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).
ISO 50 vs ISO 100 Sensitivity Settings
The SLT-A77's ISO 50 mode is - as expected - an 'extension' ISO sensitivity setting, which gives roughly 0.7 EV less highlight dynamic range than ISO settings of 100 and higher. From ISO 100 right up to its maximum of ISO 16,000, the A77's highlight dynamic range is exactly the same, as we'd expect, although shadow detail gets seriously swamped by noise at its highest ISO settings.
At default settings, the A77 gives around 1.3EV more highlight range than the Pentax K-5, and around 0.7EV more than the Canon EOS 7D. The A77's dynamic range is very close to the Nikon D7000, although the highlight 'roll-off' is much smoother - the D7000's steeper curve represents a tendency for highlights to clip to white rather suddenly.The performance gaps indicated here are essentially closed when the dynamic range expansion modes in the A77's various competitors are activated. With HTP turned on in the Canon EOS 7D, and Highlight DR activated in the K-5, the curves overlay almost exactly with the A77. The D7000's Active D-Lighting (ADL) mode is adaptive in the sense that its intensity varies scene-by-scene, so don't read too much into the results that you can see here. The important thing to notice is that with ADL turned on, highlight brightness is significantly retarded, which gives you a better chance of retaining detail in bright scene elements.
The effect of Sony's 'DRO+' settings also 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 incrementally the amount of mid tones by lifting the shadow areas, to get the most detail out of these areas from a single exposure.
The SLT A77 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 no surprise to note that the A77's dynamic range curve exactly overlays that of the Alpha NEX-5N, since Sony uses the same tone curve across its entire Apha range of cameras.
Some of the Creative Styles increase contrast a little - shooting in the Vivid and Landscape styles ensures very 'punchy' results. The A77's 'Monochrome' Creative Style also gives slightly higher contrast results than 'Standard'.