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.
The Df uses the same well-judged tone curve that we previously saw on the D800, which offers a smooth and gentle transition from bright areas across to totally over-exposed regions, which gives a slightly more natural-looking image compared to the D600 (it's abrupt transitions to 'clipped' regions, as much as the clipped regions themselves that make images look unpleasant).
The range of tones included in the camera's JPEGs can be increased by engaging Active D-Lighting, which not only attempts to intelligently adjust the tonal balance of the image, but it also adjusts the exposure parameters to capture more highlight tones.
The Df sees the introduction of the highest Active D-Lighting setting we've yet seen - Extra High 2. As usual, this uses a combination of context sensitive brightness adjustments (which brighten shadow regions but do so in a way that preserves local contrast), and exposure adjustments that capture extra highlight information.
Compared with the ADL Off setting, the camera here reduced the exposure by 1.7EV to shoot the Extra High 2 image, then applied radically different processing so that the mid tones of the image still appear as mid tones. The real-world effect of this setting can start to look a bit extreme, but it avoids the worst excesses of low contrast or unnervingly artificial results that can stem from clumsy tone-mapping.
Extended ISO 'Low'
As with many cameras offering an extended ISO setting at the bottom of the camera's range, the Df achieves ISO 50 equivalent by essentially shooting its ISO 100 state but with more light let in. A different tone curve is then used to ensure the JPEG has the correct brightness (which is what defines ISO). The result is an image that clips highlights a stop earlier (and much more abruptly), but should exhibit less noise in shadow regions (not something that's likely to be noticeable on the Df).