At long last, we come to the nitty-gritty of the conversion from Camera Raw to gamma-corrected RGB. In Chapter 5, Hands-On Camera Raw, we'll look at the various ways it makes sense to use the controls Camera Raw offers. Here, though, we'll look at how they actually apply to the raw conversion.
Demosaicing and Colorimetric Interpretation
The first stage of the process, demosaicing, introduces the color information, turning the grayscale image into an RGB one. This stage is also where the initial colorimetric interpretation occursthe grayscale is converted to a "native camera space" image, with linear gamma and primaries (usually, but not always, R, G, and Bsome cameras add a fourth color filter) defined by the built-in profiles that define each supported camera's color space. (See the sidebar "Camera Raw and Color" for more details on how Camera Raw handles the tricky task of defining camera color.) The demosaicing and colorimetric interpretation happen automatically to produce the default rendering you see in Bridge and the larger one you see when you open the image in Camera Raw.
Operationally, the first step is the colorimetric interpretation. The demosaicing is then performed in linear-gamma camera space. A little noise reduction, and any chromatic aberration corrections, are also done in the native camera space. (Chromatic aberration corrections could cause unwanted color shifts if they were done later in a non-native space.)
White Balance and Calibrate Adjustments
White Balance (Color Temperature and Tint), in addition to any adjustments made in Camera Raw's Calibrate tab, actually tweak the conversion from native camera space to an intermediate, large-gamut processing space. (This intermediate space uses ProPhoto RGB primaries and white point, but with linear gamma rather than the native ProPhoto RGB gamma 1.8.)
These operations work by redefining the colorimetric definition of the camera RGB primaries and white rather than by redistributing the pixel values. It's simply impossible to replicate these corrections in Photoshop, so it's vital that you take advantage of Camera Raw to set the white balance and, if necessary, to tweak the calibration for a specific camera. (I'll save the detailed description of how to use these controls for Chapter 4, Camera Raw Controls.)
Most remaining operations are carried out in the intermediate linear-gamma version of ProPhoto RGB. You may be able to achieve a similar appearance by editing in Photoshop, but the Camera Raw controls still offer some significant advantages. The tone-mapping controlsExposure, Shadows, Brightness, Contrast, and the Curvepresent the most obvious case. The Exposure control is paramountif you don't use it, you simply aren't making the best use of your bitsbut the others are important too.
The tone-mapping controls work together to let you tailor Camera Raw's conversion from linear capture to gamma-encoded output. Collectively, they have a huge influence of the overall tonality of the image. Even if you plan to do significant post-conversion editing in Photoshop, it's well worth using Camera Raw's tone-mapping features to get the image as close to the desired end result as possible.
Why? Because doing so produces a gamma-encoded image in Photoshop with the bits distributed optimally. That means that the image will better withstand subsequent editing (see "Losing Data and Limiting Options," earlier in this chapter) and you have less work to do after the conversion in Photoshop.
The adjustments made by Exposure, Shadows, Brightness, Contrast, and the Curve tab are applied as a single operation on the raw conversion, so the order in which you make the adjustments doesn't matter from a quality standpoint. We'll discuss the workflow reasons for making adjustments in a specific order in Chapter 5, Hands-On Camera Raw.
The Exposure slider is really a white-clipping control, even though it affects the whole tonal range. You can achieve superficially similar results using Exposure or using Brightness, but even though Brightness values greater than 100 can produce white clipping, Brightness is at heart a midtone adjustment.
At positive values, the Exposure slider behaves very much like the white input slider in Photoshop's Levels command, or the Exposure slider in Photoshop's new Exposure command, clipping levels to white. But since it's operating on linear data, it's gentler on the midtones and shadows than white clipping in Photoshop on a gamma-corrected image, and it offers finer control over the white clipping than do Photoshop's controls.
When you set the Exposure slider to negative values, the story is very different, because one of Camera Raw's most remarkable features comes into play. Unlike most raw converters (or Photoshop's Exposure command), Camera Raw offers "highlight recovery." Most raw converters treat all pixels where one channel has clipped highlights as white, since they lack complete color information, but Camera Raw can recover a surprising amount of highlight detail from even a single channel. It does, however, maintain pure white (that is, clipped in all channels) pixels as white (unlike most converters that turn clipped pixels gray), and darkens the rest of the image using special algorithms to maintain the nonwhite pixels' color. See the sidebar "How Much Highlight Detail Can I Recover?" for more technical details, and see Figure 2-8 for a real-world example.
Figure 2-8. Highlight recovery
This image is overexposed, as indicated by the white spike at the right end of the histogram.
Reducing the value of the Exposure slider to -0.75 stops brings the highlights back into range. The amount of headroom varies from camera to camera, but this camera easily allows a three-quarter-stop recovery on this image.
Increasing the Brightness slider value to 60 and the Contrast slider value to 64 counteracts some of the darkening effect of the Exposure adjustment. Raising the Shadow slider to 6 puts some punch back in the shadows.
