Apr 6, 2005

The Power of the Curve–Camera Raw 3

Like any new Photoshop release, Photoshop CS2 has plenty of new features and enhancements to old ones, but if I have to pick one single feature that has changed the way I use the program, it has to be the addition of a Curve tab in the Camera Raw plug-in. It may seem a small thing, but this feature has completely flipped the balance between the amount of time I spend in Camera Raw and the amount of time I spend in Photoshop. My friend and colleague Jeff Schewe has jokingly referred to Photoshop as a plug-in for Camera Raw. I can only say that never has a truer word been spoken in jest!

Camera Raw’s Curve tab doesn’t replace the tone-mapping sliders; Exposure, Shadows, Brightness, and Contrast — in the Adjust tab. Rather, it lets you fine-tune the results of those sliders by letting you manipulate the contrast in specific tonal ranges. You still need to do the heavy lifting using the sliders — see “Under the Hood,” later in this story.

Camera Raw’s Curve is a luminosity curve that looks very much like the composite curve in Photoshop’s Curves command, with which it shares many conveniences — see Figure 2. The dialog also has an underlayed luminosity histogram.

Figure 1: The Camera Raw 3.0 Dialog
(click image for full size view)

Figure 2: Camera Raw’s Curve tab provides a familiar interface for tonal editing.

Of course, you can place curve points by eye, then drag them with the mouse, but that’s a relatively inefficient way of using the curve editor. Some of the conveniences the Curve tab shares with Photoshop curves can help you work quickly with the curve editor.

When you Command-drag/Ctrl-drag in the image window, a small white circle appears on the curve showing you where on the curve the pixels below the image lie. You can use this feature to determine which segments of the curve you want to make steeper (to add contrast) and those you want to make flatter (to reduce it) — see Figure 3.

Figure 3: Command-drag/Ctrl-drag over the image to see where on the curve the pixels below the cursor lie.
(click image for full size view)

When you’ve determined where you need to place curve points, you can do so by simply Command-clicking/Ctrl-clicking in the image. This places a curve point where the white circle appeared.

To adjust the curve points, you can drag them with the mouse, but an easier and more precise way is to use the arrow keys. Up, down, left, and right arrows change the values by one level, adding shift moves in increments of 10 levels. You can select the next curve point by pressing Ctrl-Tab or the previous one by pressing Ctrl-Shift-Tab on both Mac and Windows OS, so you don’t need to move your hands away from the keyboard once you’ve placed the curve points.

Note that unlike Photoshop’s curve, Camera Raw’s doesn’t update in real time (see “Under the Hood” for an explanation as to why), so adjusting curve points from the keyboard, and waiting for the preview to update, is much easier than dragging points with the mouse. Figure 4 shows the image after editing with the curve. I increased the highlight contrast and darkened the distracting background.

Figure 4: The curve allows me to increase the highlight contrast and darken the background.

Figure 5: The actual curve with points placed to product the results in Figure 4.

Under the Hood

Camera Raw’s curve appears to work very much like Photoshop’s, but producing that appearance actually requires considerable under-the-hood trickery. Photoshop’s curve operates on gamma-encoded images that have been adjusted so that the digital values represent brightness approximately the way humans see it. The center point of the curve adjusts something we recognize as the midtone.

Camera Raw’s curve, however, is actually operating on the raw linear-gamma data. The slider adjustments in the Adjust tab and the adjustments made by the curve are concatenated into a single operation during the raw conversion. But to the user, it seems to be operating on gamma-corrected data — moving the center point adjusts the midtone, even though the midtone in a gamma 1.0 tone response curve is actually way down at level 50 or so. (For those who need all the details, the curve simulates a gamma 2.2 curve, irrespective of the gamma encoding of the selected output space.)

To accomplish this feat, Camera Raw’s curve has to do a lot of processing under the hood, which is why it doesn’t preview in real time — the gamma 2.2 adjustments you make with the curve are translated into linear space, concatenated with the Adjust tab slider adjustments, then translated to your selected output space to give you the preview.

Why Not Use the Curve for All Tonal Adjustments?

If you come from the mindset that amateurs use sliders and pros use curves, you may be tempted to bypass the Adjust tab slides and try to do all your tone-mapping with the curve. If you do, you’ll likely be disappointed. The curve picks up where the sliders leave off, so you’ll get much better results if you use the curve to fine-tune the slider results, using the sliders to do the initial redistribution of the digital values, than if you try to do everything with the curve.

The curve is effective at all parts of the tonal range, but it’s particularly useful for shaping highlight detail. One of the inherent properties of digital captures is that, because they’re linear, they devote a great many more bits to describing the highlights than they do to describing the shadows. We almost always have more highlight detail than we really know what to do with. The slider controls in the Adjust tab don’t offer enough precision to fine-tune the highlight range, and this is where the curve really comes into its own. Figure 6 shows an image that appears to have little or no detail in the highlights, but in fact there’s a great deal of detail there just waiting to be pulled out.

Figure 6: Is there detail in these highlights?
(click image for full size view)

Pulling back the highlight range on the curve, using the techniques described earlier in this story to place and adjust the curve points, produces the result shown in Figure 7.

Figure 7: Yes, there is detail in these highlights!
(click image for full size view)

Obviously, there’s a great deal of highlight detail lurking in the image. This is true of most digital captures, and if you want to exploit that detail, the best place to do so is in Camera Raw’s Curve tab. Once the image is converted, a lot of that highlight detail disappears as different values in the capture get mapped to the same value in the converted image. Moreover, making the adjustment on the raw image means that it gets applied to all the iterations you create from that raw image, leaving you less work to do post-conversion in Photoshop, so it’s a win-win situation.

Figure 8: The original image is on the left. The image on the right has been fine-tuned to bring out additional highlight detail.

With the advent of the Curve tab, I can do all my global tone-mapping right in Camera Raw, and do it once for each image. I use Photoshop for sharpening, for localized tone and color adjustments, and for saving output files for different output processes, and a great deal of that work can be automated. The Curve tab may not lend itself to sexy demos the way other new features like Vanishing Point do, but it’s the CS2 feature that makes the biggest difference to my work.


Bruce Fraser is the noted author of the best-selling Real World Camera Raw with Adobe Photoshop CS.

He is also the co-author of the books; Real World Photoshop and Real World Color Management from Peachpit Press. Bruce also writes for CreativePro where he pens his “Out Of Gamut” articles.

Bruce is a founding member of PixelGenius, developers of the PhotoKit series of Photoshop plug-ins and was lead designer for PhotoKit Sharpener.

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