White Balance and Camera Profiles
Adjusting the Black Point, White Point, and Contrast
Correcting Aberrations
Sharpening the Capture
Converting the Image to Black-and-White
Output Formats and Bit Depth
Batch Processing
Demosaicing
Demosaicing the color array is the process of filling in the incomplete color information resulting from the brightness data gathered through the CFA (color filter array). That is, the demosaicing process supplies, for each pixel, the two channels of color information not recorded through the specific photosite’s color filter. The RAW converter does so by interpolating from information recorded at neighboring photosites. Because demosaicing requires the estimation of missing color information, and each converter uses a different algorithm, the color rendering of a RAW file will vary from one converter to the next. In other words, the computer fills in the red and green components for each site with a blue filter (and the corresponding channels for the other photosites) based on data from nearby photosites. The demosaicing process results in three grayscale images, one for each of the red, green, and blue color channels, which are integrated by the software into a full color photograph.
White Balance and Camera Profiles
Where the white balance of color film is set by the film’s emulsion and the use of color correction filters (Chapter 6 and Chapter 7), the RAW file is not color balanced. Instead, the camera’s white balance setting is saved as a part of the metadata that accompanies the RAW capture and becomes the default white balance setting in the RAW converter. Importantly, as with the other setting in Adobe Camera Raw, no color correction will be made until the end of the conversion process, so you are free to change white balance settings, select intermediate color temperatures, preview the results, and change again, all without incurring any losses. This gives you immense control at any time, allowing you to change things as your seeing and thinking evolve.
In ACR, the color balance settings are controlled by the Color Temperature and Tint sliders found on the Basic tab. Figure 11-10 shows a RAW capture rendered in ACR using the As Shot color temperature. That is, ACR defaults to the camera’s color temperature setting recorded in the metadata accompanying the capture, in this case the daylight setting of 5500°. If you measured the color temperature when you made the capture (a process discussed below), the measured value will become the default setting in ACR.
The ACR’s White Balance drop down menu offers a number of preset color temperatures including Daylight, Cloudy, Shade, and Tungsten, and you can select any value you wish by moving the Temperature and Tint sliders or by directly entering numeric values. Try the presets, move the sliders, and use the settings that render the image as you wish. As with the other settings in ACR, trying different white balance settings does alter the RAW data and is applied to the processed TIFF or Photoshop format file only when you open or save the image. When you save the converted image the file will be saved as a new file. The RAW file retains all of the information from the capture should you want to go back to your original and try other converter settings.
For images that require precise color fidelity (suppose you’re photographing a 17th century painting for preservation purposes), you can first photograph a neutral gray or white target in the prevailing light. If you are using a DSLR or one of the more advanced point-and-shoot cameras, the camera will include the measured color temperature in the metadata, and the field determined color temperature will become the default white balance setting in the RAW converter.
Color temperature and balance are but two elements of the larger subject of color management—the science and art of causing each color in the scene to be rendered identically throughout the digital process. It is the goal of a color management system to cause the colors in front of the camera to be accurately rendered on your computer display, on the Internet, and on the final print. In reality, the goal cannot be achieved fully because each physical device has a limited universe of colors that it can capture, display, or print. Furthermore, reflective images (prints) have different properties than do light-transmitted images (such as computer displays and transparencies). The universe of color that any device can record or reproduce is known as its “gamut” and, in the real world, the gamut of the camera is larger than the gamut of the computer display, and the gamut of the display is different from and often larger than the gamut of an inkjet printer. Figure 11-12 shows a graphic depiction of the gamuts of the visible spectrum,, the Adobe RGB color space, the smaller sRGB color space, and the gamut for a typical inkjet printer.
Figure 11-11. This shows the same capture rendered using the Shade white balance setting
The task of maintaining color consistency across physical devices is the function of the color management system. Needless to say, the subject of color management is highly technical, and beyond the scope of the chapter. However, there is one part of color management that you should be aware of as you convert your RAW captures and that is the subject of camera profiles. Camera profiles are data tables used by the RAW conversion and image editing software to cause the computer display to render color as it appeared at the time of the capture.
(Graphic created by Stephen Laskevitch.)
Figure 11-12. Graph of gamuts for human vision, Adobe RGB, sRGB, and a typical inkjet printer
Camera manufactures and software vendors include profiles in their proprietary RAW converters, thereby insuring consistent color rendition across its family of cameras. ACR offers a default profile for almost all cameras and, for the ultimate in color control, allows the photographer to apply a custom profile for any specific camera and lens combination. If you are working with ACR, you can access any custom profile you may have under the Camera Calibration tab. As you work with camera profiles and white balance, keep in mind that color rendering is subjective and is therefore fair game for the exercise of artistic judgment. There are no absolutes; there are no rules. Always keep your options and goals in mind, for they’re the heart of expressive photography (and, in fact, of all art).
Adjusting the Black Point, White Point, and Contrast
The response of the camera’s sensor is linear, meaning that a change in brightness produces a proportional change in electrical response. Because the linear response of the sensor, without modification, produces a low-contrast, dull photograph, the RAW converter must modify the contrast to render an acceptable image. In addition, the RAW converter must set the black and white points, that is, the brightness levels which will be rendered pure black and pure white. In ACR, the settings that control what levels are to be rendered pure black, pure white and contrast are found under the Basic tab. It is generally useful to modify the settings in the order in which the sliders appear in the user interface. Here is where you can take the washed-out image that you see on your camera display and change it to get the full tonal range you want in the final print. On the Basic panel start by adjusting the Exposure, capture important highlight values using the Recovery slider thereby setting the white point, use Fill Light to lighten shadows, and Blacks to set the black point. Then proceed to the Clarity, Vibrance, and Saturation sliders for fine-tuning. Although editing the washed-out RAW capture can be postponed until you edit in Photoshop, it is generally best to get as close to the final image as early in the process as possible, and make your refinements later on.
In ACR, the Exposure and Recovery sliders set the white point; the Blacks slider sets the black point. Together, the three sliders provide the pure black and pure white endpoint references from which the other tonalities are measured, however, the settings do not compel you to include pure black or pure white in your photograph. If you want to produce a high key image, increase the Exposure and Recovery settings to render the darkest portions as midtones or light tones. Don’t confuse the setting of the black point with having a black in the image. ACR will set a black point, or you may well set a black point, but a high key image may still have nothing darker than a midtone gray. In other words, the black point is a reference from which other tonalities are measured, but you do not have to have any black in your image. So, setting a black point doesn’t force you into having a black if you don’t want one. Nor, of course, does setting a white point force you to have a white when none is desired. (See Figure 3-4 and Figure 3-5, respectively, as examples of images lacking a black or a white). Figure 11-13 shows a RAW capture and histogram converted using the ACR default settings.