In this final article of his three-part series on "Digital Photography for Print," Professor Sharma delves into raw image processing.
By Abhay Sharma, PhD
Last month, we described how the format of a camera raw files depends on whether it is a Canon, Nikon or Fuji file. In addition to the format, the decoding of raw files also can vary from process to process. There may be a difference between Adobe Photoshop Camera Raw, third-party processing and the vendor's own processing.
The whole problem with proprietary formats is that each one has to be incorporated individually. Thus, Adobe Camera Raw must be continuously updated to accept every new camera raw. Since last month, its latest CS2 plug-in, v.3.6, has been released, extending the raw file support to 13 additional digital camera models. (Free downloads are available at www.adobe.com.) If there was a generic camera raw format, however, we would not need to continuously update. And, more importantly, a generic file format would ensure that raw files are a safe option in the future.
The file format of a raw image is not standardized— and there is no universal method to open and render the raw image data. This article, the last of GAM's three-part series on "Digital Photography for Print," explains raw file processing options and shares the results of a colormanagement experiment.
There are a number of ways to open a raw file. You can use the manufacturer's program; to open raw images created with Nikon cameras, for example, you can use Nikon Capture, or the Nikon NEF plug-in for Photoshop. Many photographers already use Photoshop's Camera Raw; Macintosh users are coming to grips with Apple Aperture; and some photographers may be testing Adobe's Lightroom. So we see that there are different ways to process a raw image, and each one can give a different look to your images. In the same way that photographers in the past chose Fuji or Kodak film, photographers today may want to experiment with the different ways to process your raw files and then stick to a particular process that gives the desired result.
In an experiment, we shot the 24-patch Macbeth ColorChecker target in raw mode using a Canon EOS SLR. The same raw image was opened in Photoshop's Camera Raw and also in Canon's File Viewer utility, and then were compared. Both vendors made the image more colorful compared to the original. There were also changes to the all-important neutral axis. Each product may do a different color rendering to produce images that it believes are "pleasing"—and these may or may not coincide with what the user believes is pleasing.
Different methods of opening raw files are likely to give different results in image sharpness, noise and color quality—which may be critical for professional users. Photographers should experiment with the different products and then stick to the workflow that is best suited to the job they are doing, akin to choosing a film for its warm response or its good flesh tones.
When processing a raw file, there's the choice to tag the image with an ICC profile, usually either sRGB, Adobe RGB (1998) or ProPhoto RGB. It is correct to associate this profile with the rendered image. The raw file contained information relating to the camera's color response, including spectral sensitivities of the sensor elements. Processing software is able to model the color response of the camera, apply that to the image and process it into the appropriate color space.
Thus, when selecting sRGB or ProPhoto RGB, the image is correctly converted from the camera's color space encoding into sRGB or ProPhoto RGB. The image is then tagged with the sRGB profile and correctly describes the color characteristics of the image. But remember, all sRGB files are not equal; users can expect different cameras to produce different sRGB images due to the different rendering processes described earlier.
To custom-profile a digital camera, you need an RGB image of a target. The target may be the GretagMacbeth DC or semi-gloss (SG) targets, though it's possible to use the GretagMacbeth traditional ColorChecker or even the IT8.7/2 target. Camera (input) profiles can be made in a manner similar to a scanner profile, but in camera profiling it is important to consider lighting conditions. Ideally, there should be no change in the illumination during capture of the chart image and subsequent capture of scene images.
The other issue to consider is that to get RGB values for camera profiling it is preferable to use RGB values that are not processed or color adjusted, so JPEG images are not ideal. The reason for this is that the custom ICC profile you are about to make will provide the rendering. It is necessary, therefore, to have RGB values that are not rendered or color-corrected as input to the profile-making process.
There are a few ways to get unprocessed RGB data from a digital camera. You can use software such as Bibble (www.bibblelabs.com) or dcRaw, available in various forms from command line Linux to Mac OS X. Finally you can trick Adobe Camera Raw into producing an unrendered RGB file by setting shadows, brightness and contrast to zero, and curve to linear. The correct use of ICC profiles in your digital camera workflow allows you to deal with the gamut limitations of print and thus be able to correctly predict the changes that will occur to the image when it is reproduced on the smaller color gamut of a printing press.
In conclusion, raw file formats are becoming increasingly popular for photographers because they offer increased flexibility, quality and control compared to traditional JPEG processing. The problem with the raw format is that cameras can use many different raw formats— with specifications that are not publicly available. This limits photographers to using the manufacturer's software and, worse, may threaten access to the raw images if the software (or the company) becomes obsolete.
The Openraw website, www.openraw.org, is an example of a group that wants camera manufacturers to publicly document their raw image formats—past, present and future. Adobe's Digital Negative specification provides a solution to this growing problem. Something must be done lest some of us are left with the equivalent of the Betamax videotape or the Philips Videodisc. We urge you to understand the issues, experiment with the different converters and join the debate about the future of digital camera files.
Dr. Sharma, author of Understanding Color Management, chairs the School of Graphic Communications Management at Ryerson University, Toronto.
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