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 5D Mark III exhibits a tone curve that is virtually identical to its predecessor, the 5D Mark II. By default the Highlight Tone priority feature is switched off but the highlight roll-off is smooth, with almost 4 EV of highlight range. Turning HTP on gets you an additional stop and even flatter tone curve in the highlights. This makes for a very gentle transition from highlight detail to clipped white. Unfortunately the camera's Auto Lighting Optimizer (ALO) feature is not triggered by our dynamic range test scene but you can see its impact in a real-life sample on the features page of this review.
The EOS 5D Mark III offers seven different 'Picture Styles', which are essentially color response presets applied to in-camera JPEGs. As we have seen in previous generation EOS cameras, the various Picture Styles use either one of two tone curves, the first more contrasty curve for the Standard, Portrait, Landscape and Monochrome Picture Styles, and a slightly flatter curve for the Neutral and Faithful Picture Styles. Neither curve delivers more dynamic range and they both clip highlights at the same point.
The Canon EOS Mark III's base ISO setting is 100. However, the camera also offers an extended 'L' setting which is equivalent to ISO 50. This lower sensitivity setting is essentially ISO 100 with 1 EV additional exposure and a tone curve adjustment to arrive at a final exposure with the correct brightness. As you can see in the graph above the penalty is a significant decrease in available highlight range. Given this difference, we would advise shooting at or above ISO 100 to maintain maximum dynamic range, using ND filters instead for situations that require a longer exposure.