It's simply impossible to match Camera Raw's highlight detail recovery in Photoshop on a gamma-corrected image. In linear space, half of the captured data describes the brightest f-stop, so you have a large number of bits describing the highlights. Once the image is converted to a gamma-corrected space, you have far fewer highlight bits to play with.
The Shadows slider is the black clipping control. It behaves very much like the black input slider in Photoshop's Levels command, but its effect tends to be a little more dramatic, simply because it's operating on linear-gamma data, which devotes very few bits to the deepest shadows. In the first edition of this book, I characterized the Shadows control as "a bit of a blunt instrument," but changes to the logic in Camera Raw 2.3 and later have made it a much more sensitive tool. I now use it fearlessly to set the black point.
Check clipping at 100% view. At zoomed-out views, you may wind up clipping pixels you didn't intend to. Always check the 100% view before doing the conversion to make sure that you aren't clipping pixels you wanted to preserve.
Brightness and Contrast
The Brightness and Contrast controls let you tweak the conversion of the intermediate tones from the linear capture to the gamma-corrected output space. They work completely differently from the similarly named Photoshop Contrast and Brightness controls. Instead, they behave similarly to Photoshop's Levels and Curves, respectively (Brightness is a midtone adjustment, Contrast is an S-curve) but with one important difference. The Camera Raw controls use an algorithm that preserves the original hue, whereas hard curve adjustments to the composite RGB curve in Photoshop can cause slight hue shifts.
If you make little or no adjustment with the Exposure slider, it's mildly advantageous to use Camera Raw's Brightness and Contrast sliders rather than using Photoshop's tools. But the bigger your Exposure adjustments, the more essential making matching Brightness and Contrast moves becomes (see Figure 2-8, earlier in this chapter).
The Saturation slider operates similarly to the master saturation slider in Photoshop's Hue/Saturation command, but does a slightly better job of avoiding hue shifts. As with the Exposure and Shadows controls, the Saturation slider can introduce clipping, so exercise caution. I'll discuss how to spot saturation clipping in detail in Chapter 4, Camera Raw Controls.
Camera Raw allows you to convert images at the camera's native resolution, or at larger or smaller sizesthe specific sizes vary from camera model to camera model, but they generally correspond to 50 percent, 66 percent, 100 percent, 133 percent, 166 percent, and 200 percent of the native size.
For cameras that capture square pixels, there's usually very little difference between resizing in Camera Raw and upsizing in Photoshop using Bicubic Smoother or downsizing in Photoshop using Bicubic Sharper. However, if you need a small file, it's usually more convenient to convert to a smaller size in Camera Raw than to downsample in Photoshop after the conversion.
For cameras that capture nonsquare pixels, the native size is the one that most closely preserves the original pixel count, meaning that one dimension is upsampled while the other is downsampled. The next size up preserves the pixel count along the higher-resolution dimension, upsampling the lower-resolution dimension to match and create square pixels in the converted image. This size preserves the maximum amount of detail for non-square-pixel cameras, and it typically produces better results than converting to the smaller size and upsampling in Photoshop.
The one size up is also useful for Fuji SuperCCD cameras, which use a 45-degree rotated Bayer pattern. The one size up keeps all the original pixels and fills in the holes caused by the 45-degree rotation. The native pixel count size actually uses the rotation and filling in from the one-size-up processing, and then downsamples to the native pixel count.
Camera Raw's sharpening is relatively unsophisticated, with only one parameter: strength. It's handy for doing quick-and-dirty sharpening for preliminary versions of images, but it's not as flexible as Photoshop's sharpening features because it's applied to the entire image, and it lacks a radius control to let you tailor the sharpening to the image content.
Camera Raw offers the option to apply sharpening to the preview image only, leaving the converted image unsharpened. This option is useful in helping you set the overall image contrast, because a completely unsharpened image generally looks flatter than one that has had some sharpening applied. Some pundits claim that sharpening should always be applied in linear-gamma space (as is Camera Raw's sharpening). Frankly, I've yet to see any major benefit in doing so, and the relative lack of control over sharpening in Camera Raw always leads me to sharpen post-conversion in Photoshop unless speed outweighs quality.
Luminance and Color Noise Reduction
While the Sharpening control is mildly convenient, the Luminance Smoothing and Color Noise Reduction controls in Camera Raw are simply indispensable. Luminance noise manifests itself as random variations in tone, usually in the shadows, though if you shoot at high ISO speeds it can spread all the way up into the midtones. Color noise shows up as random variations in color.
Before the advent of Camera Raw, I relied on rather desperate Photoshop techniques that involved converting the image to Lab so that I could address color noise and luminance noise separately, usually by blurring the a and b channels to get rid of color noise, and blurring or despeckling the Lightness channel to get rid of Luminance noise. Compared to the controls offered by Camera Raw, these techniques were very blunt instruments indeedthe round trip from RGB to Lab and back is fairly destructive due to rounding errors, and working on the individual channels is time-consuming.
Thanks to some nifty algorithms, Camera Raw lets you address color noise and luminance noise separately without putting the data through a conversion to Labthe processing is done in the intermediate large-gamut linear RGB. Camera Raw's noise reduction controls are faster, less destructive, and more effective than anything you can do in Photoshop. So use them!