Digital Camera RAW mode

by Russ Price

This is a picture of a semaphore signal in Urbana, IL, taken on March 7, 2004. It was late afternoon, with a semi-overcast sky, and I was facing northwest - a difficult lighting situation.

The camera used is a Canon EOS Digital Rebel (a.k.a. 300D in Europe, KISS Digtal in Japan), shooting in RAW mode. The RAW shooting mode embeds a JPEG into the resulting .CRW files, so I included the embedded image for comparison.

Exposure data:
ISO 200, 1/25 sec. at f/11, with a Canon 50mm f/1.8 II lens.
Exposure compensation was not set.

100% crop of embedded JPEG in the .CRW file	Same size image postprocessed from raw data

Software used:
The picture was converted to 48-bit TIFF using dcraw and ImageMagick, and postprocessed using Cinepaint. The GIMP was used to rotate and crop the embedded JPEG; no other adjustments were made to it.

Postprocessing was done under Debian GNU/Linux.

Steps in processing:

Convert to 48-bit TIFF.

dcraw -v -c -w -4 file.crw | convert ppm:- file.tif

Load the TIFF into Cinepaint. The initially-converted image will appear nearly black.

Apply gamma=2.2. This results in a washed-out image that needs more work.

Apply a curve.

Apply levels adjustments.

Apply unsharp masking.

Resize to match embedded JPEG that was extracted from the .CRW file.

This is a good example of how using RAW format can allow you to bring out details under difficult lighting situations. On a Digital Rebel, the .CRW file stores 12 bits per color channel per pixel. JPEGs can only store eight bits per channel per pixel, and the camera's JPEG conversion might not do what you want. Note, for instance, the unlit lenses on the semaphore, which disappear in the embedded JPEG from the camera.

Norman Koren has an excellent tutorial with tips on getting the most from your RAW format images. I based my technique on his advice; in particular, his page helped me understand how to use curves.

I might try tweaking the image a bit more to bring out the sky a little better...