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).

Cameras Compared

At default settings (Highlight Correction Off, 'Bright' Custom Image Mode) the K-30's tone curve is rather steep and more or less identical to previous Pentax models such as the K-01 or K-5. It measures a total dynamic range of approximately 8.5 EV, just 2.9 EV of which are in the highlights with a rather abrupt clip to white. This is slightly less than what we see from the K-30's direct competitors in the mid-level DSLR bracket of the market. However, additional dynamic range can be obtained by activating one or both of the K-30's dynamic range tools - Highlight Correction and Shadow Correction (see below).

Custom Image Modes

Like on the K-30 and K-5 the Bright, Natural, Portrait, Landscape, Vibrant, and BW settings use essentially the same curve and therefore produce the same amount of dynamic range. The Muted setting uses a more linear, less contrasty curve. Bleach Bypass and Reversal Film on the other hand create images with more contrast. However, all the settings clip highlights at the same point. Cross Processing (which was previously available as a filter) has now moved into the Custom Image menu and has a very contrasty tone curve.

Dynamic Range Expansion

You can expand the dynamic range in your images by activating one or both of the K-30's dynamic range tools - Highlight Correction and Shadow Correction. These features exemplify the two different methods widely used by cameras to improve the dynamic range conveyed in the final image.

Technically, Highlight Correction is like underexposing by a stop to avoid blowing highlight detail, and then pulling up the midtones and shadows in post-processing to compensate, giving an image of the correct overall brightness. This results in an increase in the camera's lowest-available ISO to 200 when Highlight Correction is turned on.

Shadow Correction is a much simpler concept in that it simply boosts the brightness of shadow regions to create a more balanced image. The level of shadow correction is user definable in four steps (Off to 3). There's also an Auto setting in which the camera applies one of these four settings, depending on the contrast in the scene. None of these options increase the Dynamic Range, per se, but can pull detail out of deep shadows so that it's visible in the final image.

Shadow and Highlight Correction apply to both JPEG and Raw images. A Raw converter that fully supports the K-30 should recognise the need to use a different tone curve to process the result. At worst, with Highlight Correction switched on a non-mainstream converter you might have a shot that initially looks underexposed and needs 1EV of extra exposure applied.

You can also combine Highlight Correction with any level of Shadow correction which gives you quite a few possible combinations to play with. In the widget above we have included the tone curve for Highlight Correction On and Shadow Correction High. As you can see the image has both - more detail in the highlight areas and slightly lifted shadows although the effect at both ends of the spectrum is not as pronounced