Blending modes produce an effect by combining the colors of the current layer with the colors of underlying layers. Consider using blending modes when you've already tried setting opacity but wish the results had more or less contrast than you get by setting opacity alone. Blending modes can be difficult to understand, because what makes each mode different is that they use different mathematical formulas to compare a layer's colors with the layers behind it. Like opacity, blending modes are available for layers, painting tools, layer styles, and features like filters. In the Layers palette, blending modes appear on a pop-up menu next to the Opacity value. On the options bar, the blending modes pop-up menu is labeled Mode.
Although manipulating transparency isn't the main purpose of blending modes, applying various blending modes reveals underlying layers in different ways, creating the appearance of transparency (Figure 13.9). Because of this side effect, blending modes can be a quick and creative way of creating a transparency effect. (When blending modes aren't used to manipulate transparency, they're typically used for advanced image processing, as part of special techniques for color correction and sharpening.)
Figure 13.9. Using the same two original images (top left and top right) as layers, I try two different transparency effects by simply changing the blending mode of the top layer to Screen (bottom left) and Linear Burn (right).
It isn't easy to predict exactly which blending mode will produce the results you want, because of the math behind blending modes and the many ways in which different layers can affect the results. Still, you don't have to be an engineer to use blending modesyou can try two easy ways to quickly try them out: Simply cycle through them until you see the one you want or use guidelines to take an educated guess.
To cycle through blending modes, select one layer and press Shift+=. If the layer blending mode doesn't change, you're probably changing the blending mode of a brush tool in the options barchoose a tool that doesn't use a blending mode before trying again.
To choose a blending mode through an educated guess, refer to Table 13.3, which summarizes what each blending mode does. You'll notice that blending modes fall into groups. Because it's not easy to predict how a blending mode will interact with different layer combinations, you can take a first pass by choosing a blending mode from the group that generally does what you're trying to do, and if it doesn't look right, try other blending modes in the same group. The differences between blending modes in the same group usually comes down to how much contrast they produce in tones and colors; for example, Vivid Light is similar to Pin Light but produces more intense colors.
Blending modes create the appearance of transparency, but they don't create areas that are actually transparent like areas created by a mask. If you want to create transparent areas for other programs, use layer masks or (if you must) erase areas of a layer.