But as the GH2 started to come into the hands of early users it became clear that there was something very special going on – a video mode called EX Tele Conv. Since this as such an awkward name I'm simply going to call it ETC from now on.
When shooting in 1920X1080 HD (including 24P Cinema mode) and when ETC mode is engaged, the GH2's 4608 x 3456 sensor is "cropped" to 1920X1080. In other words, only the central 2 Megapixels of the sensor's overall 18 Megapixels is recorded.
This has significant implications, almost all of them positive. Firstly, no line skipping, binning, or digital interpolation are needed. It's as if the sensor was only 2MP rather than 18MP. As a consequence the image recorded has a crop factor of 2.6X over the full Micro Four Thirds format. The best way to think of this is being akin to the crop factor of APS-C over full frame 35mm; which in that case is either 1.5X or 1.6X.
As a consequence, any lens put on the GH2 when shot in ETC video mode has an effective focal length of 2.6X of what's marked on the barrel. The new 100–300mm zoom therefore becomes a 260–780mm, with no loss of light! It's like shooting with a 2.6X teleconverter but without the image quality loss and without the light loss. There is essentially no IQ loss because the original sensor output is used; just a cropped portion of it. There is no light loss, because though it acts like a teleconverter, it isn't. There are no optical elements involved and no upressing or downressing.
SIDEBAR. Since a Four Thirds sensor already produces a 2X field of view as compared to full frame 35mm, this means that in ETC video mode the focal length marked on the lens is actually 5.2X what it would be in full-frame 35mm terms. So the long end of the new 100-300mm zoom is equivalent to 1,560mm in Extra Tele Conv video mode. Wildlife cinematography anyone?
Please also note that this review only applies to when shooting video in ETC mode. In stills mode there is degradation due to a smaller area of the sensor being used./QUOTE]
http://www.luminous-landscape.com/reviews/cameras/panasonic_gh2_11_mode_revealed.shtml
