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RAW headroom

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 the default Adobe Camera RAW conversion delivers less dynamic range than JPEG from the camera (a more contrasty tone curve and less noise reduction in shadows). It's possible to get considerably more than this out of the file, (for example, our ACR 'Best' parameters) but doing so results in a very 'flat,' unrealistic image. The point is that this additional information is there if you wish to selectively recover and blend-in this detail from a series of differently processed versions of the raw file.

  • ACR Default: Exp. 0.0 EV, Blacks 5, Contrast +25, Curve Medium
  • ACR Auto: Exposure -0.95 EV, Recovery 8, Blacks 0, Brightness 0, Contrast 0, Curve Medium
  • ACR Best: Exposure -1.85 EV, Recovery 0, Blacks 0, Brightness +118, Contrast -50, Curve Linear

Settings Usable range
ACR Default 7.2 EV
ACR Auto 9.5 EV
ACR 'Best' 12.7 EV

To test how much exposure latitude the raw file actually gives, we intentionally over-exposed an image (by 3EV) to ensure that highlights were clipped well beyond the level that could be incorporated into the JPEG file. We then used digital exposure compensation on the raw file to see to what extent those highlights could be recovered. We found that you got no additional information beyond around -1.7 EV, and even at this point the color accuracy of the recovered areas is imperfect, as you can see in the brickwork over the window in the crops below.

To check that this incorrectly rendered detail should have been recovered by -1.7EV of exposure compensation, we compared this RAW conversion to another version of the same scene that had originally only been overexposed by 2EV (i.e. one stop darker than the previous image). In this less exposed RAW file, all of the color in the brickwork could be brought back with -0.7EV digital comp, so it's clear that there is less than 1.7EV of reliable information beyond that which appears in the default output.

Given that the default ACR conversion yields around half a stop less highlight dynamic range than the default JPEG, it's fair to say that the best you're likely to get out of a RAW file exposed based on the camera's metering is around one extra stop of dynamic range in the highlights.

ACR default conversion (of +3EV image) 100% Crops
ACR with -1.7 EV digital comp. (of +3EV image) 100% Crops
ACR with -0.7 EV digital comp.(of +2EV image) 100% Crops
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