Photoshop CS3: Adjusting Color | Page 2
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Photoshop CS3: Adjusting Color
Making Other Adjustments
As you've seen, Variations is the quick way to adjust color, but sometimes it doesn't give you enough control. Other times you just want to experiment. Maybe you have a picture that's mediocre, but if you play with the colors in it and beef up the contrast, you can make something out of it. These are the times when you'll want to work with individual adjustment settings.
Consulting the Histogram
Photoshop's Histogram palette was once a dialog box. It doesn't actually do anything by itself, but if you learn how to use it, you can save yourself lots of time. If you ever took a course in statistics, you already know that a histogram is a kind of graph. In Photoshop, it's a graph of the image reduced to grayscale, with lines to indicate the number of pixels at each step in the grayscale from 0 to 255.
You might wonder why this is important. The main reason is that you can tell by looking at the histogram whether there's enough contrast in the image to allow you to apply corrections successfully. If you have an apparently bad photo or a bad scan, studying the histogram will tell you whether it's worth working on or whether you should throw away the image and start over. If all the lines are bunched up tight at one end of the graph, and the image isn't supposed to be very dark or very light, you probably can't save the picture by adjusting it. If, on the other hand, you have a reasonably well-spread-out histogram, there's a wide enough range of values to suggest that the picture can be saved. Watch out for gaps in the middle of the graph, and for ends that cut off suddenly rather than tapering down to zero. Figure 5.4 shows the histogram for a reasonably well-exposed photo.
There are plenty of lights and darks in the picture this histogram represents.
The Histogram command has another use, which is to give you a sense of the tonal range of the image. This is sometimes referred to as the key type. An image is said to be low key, average key, or high key, depending on whether it has a preponderance of dark, middle, or light tones, respectively. A picture that is all medium gray would have only one line in its histogram, and it would fall right in the middle.
All you really need to know is that, when you look at the histogram, you should see a fairly even distribution across the graph, if the image is intended to be an average key picture. If the picture is high key, most of the lines in the histogram are concentrated on the right side with a few on the left. If it is low key, most of the values will be to the left with a few to the right.
Adjusting with the Levels Dialog Box
Adjusting levels is a method of changing the brightness of an image. As you can see in Figure 5.5, the Levels dialog box has a copy of the histogram, along with some controls that you can use to adjust the values.
Be sure to check the Preview box so that you can see the effect of your changes.
Setting the black point (the point at the left of the histogram that represents absolutely saturated black) to match the concentration of darkest levels in the image, and setting the white point (at the right, indicating completely unsaturated white) to match the concentration of the lightest levels in the image, forces the rest of the levels to reassign themselves more equitably. The photo I'm using in these examples happens to be quite dark, but there's still ample detail. (You can download the uncorrected image from the book's website; the file is called chinadoll.jpg.)
Try It Yourself: Adjust Brightness Using Levels
When the colors are right, but the photo seems dull or dark, adjusting the brightness helps. Follow these steps to do that using Levels:
Choose Image>Adjustments>Levels, or press Cmd-L (Mac) or Ctrl+L (Windows).
Click the Preview box so that you can see your changes in the image window. Just for fun, you can watch the Navigator and Layers palettes change, too.
Create the desired level adjustments by moving the three sliders below the histogram to the left or right. The following are some tips for getting the effect you want:
To set the black point (the darkest black) in the image, move the slider at the left side of the Input Levels histogram to the point at which the dark lines begin to cluster.
Set the white point (the whitest tone) by moving the right Input Levels slider to the point where the light pixels begin to rise.
Adjust the midrange by watching the picture while you move the Input Levels middle slider left or right. Figure 5.6 shows the settings for this picture.
Adjusting the darks helps bring out shadow detail.
To adjust the contrast in the image, use the sliders on the Output Levels bar. The black slider controls the dark tones; moving it toward the center lightens the image. The white slider controls the light tones; moving it toward the center darkens the image.
Click OK when you're done. My corrected version is included in the color photo section. It's called China Doll, Color Figure 5.6.
Channeling Colors - In a color image, you can adjust the composite RGB or CMYK color image, or individual colors, by using the Channels pop-up menu. For now, stay with the composite. (You'll learn more about channels later in this hour.)
You can also use the Eyedroppers to adjust the levels. Click the white Eyedropper (on the right) and click the lightest part of your image. Then click the dark-tipped Eyedropper (on the left) to select it and click the darkest point on the image. If you're working on a grayscale image and there's an area in the image that seems to be right in the middle, click it with the midrange Eyedropper (in the middle). Avoid using the midrange Eyedropper in a color image unless it has an area that's supposed to be a neutral grayneither reddish (warm) nor bluish (cool); if you click in an colored area, Photoshop will adjust all the image's colors so that the area you clicked in doesn't have any color.
You "Auto" Try it - If you click Auto in the Levels dialog box or choose Auto Levels from the Image>Adjustments menu, Photoshop adjusts the levels based on its evaluation of the tonal range. However, this is usually not satisfactory. Try it, but be prepared to undo.
Adjusting with the Curves Dialog Box
Adjusting curves is much like adjusting levels, although a bit subtler. You can use the Curves dialog box instead of the Levels dialog box to adjust the brightness. The big difference is that, instead of adjusting at only three points (black, middle, and white), you can adjust at any point (see Figure 5.7).
On this kind of graph, the zero point is in the middle.
When you open the Curves dialog box, you won't see a curve. Instead, you see a different kind of graph, one with a grid and a diagonal line. The horizontal axis of the grid represents the original values (input levels) of the image or selection, whereas the vertical axis represents the new values (output levels). When you first open the box, the graph appears as a diagonal line because no new values have been mapped. All pixels have identical input and output values. As always, be sure to check the Preview box before doing anything else so that you can see the effects of your changes.
As with the Levels dialog box, you can click Auto or use the Eyedroppers to adjust the values. Because the Curves method gives you so much more control, you might as well take full advantage of it. Hold down the mouse button and drag the cursor over the portion of the image that needs adjusting. You'll see a circle on the graph at the point representing the pixel where the cursor is. If there are points on the curve that you don't want to change, click them to lock them down. For instance, if you want to adjust the midtones while leaving the darks and lights relatively untouched, click the light and dark points on the curve to mark the points at which you want to stop making changes. Then, drag the middle of the curve until the image looks right to you. Dragging up lightens tones, whereas dragging down darkens them. Figure 5.8 shows what this actually looks like (this figure is also shown in the Color Gallery). To get rid of a point that you have placed, click and drag it off the grid.
You can add up to 16 points on the curve.
A Fine Thing - To see the curves displayed in a finer grid, press and hold Option (Mac) or Alt (Windows) and click the grid.