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Dynamic Range

Our 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 camera's) 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.

Creative Style options

The graph below shows the dynamic range response from the A550's Creative Styles. They all have slightly different tone curves but deliver similar amounts of dynamic range. While all styles clip at the same point in the highlights the slightly higher contrast used by the B&W, Vivid and Landscape modes means they clip a little earlier in the shadows (and the Portrait setting's lower contrast means it gives a touch more shadow range).

Contrast settings

The A550 offers a range of contrast settings for each of its creative styles (the graphs below show the Normal style). As you can see the range from -3 to +3 isn't huge but it does at least allow you some control.

ISO Sensitivity and Dynamic Range

The dynamic range falls steadily as you move up the ISO range and noise eats away at the shadows - highlight range is essentially unchanged. As we've seen with other Sony Alpha models the JPEG dynamic range - particularly in the highlights - is excellent, better than most competitors. Note that in all cases these tests were performed without any of the A550's dynamic range optimization features enabled DRO off).

Sensitivity Shadow range Highlight range Usable range
ISO 200 -5.2 EV 4.2 EV 9.4 EV
ISO 400 -4.9 EV 4.2 EV 9.1 EV
ISO 800 -5.1 EV 4.2 EV 9.3 EV
ISO 1600 -4.7 EV 4.2 EV 8.9 EV
ISO 3200 -4.3 EV 4.2 EV 8.5 EV
ISO 6400 -3.4 EV 4.1 EV 7.5 EV
ISO 12800 -2.7 EV 4.0 EV 6.7 EV

Dynamic Range compared

As mentioned above the Alpha 550 is one of the better performers on the market, in terms of dynamic range in JPEGs.

Camera
Shadow range
Highlight range
Usable range
Sony DSLR-A550 (ISO 200) -5.2 EV 4.2 EV 9.4 EV
Canon EOS 500D (ISO 100) -5.1 EV 3.4 EV 8.6 EV
Nikon D5000 (ISO 200) -4.8 EV 4.0 EV 8.8 EV
Pentax K-7 (ISO 100) -5.7 EV 2.9 EV 8.6 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).





RAW headroom

Experience has told us that there is typically around 1 EV (one stop) or more 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 the default Adobe Camera RAW conversion delivers less dynamic range than JPEG from the camera (with a more contrasty tone curve that clips highlights earlier, and fractionally less noise reduction in shadows). The best we could achieve was 10.7 EV of total dynamic range, including around a stop of extra highlight range.

  • ACR Default: Exp. 0.0 EV, Blacks 5, Contrast +25, Brightness 50
  • ACR Auto: Exp. -1.0 EV, Blacks 0, Contrast -0, Brightness 0, Curve linear, Recovery 9

Settings
Usable range
JPEG

9.4 EV

ACR Default
7.5 EV
ACR 'Auto'

10.7 EV

WARNING: 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.

As the graphs above show the A550 - like many Sony SLRs - offers pretty good highlight range (the chances are that the A550's ISO 200 is performing the same trick as Nikon and Olympus cameras do, and that Canon offers as an option with its Highlight Tone Priority feature - see this blog post for more information). Although shooting raw can only go so far in recovering truly clipped highlights (which essentially have no tonal data in them to recover) we did find the A550's files to be remarkably pliable when it comes to mild - or even extreme - over exposure, allowing us to pull back what appeared to be featureless highlights by as much as three stops without significant problems. Of course you can't do anything with areas that are totally clipped (in a correctly exposed image there's at most a stop of extra highlight detail), but for overexposed shots it's surprising how much you can rescue from the raw file.

As you can see in the first example below the very brightest clipped areas (the highlight on the metal in the middle right of the crop) aren't recovered, and exhibit the usual color shifts / posterization, but everything else - including areas that appeared to be clipped - have plenty of tonal information hidden away that's easily recovered using negative exposure compensation at the raw stage. This is important, as the A550's metering is far from reliable, often overexposing outdoor scenes by at least a stop. Interestingly you'll also get fractionally less noisy ISO 200 images if you intentionally over-expose and pull it back in raw mode (the sample at the bottom of this page has none of the mild noise often visible in blue skies in correctly exposed ISO 200 raw shots.


Adobe Camera RAW
default conversion
Adobe Camera RAW
-2.5 EV digital comp.


Adobe Camera RAW
default conversion
Adobe Camera RAW
-3.0 EV digital comp.
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