Sony SLT Alpha A55 Dynamic Range (JPEG)
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 SLT A55 doesn't throw up any surprises as far as its dynamic range is concerned, and as you can see from the data in the graph above, in the 'Standard' Creative Style it has an impressive dynamic range which compares well to its DSLR competitors. Total dynamic range is almost 9EV, which is about as good as things get from the current crop of APS-C and full-frame DSLRs. It is interesting to note that the A55's dynamic range curve exactly overlays that of the Alpha NEX-5.
Some of the Creative Styles increase contrast a little - shooting in the Vivid and Landscape styles ensures very 'punchy' results, but with increased risk of highlight clipping as a result of the decreased dynamic range. The A55's 'BW' Creative Style also gives slightly higher contrast results than 'Standard'.
The effect of Sony's 'DRO+' settings differs depending on the scene, so this test, performed using our 18 step wedge, isn't necessarily an accurate indication of 'typical' performance with a real-world subject. It does clearly show, however, the way in which DRO+ is designed to work, extending by increments the amount of mid tones by lifting shadow areas to get the most detail out of these areas in a single exposure.