However, there is a rumour that the GH3 can record video at one stop more, ISO 12800, by dialling in +3 in exposure compensation. So, is this true? The quick answer is: Yes. But it's a somewhat strange process. I'll take a look here.
In A (aperture priority) mode
What's going on is this: In the Creative Movie mode, A exposure mode, you set the aperture and the ISO, and the camera sets the shutter speed. When setting the ISO to the max, and then dialling in a positive exposure compensation, nothing happens until you reach +2. Then, you get one more stop of gain (light) until you reach +3. So, it might seem like the camera gives you one more stop of ISO for free.
But what you don't see is the shutter speed. I think it is a big problem with the GH3 that it does not state the shutter speed during video recording, except in full manual M mode. This makes the camera much more difficult to use for video recording.
However, for the moment, you can trust me that the camera tries to give you a 180° shutter in video mode, meaning that the shutter is open half the time, or a shutter speed of 1/50s if you are using a 25 fps video mode. Except if you dial in +3 exposure compensation, then it reverts to a 360° shutter, one full stop more exposure. You see this as a significant gain in brightness, if you are video recording in a too dim environment.
This can be easily demonstrated. See the video below in the next section.
In S (shutter priority) mode
Set in a similar setting, where the lightning is not enough at ISO 6400, full aperture, and 1/25s (or 1/30s for NTSC camera), when dialling in +3 exposure compensation, you get around one more stop of brightness, as compared with the maximum exposure in M mode.
Hence, it is true to say that you effectively get around ISO 12800 by using the Creative Movie mode in S (shutter priority) mode and dialling in +3. This only makes sense when the lightning is very dim. Otherwise, you simply get a very overexposed scene, which is not what you want.
Here is a video demonstrating the features of the A and S modes:
Here are also comparisons of the exposure levels in the video. From left to right, we have the A mode, M mode and S mode. We see that when using the Aperture Priority (A) mode with no compensation, it corresponds to 1/50s (180° shutter). When dialling in +3, though, it corresponds to 1/25s exposure (360° shutter). In S mode, though, we get around one stop more gain at +3 exposure compensation, effectively around ISO 12800:
The GH3 behaviour is a bit strange here. In A mode, it runs a 180° shutter by default, i.e., one stop less than the maximum exposure. Only when you dial in +3, you get the full exposure, one stop more. Specifying a 360° shutter manually in M mode gives exactly the same exposure, i.e., there is no "magical" extra sensitivity when dialling in +3.
The feature might still be useful. If it is too dark for ISO 6400, and you want the camera to use a shutter speed as slow as possible, you can set the exposure compensation to +3. You might find that easier than switching to the M exposure mode, where you could do the same.
In S mode, though, you can also dial in +3, and get one extra stop of gain, but at the same shutter speed. This appears to be a way to get around ISO 12800 in video mode. Note that this only works when the lens aperture is set to the maximum. Otherwise, the camera would rather first increase the aperture to achieve the exposure compensation, rather than increase the ISO sensitivity.
If you are stuck in ISO 6400, maximum shutter speed (1/25s in 25fps or 1/30s in 30fps, depending on PAL/NTSC region), and you still need more exposure, what can you do? There is a solution, actually. There is another, semi-hidden feature of the GH3. It can record video at even slower shutter speeds, to capture more light in each frame.
For example, I recorded a video at 1/13s shutter speed at a concert, since I needed one stop more brightness even at ISO 6400. Of course, this gives you only 13 unique frames per second, even if the video stream is still 25fps. And you get significantly more motion blur, since each exposure is slower.
Here is a short description of how it was done, and a comparison between 1/25s and 1/13s. You must set the mode dial to "Creative Movie" mode, and then set manual focus, and the "M" exposure mode. Then you can set slow shutter speeds, all the way down to 1/2s. At 1/2s shutter speed, you only get two unique frames per second, of course, so the video might not be very useful.
As you see, 1/13s gives more exposure, and better brightness in the dim lightning, but more motion blur.
Here you can see the full concert videos as well. The one recorded at 1/13s:
And the one recorded at 1/25s, which is dimmer, but has less motion blur:
Both these videos were recorded using the Samyang 7.5mm f/3.5 fisheye lens. The maximum aperture is only f/3.5, which is quite small for a prime lens. On the other hand, it is normal for a fisheye lens. If I had used a lens with a larger aperture, for example the Lumix G 20mm f/1.7 pancake lens, I would not have needed to use a slow shutter speed.
This feature was also available with the GH1 and GH2. I used it to record fireworks with the Samyang fisheye lens, at ISO 3200, f/3.5, 1/13s. Since the GH2 topped out at ISO 3200 in video mode, this was my only option to be able to capture it. Using the GH3, I would probably have used ISO 6400 and 1/25s exposure.