Tag Archives: -3db

Why using negative gain can be bad, unless you have an F3.

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 or limit your low key parts of the image. The maximum illumination level that a camera can capture is limited by the sensor or the amount of data used to transfer the signal from the sensor into the processing circuits, the cameras DSP (Digital Signal Processor). The black level or darkest part of the image is the point where the design engineers have deemed that the ratio of actual image signal to sensor noise is high enough to give a suitably noise free image (also known as noise floor). So the dynamic range of the camera is normally the range between the sensors noise floor and saturation point.

The gain of the camera controls the video output level, relative to the sensors signal level. If you use -3db gain you attenuate (reduce) the relative output signal. The highlight handling doesn’t change (governed by the sensor) but your entire image output level gets shifted down in brightness and as a result you will clip off or loose some of your shadow and dark information, so your overall dynamic range is also reduced as you can’t “see” so far into the shadows. 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, reducing the cameras underexposure range.

gain-curves-1 Why using negative gain can be bad, unless you have an F3.

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.
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. In effect the arbitrary black level is kept some way up the sensors output range to keep the images well clear of the noise floor. 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.  When you increase the sensitivity like this you lower you black point lower down the sensors output range closer to the noise floor. 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 and raising the black point well above the noise floor but still have the full 11.5 stops but with 6db less noise.

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