Pentax Q7 Review
JPEG tone curves / dynamic range
Like recent Pentax DSLRs, the Q7 provides Highlight and Shadow Correction tools for in-camera adjustment to tone curve. Each can be set to on, off or auto. Because of the way it works (by using a lower exposure for highlight detail retention then boosting shadows and midtones for a good overall exposure) Highlight Correction 'on' mode can only be used at ISO 200 and above. Oddly, the Q7 will let you enable 'auto' correction at any ISO.
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 Q7's default tone curve doesn't send up any red flags and looks pretty average for this sensor size. We've included the Olympus OM-D E-M5 for comparison even though it's a little out of the Q7's league - here, we're using it as a proxy for the entry-level Olympus E-PM2, which has the same sensor. The Nikon 1 J1 and Sony Cyber-shot RX100 seem to give just a little more highlight tone before clipping to white, but in practical terms these cameras are very closely matched. The E-M5/E-PM2 does about a half stop better than the Q7 at the highlight end of the range, with a smoother rolloff to white.
'Bright,' Pentax's default JPEG processing mode, looks very close to Natural processing mode in terms of tone curve, though in shadow tone natural mode seems to offer almost an extra half stop advantage. Landscape and Bright modes are virtually indistinguishable in terms of tone curve.
Muted offers the broadest range of dark tones, and might be a good option for portraiture, or a JPEG shooter who wants the option of 'tightening up' the tones later, on a PC. Radiant mode has a smoother rolloff to white and will produce punchier shadows, while Reversal Film has the steepest drop to shadow tone. Certain modes like Muted and Radiant have a brighter middle gray point, producing an image that's much brighter overall with a somewhat washed out appearance. You can take a look at the effect of each of these modes on the Features page of our review.
Highlight Correction alone provides about an extra stop of highlight tone, and Shadow Correction by itself in turn provides about an extra stop and a half of shadow tone compared to default images. With both corrections enabled, there's virtually no difference in highlight tone but our test shows a loss of shadow tone. That's due to increased shadow noise, a by-product of highlight correction, also amplified by shadow correction. Right down there in the very bottom-left of the graph is pretty well black, and this minor difference is not visible in our real-world shooting.
Below you can see the real world results off highlight and shadow correction 'on' and 'off' settings applied to a high contrast scene - not our loveliest Seattle scenery but the wide tonal range made it a good subject for this test.
HC Off, SC Off
HC On, SC Off
HC Off, SC On
HC On, SC On
Highlight Correction does a nice job of exposing the blue tone in the sky and some of the tone in the buildings in our test scene that was otherwise lost to clipping. Likewise, shadow reduction brings back tone and a little detail in the greenery and gravel in the deep shadows.
'Auto' Highlight and Shadow Correction analyze a given scene and in our testing, applied a bit more conservative amount of correction than full 'on' modes did. Contrasty scenes in full auto were evened out with some darker shadows intact; turning both corrections 'on' tends to result in a slightly 'flat' image. The Q7's in-camera Raw editor will allow adjustment to Shadow Correction post-capture; there's no option to apply Highlight Correction after the fact, though. Shooting with Highlight Correction enabled at all times wouldn't be a bad idea for JPEG shooters, though it could cause issues with exposure in especially bright situations.