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 RX100 doesn't have any dynamic range expansion features like most of its peers (where halving the exposure and pushing the tone curve gives an extra stop of DR), but it does have DRO to make the most of what is captured. However, even without this feature, the RX100's native exposure/tone curve relationship does a good job of capturing a good amount of highlight detail and providing a smooth roll-off - avoiding abrupt clipping in over-exposed regions.
As already shown, the DRO mode makes no attempt to capture a great range of tones (it doesn't affect the underlying exposure), but it does make the most of those that have been captured. The results in the real-world give more balanced images while retaining overall contrast.
Expanded low ISO settings
The RX100 has two ISO settings below its native 'base' ISO of 125 and, as you would expect, these both result in lower highlight dynamic range (since they are effectively the same as ISO 125 but with greater exposure). Any slight advantage in noise performance will be undermined by the tendancy to clip highlights earlier. Sadly there's no way of using an ND filter with the RX100 to enable the use of long shutter speeds without sacrificing dynamic range.