Sony Alpha DSLR-A100 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 Sony DSLR-A100 produced a fairly standard 8.1 EV (just over eight stops) of dynamic range between ISO 100 and 400, this dropped to around 7 EV at ISO 800 and 6 EV at ISO 1600. The reason for the drop at ISO 800 was simply that we stop measuring 'shadow range' if the signal-to-noise ratio of the patch is below our predefined value. This was also true at ISO 1600 but with also a slight 'clip' of highlights too.
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 100||-4.8 EV||3.3 EV||8.1 EV|
|ISO 200||-4.8 EV||3.3 EV||8.1 EV|
|ISO 400||-4.8 EV||3.3 EV||8.1 EV|
|ISO 800||-3.7 EV||3.3 EV||7.0 EV|
|ISO 1600||-3.1 EV||3.1 EV||6.2 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 27 represents -3.0 EV on our graph above, each step represents +/- 0.3 EV (although we use more accurate fully calibrated values). As you can see on the ISO 1600 crop it's difficult to make out the difference between the steps below step 27, 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)|
In addition to the ISO 100 to 1600 range the DSLR-A100 also provides two options labeled Lo80 and Hi200. The Lo80 ('Low Key') option provides ISO 80 with a tone curve which boosts shadow areas to maintain shadow detail in a low key shot (although appears to compromise highlight range by around 0.2 EV). The Hi200 ('High Key') option provides ISO 200 with a tone curve which maintains as much highlight detail as possible. The results of our test were very interesting, both settings doing exactly what they were expected to. The most interesting of the two must be the Hi200 option which manages to maintain highlight detail a whole stop further than normal.
Dynamic Range compared
An interesting difference between the EOS 30D and EOS 5D was that the 5D managed to deliver slightly more 'highlight range' but we called the 'shadow range' earlier than the EOS 30D (on luminance value). The overall performance between these three cameras is virtually identical (apart from the shape of the curve), it would be difficult to make out 0.2 EV's difference.
|Camera (ISO 100)||
|Sony DSLR-A100||-4.8 EV||3.3 EV||8.1 EV|
|Canon EOS 30D||-5.1 EV||3.3 EV||8.4 EV|
|Olympus E-330||-4.8 EV||3.1 EV||7.9 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 to the nearest third of a stop (the exact range is calculated to sub-stop fractions and using calibration data).
Next we will test the additional headroom available in RAW files. 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 a pretty impressive 10.5 stops total dynamic range, most of this however is at the shadow end (the highlight range, above middle gray) remains at around 3.4 EV.
- ACR Default: Exp. 0.0 EV, Shadows 5, Bright. 50, Contrast 25, Curve Medium (Default)
- ACR Best: Exp. -1.3 EV, Shadows 0, Bright. 60, Contrast -30, 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).
Sony's DRO options
Note that we did test Sony's DRO options using our step chart but unfortunately it appears to be looking for differences between large percentages of the image area rather than a relatively small strip across the center of the image. We did however make a fairly extensive evaluation of the difference it makes to everyday shots on this page.
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