow levels, but for a like-for-like brightness they are no more noisy than any other gamma. Using the wrong terminology or miss-understanding how a video camera works is resulting in very silly mistakes. Once upon a time video camera operators used to rely on monitors and waveform displays to get exposure and gain levels right. But this isn’t cool anymore because we are all now “Directors of Photography” and DP’s don’t use gain or shutter speeds they use ISO and angles. This is nonsense and the desire to sound hip and cool is resulting in poor or miss-guided camera operation.