Our new 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 cameras) black to clipped white (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' 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 the camera can capture before it clips to white. Shadow range is more complicated, in our test we stop measuring values below middle gray 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.
Custom Image Modes
The K20D offers a total of six Custom Image modes, a couple of which (Bright and Landscape) use slightly higher contrast, but all of which offer essentially the same dynamic range.
In addition to the image tone options there are also obviously a range of image parameters which can be adjusted, only Contrast would affect the final image dynamic range. Here we have tested the extremes of the contrast setting from -4 to +4. Unusually there is little difference at all between the various contrast settings - the shape of the S-curve changes but the highlight and shadow cut-off points are the same in each case.
The K10D had a very strange, almost linear tone response without any discernible 'shoulder' (the roll-off as you reach the brightest highlights). The K20D has more of a curve, but the lack of a gentle roll-off will inevitably lead to harsh highlight clipping in challenging shooting conditions.
That all said, overall dynamic range is pretty good, with up to 9 stops at ISO 100, though most of this is in the shadows - 3.2 stops over mid-grey is far from class-leading. Dynamic range falls fairly quickly as you move up the range and noise starts to eat away at shadow detail (though highlights are retained throughout the range).
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 100||-5.8 EV||3.2 EV||9.0 EV|
|ISO 200||-5.4 EV||3.2 EV||8.6 EV|
|ISO 400||-4.7 EV||3.2 EV||7.9 EV|
|ISO 800||-4.3 EV||3.2 EV||7.5 EV|
|ISO 1600||-4.0 EV||3.1 EV||7.1 EV|
|ISO 3200||-2.7 EV||3.1 EV||5.8 EV|
Dynamic Range compared
The K20D's rather harsh tone curve means that whilst overall dynamic range is very good compared to most of its competitors, it is, like the Olympus E-3, skewed slightly towards the shadows, meaning highlights clip around half a stop or so earlier. The solution - to underexpose slightly and use post-processing to change the curve - will introduce a little extra noise, but this is unlikely to be an issue unless you're making poster prints.
|Camera (ISO 100)||
|Pentax K20D||-5.8 EV||3.2 EV||9.0 EV|
|Canon EOS 450D||-5.1 EV||3.6 EV||8.7 EV|
|Sony DSLR-A700 (ISO 200)||-4.9 EV||3.9 EV||8.8 EV|
|Olympus E-3||-5.8 EV||3.0 EV||8.8 EV|
|Pentax K10D||-4.5 EV||2.8 EV||7.3 EV|
The wedges below 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).