Pentax K100D Review
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.
Image Tone option
The K100D provides two selectable image tones which define the tone curve and color saturation. While the Natural tone does deliver a less contrasty image it does not extend dynamic range over the default Bright option, nor does using the Bright option impact on dynamic range.
ISO Sensitivity and Dynamic Range
In other digital SLR's we have observed slightly less highlight range at the base sensitivity (this because of the increased likelihood of a 'full well' situation). This situation occurred when we tested the K100D, it delivered about 0.3 EV (a third of a stop) less highlight range at ISO 200 than at ISO 400 which could for example lead to harder clipping of highlights. The animation of the two step chart shots taken at ISO 200 and 400 demonstrates this difference.
|Animation showing the dynamic range difference between ISO 200 and 400|
At higher sensitivities the total range becomes limited by the amount of noise at the darker end of the range (shadow range). Overall performance was below average at ISO 200 and about average above this sensitivity.
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 200||-4.5 EV||3.0 EV||7.5 EV|
|ISO 400||-4.5 EV||3.4 EV||7.9 EV|
|ISO 800||-4.0 EV||3.4 EV||7.4 EV|
|ISO 1600||-3.4 EV||3.4 EV||6.8 EV|
|ISO 3200||-2.7 EV||3.4 EV||6.1 EV|
Dynamic Range compared
At ISO 200 the K100D produced slightly less highlight range (as detailed above) than the Canon EOS 350D (Digital Rebel XT), at ISO 400 things were more evenly matched.
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).
Experience has told us that there is typically around 1 EV (one stop) of extra information available at the highlight end in RAW files and that a negative digital exposure compensation when converting such files can recover detail lost to over-exposure. As with previous reviews we settled on Adobe Camera RAW for conversion to retrieve the maximum dynamic range from our test shots.
As you can see from the graph and wedges below Adobe Camera RAW in default mode actually returns a little less dynamic range, a combination of a more contrasty tone curve and more noise in shadow areas means it is cut-off by our 'lowest acceptable SNR'. The best we could achieve (with some pretty extreme ACR settings) was just over 10 stops total dynamic range, and more importantly about a stop more highlight range (although with no guarantee of color accuracy).
- ACR Default: Exp. 0.0 EV, Shadows 5, Bright. 50, Contrast 25, Curve Medium (Default)
- ACR Best: Exp. -0.85 EV, Shadows 0, Bright. 70, Contrast -25, Curve Linear
One thing to bear in mind is that although ACR was able to retrieve the 'luminance' (brightness) of wedge steps which were previously clipped there's no guarantee of color accuracy as individual channels may clip before others. (Note that the wedge below labeled as 'ISO 200' is of course a JPEG straight from the camera).
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