There’s a large difference in the image from these 3 cameras. See how they compare in this test video.
Later in the video there is also a test of the Contax Zeiss 300mm F4 in Ex-Tele crop mode (1:1 1080p from the centre of the sensor) on the GH2. In this mode you can see into people’s windows from 2 miles away sat on a hillside. It is insane!!