The use of negative gain reduces dynamic range.

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alisterchapman
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The use of negative gain reduces dynamic range.

Post by alisterchapman » July 4th, 2011, 9:34 am

One way to reduce the noise in a video camera image is to reduce the cameras gain. We all know that increasing the gain to lets say +6db will increase noise and generally the reverse holds true when you reduce the gain, the noise typically reduces and this may be helpful if you are going to do a lot of effects work, or just want a clean image.

However in most cases negative gain reduces dynamic range as it will artificially clip your low key parts of the image. The brightness range is restricted by the cameras DSP (Digital Signal Processor), it only has so many bits of data to sample the sensors output, from dark to light. If you reduce the gain the highlight handling doesn't change (DSP bit depth limited) but your entire image get shifted down in brightness and as a result you will clip off some of your shadow and dark information, so your overall dynamic range is also reduced. Dynamic range is not just highlight handling, it is the entire range from dark to light. 3db is half a stop (6db = 1 stop) so -3db gain reduces the dynamic range by half a stop.

So for cameras like the EX1 and EX3 or even PMW-500/PDW-700 using negative gain can be a bad thing to do. You need to be aware that there is a trade off of noise against dynamic range and need to be sure that the small noise benefit are worth the sacrifice of some latitude. The problem is further compounded on the EX1 and EX3 where the use of -3db gain also reduces the peak white recording level by 3db and with Cinegamma 2 this will prevent you from actually reaching peak white (unless you change the gamma curve gain).
Interestingly the PMW-F3 has an excess of dynamic range for the normal gammas and cinegammas and the processing appears to take advantage of this to keep the images very clean. When you shoot with the standard gammas and cinegammas on the F3 the cameras base ISO (sensitivity) is 400 asa at 25p. This gives a very clean, ultra low noise image with 11.5 stops of dynamic range. When you switch the camera to S-Log, which gives a greater dynamic range (approx 13 stops by my estimation) the base ISO increases to 800 asa. Looking at some of my S-Log test footage a clear increase in under exposure latitude can be seen when you use S-Log. I suspect that the "0db" point in the F3 is actually 800 asa as used by S-Log, where maximising dynamic range and using the full sensor range is the priority. Meanwhile with standard gammas, which are limited to 11.5 stops anyway, you can reduce the gain by 6db (1 stop) sacrificing one stop of underexposure but still have the full 11.5 stops but with 6db less noise.
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