The naked truth about shooting raw (very long post)

Started Nov 9, 2005 | Discussions thread
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Cresence Regular Member • Posts: 251
The naked truth about shooting raw (very long post)

I’ve been registered on the forum for all of two days, and already I’m writing eye-catching, steamy headlines. Please pardon my audacity. Please understand as well that I’m not arrogant enough to think I have the “naked truth” about RAW conversion, but I do have some observations and concerns that I would like to share. I haven’t seen them put quite this way in other threads (possibly because they aren’t valid and make no sense).

I want to shoot RAW. That’s been the case for some time, but lately I’ve become more dissatisfied with JPEG image editing and more inclined to push the performance of my Nikon D70 (and a D200 if Santa is nice). Out of curiosity, I began researching the intricacies of RAW image processing and where it might take me.

The basics seem straightforward enough. A typical sensor responds monochromatically to light, generating analog grayscale values in the form of either electron packets (CCD) or voltages (CMOS). A color filter array (CFA) over the sensor associates each photosite with a color, usually red, green or blue (but sometimes cyan, magenta, or yellow); a Bayer pattern in the CFA is typical, with rows of red and green filters alternating with rows of green and blue filters. In this way, the monochromatic sensor data can be separated into red, green, and blue data streams, sent to one or more 12-bit analog-to-digital converters (ADCs), and assigned a number (0 to 4095 for each photosite/color combination). Foveon sensors (with overlays to get R, G, and B values at each site) and Fuji’s SuperCCD sensor (with hexagonal photosites) involve significant departures, but I won’t go into those here.

When the R, G, and B data streams come from the ADC (or ADCs), they are incomplete. Each pixel has a value for one color but needs a value for all three colors to cover a full color spectrum. At this point, the data for the array of pixels resembles a mosaic. “De-mosaicing” is required to fill in the missing color data, that is, to render the image. This involves interpolation, edge delineation, adjustments for dark current, and noise reduction (elimination of artifacts).

This process is key. In most cases RAW files are constructed from the image data prior to de-mosaicing. Furthermore, the only camera adjustments which affect the data prior to de-mosaicing include shutter speed, aperture, ISO level, and possibly white balance (depending on the camera design). These will determine the image recorded in a RAW file. In-camera parameters for sharpness and contrast, hue and saturation, and possibly white balance don’t influence the image data and affect image processing only insofar as the parameters are passed through to software packages through EXIF 2.2 tags.

Thinking about all of this has led me to some working conclusions. Feel free to correct me, extend my thinking, or provide better examples than I have:

1. We can’t blithely assume that the de-mosaicing done by third-party software is equivalent to or better than what we can get out of the camera. Companies like Nikon regard the relevant image processing algorithms as proprietary. Nikon might ensure that Nikon Capture has the same algorithms as those in its application-specific integrated circuits (ASICs) in each camera, but everyone else has to reverse engineer this processing or construct an alternative set of algorithms. As the designer, however, Nikon is arguably in the best position to construct a good RAW converter. Furthermore, while third-arty software (e.g., Photoshop) may include RAW conversion, there are no clear standards for judging the quality of such conversion, nor does there seem to be any critical awareness that converters may behave very differently.

The one saving grace in this matter is that many third-party RAW converters, in programs like Photoshop, Bibble, and The GIMP (through the UFRAW add-in), can be traced to the programming of Dave Collins and his open-source utility, dcraw. In head-to-head comparisons of RAW file conversion between Capture and Bibble, the processing of Bibble (and presumably Mr. Collins) looked to me to have better detail and color. This suggests that Mr. Collins has essentially bootstrapped the industry to a very competitive level with respect to the manufacturers’ own proprietary approaches.

2. Diverting the image processing workflow into RAW files essentially devalues in-camera settings for sharpness, contrast, hue, saturation, and white balance (see above). Elaborate menus, knobs, and buttons to adjust such image qualities are less relevant as these adjustments become the provenance of software controls, which tend to be more refined and flexible than in-camera adjustments. I’m not saying that menus and buttons for such adjustments are now irrelevant—sometimes it just makes more sense to set them and shoot a JPEG—but I am saying that a commitment to RAW processing implies a greater emphasis on software and a lesser emphasis on camera features which are applied after de-mosaicing is complete.

Possibly I’m belaboring this. As an example, though, note the DP Review comparison of the Nikon D200 and the Canon 5D in which the 5D’s more precise color and sharpness controls were cited as a plus. For the purposes of RAW processing, such controls represent much less value-added than the review presumes. Actually, the value added in RAW processing comes with the expertise of the photographer in using imaging software. Photographers should welcome this. It makes them more economically viable, more sharply delineates the line between amateurs and professionals, and enhances the perceptions of the marketplace with respect to what photographers do.

(continued in a second part)

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