Working with curves

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One of Photoshop’s most powerful tools is the Curves option. If I described the Levels controls as a steak knife, this precise control is the digital darkroom’s version of a razor-sharp scalpel! Before opening the Curves dialog box, choose File Window Info to open the Info Palette. Position this palette so it is near one of the corners of the screen. Choose File Image Adjustments Curves. This opens the Curves control dialog box. Alternatively, you can press the keyboard shortcut Command+M (Mac) or Ctrl+M (PC). After you open the Curves dialog box, you may notice that Photoshop’s tool palette jumps from whatever tool you were using to the Eyedropper tool. This is because you’re going to use the eyedropper to find the black, white, and gray points of the image. (Note the little eyedroppers on the lower-right part of the dialog box, with the corresponding black, gray and white point choices.) This is where having a two-monitor set-up on your computer is worth its weight in gold, by the way. With two monitors, you can have the Info palette and Curves dialog box open on one monitor while you enjoy an unobstructed view of your image on the main monitor. Fortunately, you can move the Curves dialog box around the screen, you just can’t move the Info palette around while the Curves dialog box is open.

At this point, the image needs some color baselines set. First on the list is telling Photoshop what the darkest black and brightest white points are in an image. Then, if possible (it frequently isn’t), you want to show the program where a neutral gray appears in the image. With this information, Photoshop can adjust tonal information and color balance more accurately. Follow these steps to set the white, black, and gray points:

1. Search for the image’s correct white point. This is the area of pixels as close to 256, 256, and 256 on the Info palette as possible. Be careful though. Avoid metallic reflections or glare because these don’t provide an accurate representation. In an underexposed photo, such as the example I’ve been using in this article, the white point will be fairly far away from the 256, 256, 256 coordinates. Select the white eyedropper and then maneuver the cursor over the brightest spots of the photo, noting the readings in the Info palette. This method helps you search with more precision than just viewing the image and deciding one point is brighter than another. You can undo a reading simply by choosing File ➪Edit ➪ Redo Color Sample (Command+Z on the Mac; Ctrl+Z on the PC). After you find the brightest spot, click the mouse to have the program set the white point. For this image I found the brightest spot to be at 201, 201, 199 in the sky above the trees, slightly left of center.

2. Find the image’s black point. Select the black eyedropper and move the cursor over the dark areas of the photo looking for a value near 0, 0, 0. Because this image is underexposed and reasonably contrasty, odds are good that you can get pretty close. I found a shadow area to the lower right of the image within one cluster of trees with a value of 5, 7, 2. Click on whichever point you find that comes closest to 0, 0, 0. This will adjust your image once again.

3. Look for the mid-tone gray point. Although you should always set a white point and a black point for an image, finding the middle gray isn’t as vital—in part because not all images will have an appropriate gray value. It’s still good to look for one, though. Select the gray eyedropper and examine the image looking for a value in the neighborhood of 90, 90, 90. I found a value of 88, 76, 71 and embedded (clicked my mouse on) that point for middle gray for a subtle color correction. By the way, if you use a gray card (available at any photo store catering to serious photographers) in your photography, you can place one at the edge of the image and make the photograph. Make your middle-gray reading off the card and then crop it out of your image.

After you set the white, black, and gray points, you need to adjust the actual curve for best exposure. The Curves dialog box lets you set individual points on the line from black to white. You can then make corrections to specific tonal ranges of the image. To adjust the curve, follow these steps:

1. When you open the Curves dialog box (it’s under Edit ➪Adjustments ➪ Curves), you see a grid with a diagonal line going through it. You can use your mouse to embed multiple points on this line and move small portions of the line to affect the contrast in different areas of your photograph. Unlike the Levels control, which offers three control points (Highlights, Midtones, and Shadows), Curves offers as many as 15 user-managed control points. (You seldom use more than 3 or 5 on an image, though.)

2. Start by clicking the mid-point of the diagonal line. You can then click and drag this point. Move it upwards and it lightens the image, move it downwards and it darkens it. (Whoa! Just a small amount, dragging the point halfway up or down the box will really mess up your image.)

3. Embed additional points above and below the center point. You can then move these points around individually to further tweak your image (small adjustments work best). While working in RGB format, your adjustments affect contrast and tonal values. You can also use the Curves box to make color adjustments.

4. At the top of the Curves dialog box is a pull-down menu marked “RGB.” If you click on this menu and pull it down, you’ll see individual choices for Red, Green, and Blue, in addition to the RGB offering. These choices refer to individual color channels (channel is the way Photoshop distinguishes color layers from regular layers). You can work in each of these individual colors separately from the rest, enabling you to get rid of color casts or make other changes in the color quality of your image. Color adjustments are made the same way tonal ones are, by embedding control points on the diagonal line and moving them around to change the color value of the particular channel.

