Out of Gamut: Calibrating Camera Raw in Photoshop CS
For my money, one of the most-underrated features in Adobe Photoshop CS is the Camera Raw plug-in, which reads raw images from just about every camera on the market that supports that format. (Version 2.2, which adds support for several new models thus bringing the total to more than 60 cameras, was released in late April.)
Camera Raw goes to work as soon as you point Photoshop's File Browser at a folder full of raw files, building previews and thumbnails, and reading image metadata. It also kicks in when you open a raw image -- the Camera Raw dialog box appears automatically.
Each camera vendor uses its own proprietary format for raw images, and supplies software for processing them, but in my experience, the vendor-supplied solutions leave a lot to be desired in terms of speed and stability. And of course, each vendor's solution is limited to processing raw files from its own cameras. Camera Raw, on the other hand, is not only fast and well behaved, it's also the nearest thing we have to a universal raw converter. Last but by no means least, it delivers images right where I need them, in Photoshop.
But one of the frequent knocks I hear about Camera Raw is its inability to make use of custom camera color profiles, thus ensuring more-consistent color results from the camera. It's true: Camera Raw can't use custom camera profiles. But having wrestled with many camera-profiling tools, I'm tempted to regard this as a feature rather than a limitation. If you shoot in the studio under tightly controlled lighting, and you spend four figures on camera-profiling software, you may be able to create a profile that accurately describes your camera's color behavior, at least under that lighting condition, but all my efforts have met with limited success at best.
Many of the objections I hear to Camera Raw's lack of support for custom camera profiles seem to be based on theory rather than practice -- when I ask the objectors if they've tried dialing in Camera Raw's color interpretation using its built-in Calibrate features, I'm usually met with a glassy stare, hence this column. But before delving into dialog boxes, it's worth describing briefly how Camera Raw handles color.
Camera Raw and Color
Even though you see color when you look through the viewfinder, a digital raw image is a grayscale file! Most cameras use sensors with red, green, and blue filters placed over each element in the sensor in a Bayer pattern, with twice as many green filters as red and blue, because our eyes are most sensitive to the green wavelengths. The raw file contains grayscale pixels, each one of which represents only red, only blue, or only green. The principle task of a raw converter is to interpolate the missing color information for each pixel (a process known as demosaicing) to produce a color image.
Camera Raw does this, and much more, but one of the features (besides speed and stability) that distinguishes Camera Raw from other raw converters such as the camera manufacturers' proprietary software is the way it defines color. For each supported camera, Camera Raw contains not one but two built-in profiles: one built under a tungsten light source, the other under a daylight light source. Camera Raw's White Balance tools -- the Temperature and Tint sliders -- interpolate between (or even extrapolate beyond) these two built-in profiles.
Why two profiles? Simply, the vast majority of cameras respond very differently to tungsten light than they do to daylight. Digital cameras are inherently much more sensitive to red light than to blue, and the makeup of tungsten light, which contains a great deal more energy at red wavelengths than at blue ones, exaggerates this response. So a single profile may do a good job of representing the camera's behavior under daylight or tungsten, but probably not (in my experience, never) both. Kudos to Thomas Knoll, Camera Raw's creator, for producing an elegant solution to a tough problem.
But the profiles built into Camera Raw are generic profiles for the camera type. If your specific camera differs from the units that were used to build the profiles, the resulting color will be off. So Camera Raw includes controls for tweaking the behavior of the built-in profiles to better match the behavior of your personal camera.
The Calibration Process
I don't claim that this procedure is the only way to use Camera Raw's Calibrate controls, but it's relatively simple, and has worked well on the dozen or so different camera models on which I've tested it.
The first thing you need is a reasonably well-exposed image of a known color target, which you then photograph with the camera whose response you want to tweak in Camera Raw. I prefer the ubiquitous 24-patch Macbeth Color Checker to other targets, both because it doesn't have a huge number of patches to deal with and because the pigments used to create it don't shift color appearance dramatically under different lighting, which isn't true of many scanner targets printed on photographic paper.
The second thing you need is a color-accurate digital representation of that target, which you'll use as a reference to which to compare your shot of the Color Checker. You can download a Lab color version of the Color Checker from www.colorremedies.com/realworldcolor/downloads.html.
If you want to proceed purely visually, you can use the Color Checker Lab image as is, but I find that it's helpful to have RGB numbers as a guide, so I suggest converting the Lab file to your chosen RGB working space using relative colorimetric rendering to obtain an accurate RGB version. One more catch: When you're working in Camera Raw, you can't use Photoshop's Info palette to get the RGB numbers from the target file. I just bit the bullet and spent a few minutes typing in the RGB values for each patch on Photoshop's type layers. Figure 1 shows a ProPhoto RGB version of the Color Checker with the RGB numbers entered for each patch. We'll call this the reference image, since it represents the accurate color for which you're aiming.
Figure 1: The Color Checker, downloaded from the Web site and converted from Lab to ProPhoto RGB, with RGB values added as type layers.
Setting Up for Calibration
With the converted RGB Color Checker reference image open in Photoshop, open your raw capture containing the Color Checker you shot, and arrange the windows on screen so that you can see the reference image while you work in Camera Raw, as shown in Figure 2.
Figure 2: Arrange the windows containing the two Color Checkers so that you can see the reference image (top) as you work with the calibration image (bottom).
To make the Calibrate tab available, you first have to click Camera Raw's Advanced button. This makes two new adjustment tabs, Lens and Calibrate, appear, as shown in Figure 3.
Figure 3: Click the Advanced button to make the Calibrate tab appear.
Before working in the Calibrate tab, you'll need to use the controls in the Adjust tab to make the overall contrast and saturation of your capture match the reference image. I do this using Camera Raw's RGB Readout as a guide, making adjustments that try to match the tonal values of the gray patches in the bottom row of the Color Checker I photographed to those of the reference image.
Figure 4 shows the controls in the Adjust tab and the RGB Readout, which indicates the RGB values under the cursor, just like Photoshop's Info palette. The controls fall into three groups: the Temperature and Tint sliders comprise the White Balance controls; the Exposure, Shadows, Brightness, and Contrast sliders are the tonal controls; the remaining slider, Saturation, controls the saturation.
Figure 4: The Adjust tab and the RGB Readout.
The Tonal Controls
A quick explanation of Camera Raw's tonal controls is probably helpful. Taken together, the Exposure, Shadows, Brightness, and Contrast sliders define a five-point curve, as follows:
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