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Opacity and Strength, Flow and Exposure

Another way to change the performance of a paint or edit tool is to adjust the Opacity and Flow values, which also go by the terms Strength and Exposure, respectively, depending on the tool you're using. When available, these controls appear in the Options bar. Regardless of which setting you want to change, you click the triangle to display a slider bar, drag the slider to raise or lower the value, and then press Enter or Return. Alternatively, you can double-click the option box, type a value, and press Enter.

Here's a look at how these options work:

Opacity: The Opacity value determines the translucency of colors applied with the brush or pencil tool. The option is available also when using the gradient tool, paint bucket, history brush, both stamp tools, and the eraser, all of which I discuss in future chapters. At 100 percent, the applied colors appear opaque, completely covering the image behind them. (Exceptions occur when using the brush tool with Wet Edges active, which produces a translucent stroke, and when applying Mode options, discussed in the upcoming "Brush Modes" section.) At lower settings, the applied colors mix with the existing colors in the image.

Strength: When using the blur or sharpen tool, the Opacity option changes to Strength. The value determines the degree to which the tool changes the focus of the image; 1 percent is the minimum and 100 percent is the maximum. Strength also appears when using the smudge tool, in which case it governs the distance the tool drags colors in the image. Another difference between Strength and Opacity is that the default Opacity value for each tool when you begin using Photoshop is 100 percent but the default Strength value is 50 percent. Whether Strength is stronger than Opacity or these tools merely happen to know their own Strength is uncertain; but 50 percent is the baseline.

Flow: The Flow option appears when using the brush, sponge, and eraser tools, both stamp tools, and the history brush. Although it is always accompanied by the airbrush icon, you can use Flow and airbrush independently. The Flow value controls the opacity of each spot of color a tool delivers. So as a tool lays each spot of color onto the previous spot, the spots mix together and become more opaque. This means three things. First, a particular Flow setting produces a more opaque line than an equivalent Opacity setting. In Figure 5-32, for example, a Flow value of 20 percent comes in slightly darker than an Opacity value of 50 percent. Second, Flow results in a progressive effect that compounds as a brushstroke overlaps itself, also demonstrated in the figure. Third, because Flow works on a spot-by-spot basis, you can increase or decrease the opacity of a line further by lowering or raising, respectively, the Spacing value in the Brush Tip Shape panel.

Figure 5-32: Here we see the difference between Opacity, which controls an entire brushstroke, and Flow, which affects individual spots of paint. Where Opacity is consistent (left), Flow compounds wherever the stroke overlaps itself (middle). Tighter Spacing values also heighten the effect of Flow (right).

When using the brush, history brush, stamp, and eraser tools, you can combine Opacity and Flow values to achieve unique effects. You can also add the airbrush, which compounds Flow further by adding spots of color when you slow down a brushstroke or hold the cursor still.

Exposure: Available when using the dodge or burn tool, Exposure controls how much the tools lighten or darken the image, respectively. As with Flow, Exposure compounds when you corner or overlap a brushstroke, and includes an airbrush variation. A setting of 100 percent applies the maximum amount of lightening or darkening, which is still far short of either absolute white or black. As with Strength, the default is 50 percent.

Want to change the Opacity, Strength, or Exposure setting in 1-percent increments? No problem — just press two keys in a row. Press 4 twice for 44 percent, 0 and 7 for 7 percent, and so on. This tip and the preceding one work whether or not the Options bar is visible. Get in the habit of using the number keys and you'll thank yourself later.

Changing the Flow value on-the-fly is trickier but still possible. When the sponge tool is active, Flow works just like Opacity: Type a number to change the value in 10-percent increments; type two numbers to enter a specific value. But what about the brush tool and others that offer both Opacity and Flow? Typing a number changes the Opacity value unless the airbrush icon is active, in which case typing a number changes Flow. If the airbrush is turned off, press Shift plus a number key to change the Flow value. When the airbrush is turned on, pressing Shift plus a number key changes the Opacity value.