To test this feature, I rigged the Panasonic GH1, GH2 and GH3 on a tripod, using the Sigma 30mm f/2.8. I left the lens wide open, it tends to be quite sharp. To be able to compare the cameras, I set both in the Shutter speed priority mode (S), at 1/10s and auto-ISO. The camera would then select the ISO needed for the exposure to be sufficient. I used Auto White Balance (AWB).
Here are the three images:
|GH1, 1/10s, ISO 1250|
|GH2, 1/10s, ISO 1600|
|GH3, 1/10s, ISO 1600|
These are the out of camera JPEG images, with standard image settings. The histogram below shows that all cameras expose the scene pretty similarly. The GH3 exposes it a bit brighter, though:
Another thing to note is that the GH1 uses the lowest ISO value of the three, 1250. This confirms again that the Panasonic GH1 had a somewhat conservative ISO scale, compared with other Micro Four Thirds cameras.
Looking at the image quality, we can compare 100% crops from all three images. I rescaled the GH1 image, to make it comparable with the 16MP output of the other cameras.
I think we can see that the GH2 images look sharper, but at the expense of shadow details. The GH3 appears to give a higher dynamic range, with more usable details in the dark parts of the image. With the advances of high ISO image quality, I feel quite confident using ISO values of 1600, and even 3200 when needed, with the Panasonic GH3.