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 6D's tone curve is very similar to that of the EOS 5D Mark III (which isn't any great surprise) but also very similar to that of the Sony SLT-A99. The Nikon D600, by comparison, has a much straighter tone curve above middle grey, giving a fraction less contrast in the brighter tones and a slightly less of a smooth transition to the very bright highlights.
The 6D, in common with Canons DSLRs going back several years, has two features that can help respond to high dynamic range scenes. The first is Auto Lighting Optimizer, a JPEG-only feature that will brighten the shadow regions of an image if it thinks it's necessary (usually in response to significantly backlit scenes). This isn't triggered by our dynamic range test scene, so makes no difference here. The other option is Highlight Tone priority, which will be explained in more depth further down this page.
The Canon has two different tone curves associated with its different Picture Style color modes. The majority use a standard tone curve, with just 'Faithful' and 'Neutral' taking a lower-contrast approach to the highlights - much in the way we saw the Nikon perform in the first comparison on this page.
Highlight Tone Priority
The Highlight Tone Priority option 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.
Because it works by using a lower-than-usual 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.
The EOS 6D's lowest standard ISO is 100 but it can offer ISO 50 as an extended mode. Essentially this is doing the opposite of Highlight Tone priority mode - it's increasing the exposure by a stop, then using a different tone curve to pull the image brightness down to compensate. However, whereas HTP mode attempts to protect the image from highlight clipping, switching to ISO 50 makes it far more likely.
Because it's all-but impossible to recover over-exposed highlights, we'd recommend not using the camera's ISO 50 unless you have a specific reason - in everyday shooting you'd generally be better off using a neutral density filter if you need the longer shutter speeds. That's not to say that it's useless though; if you're shooting under controlled lighting and can be confident of retaining highlights, it should give the best quality.