Sigma SD1 / SD1 Merrill In-depth Review
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.
The SD1's JPEG tone curve offers about 3.3 stops of highlight range above metered middle-grey, which is a slightly pedestrian performance compared to its peers. Both the Nikon D7000 and the Sony SLT-A77, for example, offer substantially greater highlight range. The SD1 is closer to the Canon EOS 7D's default behaviour, but lacks any kind of highlight-expansion mode analogous to Canon's Highlight Tone Priority. In the shadows, all of the cameras use very similar tone curves.
The upshot of this is that when shooting JPEG with the SD1 in contrasty conditions, you have a choice of either allowing highlights to clip, or underexposing to protect them. If you shoot Raw, it's important to appreciate that all of the feedback the camera offers about exposure in playback mode - i.e. the highlight clipping warning and the image histogram - is based on the camera's JPEG processing, as its generated from the preview image that's embedded in the Raw file. We'll look at the practical implications of this later in the review.
Here we're showing that all of the SD1's Color Modes are based on essentially the same tone curve, and therefore the JPEGs clip at the same point in the highlights. Instead the modes differ based mainly on saturation and colour tone, with Landscape being warm and saturated, for example.
JPEG contrast settings compared to SPP-converted RAW
This graph compares the SD1's range of JPEG contrast settings with a Raw file converted through Sigma Photo Pro at default settings (i.e. Standard Color Mode, Contrast =0). There are some notable points here that have a real operational impact for both JPEG and Raw shooters:
- The in-camera Contrast setting doesn't just change the steepness of the tone curve, it also changes the highlight clipping point. This is mostly a problem if you increase the contrast setting; highlights will clip earlier.
- Setting contrast to -1 clips at about the same point as the default setting (or perhaps a tad higher), but does so quite abruptly, with no roll-off.
- Raw files converted using Sigma Photo Pro use a different tone curve that resembles the default contrast setting in the highlights, but the +1 setting in the shadows.
One less-obvious, but useful point to come out of this relates to the highlight clipping warning in playback, which is triggered at a distinctly conservative value of 248. If you shoot Raw, then setting contrast to -1 will make this warning more-closely reflect where highlight data actually clips at ISO 100.
However, it's also worth bearing in mind that, because the SD1 retains a stop more highlight data in its Raw files at all higher ISOs that's simply not represented in its JPEG output, the clipping warning doesn't really mean anything useful for Raw shooters at these ISOs.
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