Adjusting with Adjustment Layers
There are 12 adjustments that can be applied using adjustment layers in Photoshop, listed in the following sections. Some are adjustments you’ll use a lot; others are used less often.
The Levels adjustment layer enables you to alter the tonal range of the image and its color balance. You do this by adjusting the intensity levels of the shadows, midtones, and highlights in the image using the sliders under the histogram. A levels adjustment is one of the "bread-and-butter" corrections that you’ll want to use with most of your photos.
The Curves adjustment layer also lets you adjust the image tonal range, but it has lots more controls than the levels dialog. To make a Curves adjustment, you drag on the line in the dialog box—it can be altered to a curve or an S shape, and the adjustment can be very subtle or highly artistic (see Figure 6).
Figure 6 The Curves adjustment lets you adjust the tonal range in the image to produce some useful and artistic affects.
The Color Balance adjustment layer lets you alter the mix of colors in the image. You can select either shadows, midtones, or highlights and adjust the amount of red, green, or blue in that are of the image. This correction can be used to remove or add a slight color cast to the image.
The Brightness/Contrast adjustment layer is a way to adjust the image’s tonal range so the alteration is applied equally to all pixels in the image. It’s a simple adjustment, but most users prefer to use the Levels adjustment instead because the Levels adjustment offers more control over the results.
The Hue/Saturation adjustment lets you adjust the hue, saturation, and lightness in the image. Reducing the saturation, for example, will remove the color from the image. Increasing saturation will give the colors in the image a boost.
The Selective Colors adjustment layer enables you to alter the amount of process colors in each of the color components in an image. Although it’s more typically used with CMYK images, it can be used on your own photos—but it’s not a typical adjustment.
The Channel Mixer adjustment layer is often used to create black and white images by varying the comparative amount of the red, green, and blue channels that contribute to the final effect. If you create black and white images from color images, it’s a great tool to use.
The Gradient Map adjustment layer is an artistic tool that enables you to map the tones in the image to a gradient fill of your choice (see Figure 7). You can choose a gradient from those supplied or create your own.
Figure 7 The Gradient Map adjustment layer is an artistic adjustment that maps the tones in the image to a gradient map.
The Photo Filter adjustment layer enables you to apply a digital photo filter to the image as you might use a lens filter on your camera. You can apply a Warming or Cooling filter, or you can apply a colored filter.
The Invert adjustment layer simply reverses the colors in the image—when performed on a black and white image, the result is like a photo negative. Applied to a color image, the colors are inverted. There is no dialog box for the Invert command; it’s either applied or not and has no user configurable settings.
The Threshold adjustment layer converts an image into a high-contrast black and white image in which a pixel is either black or white (not grey). Dragging the slider to the right lets you adjust the point on the luminance scale where a pixel becomes black—pixels to the left of the marker are black; to its right are white.
The Posterize adjustment layer flattens the colors in the image. The tonal levels setting sets the number of tonal levels for each of the red, green and blue channels, the lower the setting, the fewer colors in the image.
Figure 8 The Posterize adjustment layer flattens the colors in the image—you can choose more or fewer colors with the Levels setting.
You can apply more than one adjustment to an image by creating an adjustment layer for each of them. So you might, for example, apply a Levels adjustment and then a Gradient Map adjustment on top of the Levels adjustment. You can then vary the settings for the Levels adjustment to get a better result for the Gradient Map adjustment.