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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 (Bright) the K-01's tone curve is more or less identical to the K-5 It measures a total dynamic range of 8.5 EV, roughly 3EV of which is in the highlights with a rather abrupt clip to white.

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.

The tone curve remains essentially unchanged across the ISO range (all ISOs clip at the same point) but at higher settings shadow dynamic range is limited by shadow noise.

Dynamic Range Expansion

The K-01 features two dynamic range tools - Highlight Correction and Shadow Correction, which exemplify the two different methods widely used by cameras to improve the dynamic range conveyed in the final image.

Highlight Correction is the one that actually increases the captured dynamic range - it reduces the camera's exposure by a stop, without boosting the amplification of the sensor's signal (this reduction in exposure is shown as an increase in the camera's base ISO to 200). This reduced exposure means it captures extra highlight detail but it doesn't count as 'underexposure' because a modified tone curve is then applied so that the final image comes out at the correct brightness.

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 Correction applies only to JPEG images. Highlight Correction applies to Raw as well, since it resulted in a different exposure than the one you'd have used with the feature turned off. A Raw converter that fully supports the K-01 should recognise the need to use a different tone curve to process the result. At worst, with a non-mainstream converter you might have a shot that initially looks underexposed and needs 1EV of extra exposure applied.

The K-01's sensor is almost certainly the Sony IMX071, which is famed for being able to capture detail well down into the shadow regions without contributing much to the noise. This means there's a lot of scope for pushing the exposure down to use more of that shadow region (as the Highlight Correction mode does), and for pulling the brightness of the dark regions up to reveal more detail (as the Shadow Correction mode does). Using the two methods together can start to introduce shadow noise, but the results are an impressive (and accessible) way of showing just how capable the sensor is.

 

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OnTheWeb
By OnTheWeb (8 months ago)

Forgot to mention DXOMark rates this camera/sensor and sports performance nearly top of its class of hybrid cameras.

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