Pentax K-3 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.
The K-3 offers two tools for managing dynamic range - a Highlight Correction mode that adjusts how much of the camera's dynamic range is given-over to highlights, and a Shadow Correction mode that adjusts the lower half of the camera's tone curve to brighten-up dark areas of the image.
Highlight Correction is only available from ISO 200 upwards (an 'Auto' setting is also available at ISO 100 but has no effect). This is because it is reducing the amount of amplification applied to the data coming from its sensor, leaving more room in the Raw file for highlight data. This additional 1EV of highlight data is incorporated into the image using a tone curve with a more gentle transition from near-white to totally 'clipped' whites.
The Shadow Correction feature adjusts the bottom half of the camera's tone curve. There are five settings, including 'Off' and an Auto mode. Shadow Correction reduces contrast in the shadows and also makes near-black areas of the image brighter, thereby incorporating more the sensor's dynamic range into the JPEG image.
Unlike Highlight Correction, Shadow Correction doesn't make any adjustment to the Raw file. This, in turn, means that the level of Shadow Correction can equally be applied after you've shot the image. You can therefore reprocess a Raw file in the camera with brightened shadows and achieve the same results.
The chart below shows the K-3's default tone curve compares to some of its immediate peers.
With Highlight Correction set to 'Off' the K-3 captures slightly less highlight detail than its immediate peers - with a relatively high contrast representation of highlight tones, it clips to white a little abruptly. Turn Highlight Correction on and its tonal response matches the Nikon D7100's almost exactly - giving a much more gentle transition in the brightest tones. With this mode enabled you wouldn't expect to see a lot of extra highlight information (though it is capturing 1EV more), but there should be a less jarring transition from near-white to totally-clipped white regions.
Interestingly, because the K-3's Highlight Correction feature brings its tonal response into line with the D7100, most of its peers are all able to offer at least some advantage over the K-3 when their DR enhancement modes are engaged. The real-world effects of this are subtle, since the K-3 has very clean shadows (so you can ratchet-up the Shadow Correction feature to bring a broader range of tones into the image without any significant noise penalty), but the camera's JPEG engine isn't as clever as the Nikon's in this respect.