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.
The only image parameter on the M8 which has any significant effect on dynamic range is (naturally) contrast. At its lowest possible setting of low total dynamic range is just under 9 stops (although the gain is in shadow range, no increase in highlight range), at its highest setting dynamic range is a whole stop less at 8 stops.
ISO Sensitivity and Dynamic Range
The M8 delivers a fairly typical (but still good) eight and a half stops of dynamic range at its ISO 160 base sensitivity (which remember is actually more like ISO 200). From there upwards we see a gradual drop off in dynamic range, around half an EV per ISO stop, this due primarily to increasing noise, note that highlight range doesn't change.
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 160||-5.2 EV||3.2 EV||8.4 EV|
|ISO 320||-4.7 EV||3.2 EV||7.9 EV|
|ISO 640||-4.3 EV||3.2 EV||7.5 EV|
|ISO 1250||-3.4 EV||3.2 EV||6.6 EV|
|ISO 2500||-2.7 EV||3.2 EV||5.9 EV|
Dynamic Range compared
As you can see the M8's dynamic range graph / tone curve is a sort of hybrid between the EOS 5D and D200, its highlight range (that above middle gray) quite similar to the D200, its shadow range very similar to the EOS 5D. Bear in mind that the the data on the graph below is at base sensitivity, so for the M8 that's ISO 200 equiv., the EOS 5D at ISO 125 equiv. and the D200 at ISO 100 equiv.
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 you can see from the graph below there's at least another stop of extra information available, their was a gain of around one stop of highlight range (although no guarantee of color accuracy).
- ACR Default: Exp. 0.0 EV, Blacks 5, Contrast +25, Curve Medium
- ACR Best: Exp. -1.0 EV, Blacks 0, Contrast -50, 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.
This can be seen fairly clearly in the examples below, on the right the negative digital exposure compensation has revealed some more detail in the background but this soon turns into gray as one or more of the color channels clips.
|ACR default conversion||ACR with -3.0 EV digital exp. comp.|