Every camera in Sony's Cinema Line has the ability to shoot at 2 different base sensitivity levels.
On Sony Venice, the FX9 and the FX30 the sensors used have two different operating modes that deliver 2 distinctly different sensitivities to light. In each mode the sensors retain almost identical levels of noise and dynamic range. This allows a film maker using these cameras to be able to switch between either of the dual base ISO's according to the light levels they are working under and achieve near identical results with either (there is generaly just a touch more noise at the higher level). The gap between the upper and lower base ISO tends to be between 2 and 3 stops depending on the camera you are using.
Venice 1 is 500/2500 ISO
Venice 2 with the 8K sensor is 800/3200 ISO
FX9 is 800/4000 ISO
FX30 is 800/2500 ISO
Sony's FX3 and FX6 cameras have a slightly different approach where the camera offers 2 distinctly different base sensitivity ranges, Sony don't refer to this as Dual Base ISO but instead call it dual sensitivity. I'm not exactly sure of how this is achieved, but it is likely a combination of adjustments to the way the sensor operates, it's gain and the processing applied to the sensor output. What it delivers is two different sensitivity ranges with a 4 stop gap between the upper and lower range.
With S-Log3 on the FX3 and FX6 you get 800 ISO and 12,800 ISO.
Unlike dual base ISO there is a noticable difference in the noise between these two sensitivity levels, but the noise difference is nowhere near as large as it would be if you started at 800 ISO and added a whopping 24dB of gain to get up to 12,800 ISO. There is also a small reduction to the dynamic range at the upper base ISO.
So, is one better than the other? Well, it depends on what you need. If you need a camera to shoot in extremely low light then the dual sensitivity mode of the FX3 and FX6 means you can get some remakable performance in very low light levels. But if you are shooting a drama, performance or similar sometimes the 12,800 ISO from an FX3 or FX6 will be a bit too noisy or simply more sensitivity than you really need, so you might end up shooting at 3200 or 6400 EI which will result in much less noise but will reduce your highlight range. For this kind of application Venice or the FX9 with it's dual base ISO system might be better.
The new FX30 has a dual base ISO sensor and this needs to be considered when comparing low light performance simply by looking at the specifications. On paper it may appear that the FX3 with its upper high base sensitivity of 12,800 will always perform significantly better than the FX30 where the upper base ISO is 2500. But the FX30 at 2500 produces a very low noise image while the FX3 at 12,800 is more noisy. The reality is that while the FX3 and FX6 will remain the better choice for extreme low light the FX30 really isn't that far behind and performs very well in low light.
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