After you set the white, black, and gray points, your image should be pretty close to useable. Sometimes, however, the color may seem a little bit off. If this is the case, Photoshop has a very useful tool known as variations. You choose File Image Adjustments Variations to open the Variations dialog box. This tool provides a sort of digital test strip showing lots of different versions of your image. You can just work through the examples, fine-tuning as you go. The process I just documented works well but is destructive in nature. In other words, the image’s original pixels are changed, resulting in a loss of original data. See the sidebar about “Nondestructive Editing” in this article to find out ways to keep your original images intact. Finding ways to improve digital image files while causing as little alteration to the underlying original files is currently a major drive among many Photoshop enthusiasts. As I noted earlier, many feel it’s important to preserve the original file as much as possible, so the more things you can do that don’t alter original image data, the better. Programs such as Photoshop that offer the capability to work in layers lend themselves to nondestructive editing. Many photographers start their image editing process by creating a duplicate layer of the original image. This lets them manipulate to their heart’s content, secure in the knowledge that they can always have immediate access to the original file if need be. Working on a copy layer of the file instead of the original is a good practice, but only if your computer has the RAM and processor speed to work effectively with the larger files this technique creates. If your system isn’t up to that challenge, save this method for your next machine. Just make sure you archive an original version of the image somewhere safe. Fortunately, Photoshop offers a nondestructive approach to image adjustment. This is through a feature known as adjustment layers. These are layers that can be used to apply levels or curves controls (as well as the program’s other image adjustment tools) without changing the underlying pixels. You can manipulate an image to your heart’s content with adjustment layers, and then turn the layer off and still have your original image. You can save the file as a Photoshop file or a TIFF file with layers intact, and you can come back to it a year later and still be able to change the original levels or curves adjustment as desired. Another benefit of using adjustment layers is that you tweak the opacity of the effect, giving you even more control. An even bigger advantage to working with adjustment layers is that you can activate a mask and then selectively apply the adjustment to selected portions of the image rather than the entire photo.

One of Photoshop’s most powerful features is its capability to mask out portions of an image while leaving others unaffected. Think of a mask as a user-defined overlay. You can apply the mask to certain areas (say shadow areas) so they’re protected from a particular effect, whereas leaving other areas unmasked (say highlights) so that you can change their intensity. Masks let you fix one part of an image without having that correction mess up another area. Masks are one of the things that make Photoshop great. Sometimes your overall image is pretty well balanced exposure-wise, but it has one or two areas that need a little work. Maybe a highlight’s a little too bright or a shadow area or mid-tone that’s a bit too dark. You can’t tweak levels to fix this problem, although you could create an adjustment layer and then activate the layer mask to tweak those spots. The problem is, you’re masking a lot of area just to fix a couple of little problems. As it happens, Photoshop offers some tools to deal with this sort of problem. They’re known as Dodging and Burning tools, which is what they are also called in the conventional darkroom. As a matter of fact, the principles are the same. By design, Photoshop’s creators wisely tried to keep the feel of the digital darkroom comparable to the conventional one for all of us old geezers who got our start the conventional way. Photoshop also offers a third tool—the Sponge tool—that can increase or decrease image saturation. This one’s based on the old photographer’s trick of taking a developer soaked sponge or piece of paper and rubbing it on a portion of the image to overdevelop it. In the conventional darkroom, a dodging tool is used to block light from the enlarger from reaching a specific area of the print, making that area lighter. The Dodging tool in Photoshop does the same thing, lightening the area it’s applied to. The burning tool in the conventional darkroom does just the opposite. It adds light from the enlarger, increasing the light striking the paper and making the burned area darker in the print. The Photoshop Burning tool works in the same way, darkening the area it’s applied to. Generally, it’s best when using either of these tools, to set the exposure to a small percentage and build up the effect in an area rather than using the tool at 100 percent and probably overdoing it. Using the Dodging and Burning tools results in destructive editing, by the way, as these changes affect your original pixels. Dodging and burning through Photoshop’s tools is a bit on the dicey side. For one thing, you’re changing original image information; for another, it’s difficult to be precise with this method. The following steps walk you through another way of tackling the problem that keeps your original image intact:

1. Create a new layer by choosing File Layer Create New Layer.

2. Double-click the layer to bring up the Layer Style palette. Change the Blending mode to Overlay and the Opacity to 50 percent. Leave the advanced blending part alone, but make sure the Blend If drop-down menu is set to Gray. Click OK

3. Select a brush (not the Dodging or Burning tool) a bit smaller than the area you want to work on. Set your foreground/background colors to the default black and white by typing the letter d. Use black to burn in (darken) areas of the picture that are too bright and white to dodge (lighten) areas of the image that are too dark. It’s usually best to set the brush opacity to about 50 percent or less so you don’t overdo the effect. You can always increase it by painting over the same area a second or even third time.

4. If you overdo a spot, just paint over it with the opposite color and remove some of the effect!

5. If you have a particularly difficult area to work on, consider creating a selection to isolate the part of the image you want to correct. This will prevent you from spilling over into areas you don’t want to change.