The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure noise levels; we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with the 'Standard Output Sensitivity' method described in ISO 12232:2006, the standard used by camera manufacturers.
Unlike previous Panasonic G-series models, we found that the measured ISO from the GH2 matches its indicated ISO to within 1/6EV across its entire ISO range.
Noise and Noise Reduction (JPEG)
Despite having a smaller sensor than its APS-C competitors, image quality from the GH2 stands up very well at the lower end of its ISO scale. It is only above ISO 800 that any meaningful, visual difference becomes noticeable, and as expected the GH2 does produce noisier files, especially at ISO 6400 and 12800. That said, up to ISO 3200, the GH2 holds its own extremely well, which is impressive. Default noise reduction (NR 0) deals with chroma noise very effectively, but is fairly conservative when it comes to luminance up to ISO 3200.
In our view this is a very sensible strategy, and it means that rather than detail being smeared out of existence along with luminance noise, it is preserved, but at the expense of a slight 'grittiness' which is far from objectionable by comparison. It is clear from these comparisons though that the GH2 is unable to match some of its competitors (particularly the Nikon D7000) at ISO 6400 and above. JPEGs captured at these settings are a mess, and even with NR turned down to its minimum (NR -2) fine detail is all-but obliterated by luminance noise reduction.
It is interesting to note just how much better the GH2's performance is compared to the G2. If you select the G2 as a comparison camera you will see that even at ISO 200, images from the G2 are considerably grittier - a difference that only increases higher up the ISO scale. Switching to the graph view makes this obvious - despite its smaller pixel pitch, the GH2's sensor produces much cleaner images than the G2, with lower measured noise levels at all ISO settings.
RAW noise (ACR 6.3 noise reduction set to zero)
It's a similar story with RAW noise as we saw with JPEG. Up to ISO 1600, the GH2 gives very similar amounts of measured and perceptible noise as its competitors, and it is only above ISO 3200 that the smaller sensor of the GH2 starts to show its limitations. For the first time in the short history of Panasonic's G-series, ISO 6400 is actually useable, and although ISO 12800 isn't pretty, useable images can be coaxed out of the GH2 by careful post-capture noise reduction.
Interestingly, when noise reduction is set to +2, the GH2 applies some smoothing to RAW files taken at ISO 800 and above. The effect is subtle, but the intensity of the chroma noise especially is definitely reduced. With NR +2 activated, the GH2 gives lower measured noise results than the competition at ISO 800, 1600 and 3200, before creeping up again at ISO 6400 and again at ISO 12800, where the noise is simply too intense to be meaningfully reduced in-camera.