Dangerous Curves

Color Correction with Photoshop Curves

The human eye is an amazing optical instrument. Even the finest digital cameras are hard-pressed to match its superb ability to perceive subtle gradations of color and light, and none can match its remarkable dynamic range: its ability to cope with dramatic contrast and severely imbalanced lighting. Of all its amazing talents, however, it is probably the eye's ability to see straight through atmospheric haze—an ability that cameras lack—which makes up the major difference between what you see and what your camera captures.

Ever taken a picture of a brilliant Fall landscape only to find that the resulting photo is plagued by dull colors? Ever snapped a shot of beautifully detailed mountains only to discover that the resulting picture looks flat? Results like these occur because human vision penetrates atmospheric haze and perceives colors in their full richness; the camera, in contrast, tends to record that same haze, leading to duller colors and flatter, softer details. A polarizing filter certainly helps, but there will still be plenty of times when the results are sub-par compared with the naked eye.

Well, fret no more: There is a very effective way to correct this problem with some savvy photo-editing in Adobe Photoshop. It's called "editing with Curves." Once you master it, you'll be able to transform good shots into truly spectacular photos.

What are Curves?

"Curves" is one of the many photo adjustment features that are available in Adobe Photoshop. It provides a graphical representation of the color and light values in a photograph. These values are displayed as a curved line which can be adjusted to alter particular color and light values in the photo. It is hands down one of the most powerful and useful tools for photo-editing, though it takes some time to master.

Many photographers never learn to use Curves because they don't understand its power, flexibility, or precision. Indeed, I would need to write a small book (or perhaps not so small) to explain all of the magnificent things that Curves can do. But this article is not intended to be an instructional manual. My goal is simply to introduce you to the power of Curves so that you can see what you've been missing out on if you haven't already learned to use this amazing feature.

First, a demonstration: Below, you can see a before-and-after comparison of the same shot, captured with my Nikon D50 and my $500 Tokina 12-24mm lens. Remarkably, the "after" shot has been edited only by applying Curves adjustments to select areas—nothing else! (Some colors may appear distorted or exaggerated due to differences in monitor calibration; nonetheless, the general improvement should be startlingly apparent.)

Before CurvesAfter Curves

Although the sky looks "blown out" in the "before" shot, it actually contains a lot of detail which I was able to restore (along with the rich blue color) using Curves. Also, notice the heavy green color cast on the surface of the water in the "before" shot; I neutralized it with Curves. Likewise, atmospheric haze makes the "before" shot look dull and flat, but in the "after" shot, notice how Curves makes the whole scene more vivid: The red boxcars (center), the cement bridge pillars, the rock-strewn weeds (foreground), and the white graffitti (middle right) now look as crisp and vivid as they did to my eye.

If you were to examine an 8" x 11" print of this shot, you would also notice that the detail and color in the tiny fisherman (center) is dramatically improved, along with other fine details distributed throughout the scene. Oh, and in case you're wondering, these pictures were captured and edited as JPEG files, not RAW, so yes, this remarkable transformation is possible even while working in the reportedly "limited" editing tolerances of JPEG. That's what I call a powerful photo-editing tool!

Here's a closer look at the scene, combining the "before" and "after" shots to show the remarkable difference. All told, I applied 12 separate Curves adjustments (to specific areas of the photo) to produce the "after" results.

Before and after Curves

Now do you see what you've been missing? Keep in mind that this shot was captured with a very capable lens fitted with a multi-coated UV Haze filter. The problem is not the lens's ability to capture color, but the inevitable interference of atmospheric haze. No matter how nice your equipment, you won't get the most out of your digital camera unless you learn to tweak your photos using tools like Curves.

Learning Curves

There's a pretty steep learning curve to learning Curves (pun intended). Most Photoshop users probably know that you can edit the "master" RGB Curve for the entire photo (used to adjust brightness and contrast across the whole image), but you can also edit the Curves for individual color channels or apply Curves adjustments to precisely selected areas of the photo. That's when the true power of Curves begins to really show itself, especially for color correction.

In the illustration below, you can see the unaltered master RGB Curve (which controls brightness and contrast), as well as the drop-down menu which allows you to select the individual Red, Green, or Blue channels for color-specific adjustments. Because the photo has yet to be altered, the RGB Curve displays as a straight line running diagonally from the darkest shadows (lower left corner of the graph) to the brightest highlights (upper right corner of the graph):

Curves

In the next illustration, I have pulled the master RGB Curve down so that all of the photo's RGB values darken accordingly. If you drag the curve up instead, the entire photo brightens. Also notice that if I use the eyedropper tool to click any spot on the photo (in this case, the tip of the flower petal), the precise location of that pixel's color value appears on the Curve (as indicated by the circle on the lower part of the Curve). Every pixel in the photo corresponds to a particular point located somewhere on the Curve. Raising or lowering that point (or that part of the Curve) in the master RGB channel will increase or decrease the relative brightness and contrast of those pixels.

