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RAW headroom

Typically, shooting in raw mode will ensure that you get around 1 EV (one stop) of extra information available in highlights. 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 converting raw files from the EOS 550D to retrieve the maximum dynamic range from our test shots.

As you can see the default Adobe Camera RAW conversion delivers slightly less dynamic range than JPEG from the camera (a more contrasty tone curve is the main culprit). It is, however, 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, totally unrealistic (and certainly 'unphotographic') image. The important 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 -2.05 EV, Recovery 0, Blacks 0, Brightness +150, Contrast -50, Curve Linear

Settings Usable range
JPEG 8.8 EV
ACR Default 7.4 EV
ACR 'Best' 11.7 EV

Highlight tone priority in raw mode

The 'Highlight tone priority' (HTP) function expands dynamic range in the highlights and can be activated via C.Fn II-6. With HTP turned on the minimum ISO setting increases to 200. We've looked at this feature in previous reviews and in the dynamic range section of this review, and HTP works in exactly the same way in the EOS 550 as it does in other recent Canon DSLRs, including the EOS 7D.

JPEG HTP OFF JPEG HTP ON
RAW HTP OFF RAW HTP ON

Most of the time, the effect of HTP is likely to be fairly subtle in our experience, but it can make a huge difference to some scenes, like the one we've shown here. These four images are the product of two photographs taken in Raw + JPEG simultaneous capture, at the same ISO sensitivity and exposure values. With HTP turned off, the brightest areas of this scene are obviously clipped, but when HTP is turned on, these areas are restored to their true color, and a lot of detail is revealed which is lost to clipping in the original shot.

The same goes for raw files as well. These two raw files were converted in ACR with -2EV digital exposure compensation, and the 'brightness' value increased to achieve equivalent luminance in the sunlit brickwork to the left of the image (which is the nearest thing to a midtone in this shot). You can see that the raw file recorded with HTP turned on contains a lot more detail in the highlight areas, and a much truer color tone. The raw file created with HTP turned off looks washed out by comparison, and the color of the bright areas is inaccurate.

In simple terms, what this tells us is that where you'd normally expect around 1EV of extra dynamic range in the highlights in a raw file compared to a JPEG, turning HTP on will get you around 2EV of extra range (i.e. 1EV more than a simultaneously captured JPEG).

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