JPEG Tone curves / Dynamic range
Our Dynamic Range measurement system involves shooting a calibrated Stouffer Step Wedge (13 stops total range) which is backlit using a daylight balanced lamp (98 CRI). A single shot of this produces a gray scale wedge from the camera's clipped white point down to black (example below). Each step of the scale is equivalent to 1/3 EV (a third of a stop), we select one step as 'middle gray' (defined as 50% luminance) and measure outwards to define the dynamic range. Hence there are 'two sides' to our results, the amount of shadow range (below middle gray) and the amount of highlight range (above middle gray).
To most people highlight range is the first thing they think about when talking about dynamic range, that is the amount of highlight detail above middle gray the camera can capture before it clips to white. Shadow range is more complicated; in our test the line on the graph stops as soon as the luminance value drops below our defined 'black point' (about 2% luminance) or the signal-to-noise ratio drops below a predefined value (where shadow detail would be swamped by noise), whichever comes first.
Note: this page features our new interactive dynamic range comparison widget. The wedges below the graph are created by our measurement system from the values read from the step wedge, the red lines indicate approximate shadow and highlight range (the dotted line indicating middle gray).
The GH3 uses a new tone curve compared to the GH2, which offers about 2/3 stops more highlight range and gives the smoother roll-off to clipped white that most other brands offer. This is a pretty normal response for a modern camera (and a welcome, if overdue change from Panasonic).
The GH3 uses four different tone curves for its six different Photo Styles. The Standard and Monochrome mode use the same curve, and the Portrait and Natural share another. The differences between the curves are subtle, but the overall affect is that Vivid gives contrasty, punchy results whereas Portrait is much more open in the shadows.
The GH3's dynamic range boosting mode, iDynamic appears to do something we've not previously seen fro Panasonic models - in that it adapts its exposure to capture more highlight information. It then uses a lower-contrast tone curve to squeeze this additional tonal information into the image. This is something several other brands do, and it's an intelligent way of increasing the amount of highlight information captured for the camera's JPEGs.
The upshot is that Raw images from iDynamic High may appear slightly darker when you first start processing them (risking more a touch more shadow noise appearing when brought back to the correct brightness). The differences in highlight capture we're seeing here come from around 1/3EV exposure differences, so the real-world effects are likely to be fairly subtle.
Extended ISO settings
The GH3 offers an option to extend its ISO range down to 125 and up to 25600. Looking at the tone curve, it's clear that ISO 125 offers less highlight range than the other settings. This implies that ISOs 125 and 200 use the same sensor setting, but with a different tone curve applied to the JPEGs. The result is that the 2/3EV extra exposure that ISO 125 gains over ISO 200 ends up clipping highlights 2/3rds of a stop earlier, but with the benefit of cleaner mid-tones and shadow regions.