Curves

Here, I have made several exaggerated adjustments to demonstrate how radically the Curve can be manipulated. Each point on the curve is an "anchor point" which can be used to "lock down" the Curve so that certain adjustments only affect specific tonal ranges. Also, the eyedropper is showing that the bend in the butterfly's proboscis is a middle-toned RGB value located halfway up the Curve (as indicated by the circle). By clicking different areas of the photo with the eyedropper, you can locate specific pixels on the Curve and then use anchor points to adjust only those particular values. Notice that because there are several horizontal (or near-horizontal) segments on the adjusted Curve, those tonal ranges have been "flattened" to a near-identical tonal value; hence the "color banding" which has resulted in the image.

Curves

Here, I have selected the Green color channel from the drop-down menu. I have also used the Rectangular Marquee Tool to select a specific, box-shaped area of the photo so my adjustments only affect that area. By pulling the Green Curve upward, I have increased the Green values for all pixels in the selected area. (Pulling down will decrease the Green values. The same can be done in the Red and Blue channels.) Here, the eyedropper tool is being used to show the Green value for the tip of the flower petal, as indicated by the circle on the upper half of the Curve. Anchor points could be used to adjust precise, isolated sections of the Curve if you only want to adjust the Green (or Red or Blue) values of a particular tonal range.

Curves

In the final illustration, I have used the Wand Tool to select only the orange areas of the butterfly's wings. By adjusting the Curves for the individual Red, Green, and Blue channels (Red Curve is shown), I have changed the color from orange to bright red. This is only a small taste of the remarkable control that Curves will allow. With the help of the Wand, Marquee, and Lasso selection tools, you can adjust the color, brightness, and contrast values for any particular area or single pixel you choose!

Curves

These are the basic principles of Curves-based color correction, but we still haven't even scratched the surface of what Curves can do. Clearly, mastering Curves is worth the effort—even if you only use it to remove unwanted color casts or to rescue vivid colors from the flattening effects of atmospheric haze. Skillful Curves adjustments are a key part of the magic that separates ordinary photos from extraordinary ones.

Practice Makes Perfect

Learning to make precise selections with the Wand, Marquee, and Lasso selection tools is the real key to unlocking the full power of Curves. Yet many photos can be dramatically improved with a few simple Curves adjustments applied to the whole picture. Since this requires no familiarity with Photoshop's selection tools, it's an excellent way to begin exploring the power of Curves without feeling overwhelmed.

Below, you can see a shot of the Iowa river before and after applying a Curves adjustment to the entire picture, without using selection tools. The first shot actually looks decent when it isn't sitting side-by-side with the improved version, but here, you can see what a dramatic difference the Curves adjustments have made for the contrast, richness, and color intensity of the image. Now the dead weeds and muddied rocks have regained their proper hue, instead of the washed out gray color in the original shot.

Before Curves Edit After Curves Edit

Below, you can see all of the Curves adjustments I used to transform the Iowa river photo:

Curves for the RGB, Red, Green, and Blue channels

First, I locked down the top part of the master RGB Curve so that the highlights would remain unaltered (note the four lockdown points to keep the line straight). I also pulled the lower portion of the RGB Curve down to darken the shadows and increase contrast. Next, I bumped the Red Curve up just a little to restore some of the ruddy brown color to the rocks. Then, I bumped the Green Curve up very slightly to enrich the turquoise colors on some of the rocks. Finally, I pulled the Blue Curve down a bit to remove the blue color cast that was suppressing most of the color in the image.

Notably, all of these changes were made as a single Curves edit (with the aforementioned tweaks to each individual channel) applied to the entire picture! Trickier images (like the train bridge image that began this article) will, of course, require much greater skill to select and manipulate portions of the image, but don't let this put you off to using Curves. Inexperienced Photoshop users will still be able to obtain impressive results with many of their photos simply by experimenting with Curves adjustments. The more you experiment, the better you'll understand what Curves can (and can't) do.

To access the Curves window and begin experimenting, open a photo in Photoshop and select Image --> Adjustments --> Curves... (or press Ctrl + M). When the Curves window appears, make sure the "Preview" box is checked so you can see how your Curves adjustments are affecting the image. The drop-down menu at the top of the Curves window allows you to switch between the RGB, Red, Green, and Blue channels. When you are satisfied with the results, click "OK" (or click "Cancel" to discard the changes). You can also save your Curves adjustments to a designated location if you want to apply the same adjustments to other (similar) photos.

Mastering Curves

If you want to master Curves, a reliable reference book will certainly help. My two favorite resources are Photoshop Artistry: for Photographers Using Photoshop CS2 and Beyond (Haynes, Crumpler, and Duggan, 2007) and Professional Photoshop: The Classic Guide to Color Correction, 5th Edition (Margulis, 2006).

Photoshop Artistry provides comprehensive instruction in using Photoshop, complete with DVD tutorial assignments. It includes excellent, in-depth discussions of various editing tools, sharpening techniques, color correction (and calibration), and other vital topics. Professional Photoshop offers a more extensive examination of color correction techniques (primarily using Photoshop's curves and channels features), but also discusses other vital photo-editing concepts like sharpening. Both books are terrific resources worth their weight in gold. To my mind, Photoshop Artistry is the single best "one-stop resource" for anyone who needs truly comprehensive instruction in Photoshop and the principles of digital editing.

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© 2007, Wesley Kisting


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