Canon EOS 400D / Digital Rebel XTi/ Kiss X Digital 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.
ISO Sensitivity and Dynamic Range
The EOS 400D produced a consistent 8.4 EV of dynamic range from ISO 100 to 800, dropping to 7.8 EV at ISO 1600. This performance is about 0.3 EV better than the competition, the primary gain being at in highlight range, producing 3.5 EV (stops) of detail above middle gray.
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 100||-4.9 EV||3.5 EV||8.4 EV|
|ISO 200||-4.9 EV||3.5 EV||8.4 EV|
|ISO 400||-4.9 EV||3.5 EV||8.4 EV|
|ISO 800||-4.9 EV||3.5 EV||8.4 EV|
|ISO 1600||-4.3 EV||3.5 EV||7.8 EV|
Obviously an ISO 1600 image would contain areas of shadow with a level below our 'shadow range' point however it would difficult to make out any detail because of noise, this can be seen more clearly in the 100% wedge crops below. Step 32 represents -4.7 EV on the graph above, each step represents 0.3 EV. As you can see on the ISO 1600 crop it's difficult to make out the difference below step 32, the amount of noise now overtakes any useful 'signal'. (Note that the crops below have had their brightness boosted to make it easier to see the patches, for unmodified crops click on these links: ISO 100 crop, ISO 1600 crop).
|ISO 100 (image brightness boosted, original)|
|ISO 1600 (image brightness boosted, original)|
Dynamic Range compared
The EOS 400D delivered around 0.3 EV (a third of a stop) more dynamic range than the competition. As mentioned above this was primarily in highlight range which (assuming the same exposure) would enable it to better capture areas of the image which may otherwise be overexposed. It's also clear from the graph below that Canon are using a tone curve which will produce a more gradual roll-off to highlights (Nikon in particular tend to use a more contrasty tone curve).
|Camera (ISO 100)||
|Canon EOS 400D||-4.9 EV||3.5 EV||8.4 EV|
|Nikon D80||-4.8 EV||3.2 EV||8.0 EV|
|Sony DSLR-A100||-4.8 EV||3.3 EV||8.1 EV|
|Canon EOS 350D||-4.8 EV||3.3 EV||8.1 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).
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. -1.0 EV, Shadows 0, Bright. 70, 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. (Note that the wedge below labeled as 'ISO 100' is of course a JPEG straight from the camera).
Oct 14, 2006
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