Canon Rebel T3i / EOS 600D Review
Canon EOS Rebel T3i/600D 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 600D uses the same the tone curve as we've seen many times before from Canon, giving good dynamic range (around 3.3EV above middle grey) and a gentle roll-off as you move towards the highlights. The Neutral and Faithful Picture Styles are a little different from the others, employing slightly lower contrast in the upper mid-tones but clipping a bit more abruptly to white (although at the same point relative to middle grey). Overall this is exactly what we're used to seeing from Canon DSLRs, meaning a little less highlight range by default than is typically obtained from Nikon and Sony competitors (but much the same as other brands).
The Highlight Tone Priority option (Custom Function II.6) is a method for capturing more information in the brightest parts of the scene. It does this by applying less amplification to the signal coming from the sensor, then compensating for it by using a different tone curve to ensure the correct brightness in the final image. Turn this on and the 600D captures an extra stop in the highlights, resulting in an overall range that at least matches that of Sony and Nikon models.
When used at ISO 200, the effect is the same as underexposing an ISO 100 shot by one stop, then pulling up the mid tones and shadows to compensate. The result is an image that looks right but doesn't lose the extra highlights you've captured. It means you have to use ISO 200 and upwards, because you're using the sensor at its lowest amplification setting but with one stop less exposure than in normal mode. The penalty is a touch more noise in shadow regions (as shown by the bottom limit of the HTP ON strip being slightly higher), but in return you get around one stop more highlight detail, which can be invaluable on bright, sunny days.
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