Canon EOS 650D/Rebel T4i 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.
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).
The EOS 650D uses the same tone curve we've seen many times before from Canon - including that found on its predecessor, the EOS 600D. It has a dynamic range of around 3.3 stops EV above middle grey and a gentle roll-off as you move towards the highlights. Overall this is exactly what we're used to seeing from Canon DSLRs, meaning a bit less highlight range by default than is typically obtained from Nikon and Sony competitors as well as the best of the Micro Four Thirds offerings we've seen.
The 650D, like it predecessor has five discrete Picture Styles, in addition to an Auto option. The Neutral (shown by default in the graph above) and Faithful Picture Styles offer a different tone curve compared to 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).
Highlight tone priority mode
The Highlight Tone Priority option (Custom Function II.3) offers 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 650D captures an extra stop in the highlights, resulting in a dynamic range that is at least the equal of Sony and Nikon DSLRs.
Because it works by using a lower signal amplification, HTP cannot be employed at base ISO. The minimum value that can be used is ISO 200. When used at ISO 200, the effect is the same as underexposing an ISO 100 shot by one stop, then pulling up the midtones and shadows to compensate. The result is an image at a 'normal' exposure but that retains the extra highlights you've captured. This approach - common to many other manufacturers - comes at the potential cost of increased noise in shadow regions. We discussed the real world benefits and consequences of activating HTP in our earlier review of the Canon EOS 600D.
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