Flow versus Opacity for Brushes


Today’s Question: Generally, photographers and photo retouchers will have a preference for using either Flow or Opacity to modify a brush stroke [in Photoshop]. Are there any circumstances where modifying both offer the ideal control?

Tim’s Quick Answer: Yes, there are certainly situations where you may want to adjust the value for both Flow and Opacity in Photoshop. While these two options for the brush tools have similarities, there are important differences between them that make both of them useful in different situations.

More Detail: The Flow and Opacity settings for the brush tools in Photoshop share a similarity in terms of their behavior. In fact, if you tested each out without being careful about your technique, you might conclude that they are exactly the same. But there is an important difference.

Both Flow and Opacity affect the opacity of your brush stroke. For example, if you set either Flow or Opacity to 50% and paint a single stroke using black set against a textured white background, you would effectively be painting with 50% gray based on the reduced opacity of the black “ink”. In addition, because the opacity rather than the actual tonal value has been adjusted, the textured background would show through the translucent stroke you painted.

The difference with these settings relates to what happens when you paint over the same area more than once, and how you go about applying the brushstroke.

If you reduce the value for Opacity to 50% but keep Flow at 100%, if you paint with black you will effectively be painting with the equivalent of 50% gray, but with transparency. If you keep the mouse button held down and paint back and forth over the same area, you’ll still only be painting with 50% gray, without having the painting effect building up based on passing over the same area multiple times.

If, on the other hand, you reduce the value for Flow to 50% while keeping the Opacity setting at 100%, you’ll initially get the same result in terms of painting with a translucent 50% gray. However, if you keep the mouse button held down and paint back and forth over the same area, the “paint” will build up in those areas. You don’t need to release the mouse, in other words, to apply a stronger effect in areas if you paint over them more than once.

In this respect the two settings differ primarily in terms of the behavior related to painting over the same area more than once. However, you could also use the two settings on conjunction with each other to achieve greater control.

For example, you could set the Opacity value to 50%, and set the Flow value to a lower value, such as perhaps 10%. If you paint back and forth without releasing the mouse, you’ll build up the effect in all areas that you paint over more than once, but that build up will never exceed 50% strength based on the Opacity setting. So you can paint with greater flexibility by using the Flow to enable a buildup of “paint” in areas you pass over more than once with a single mouse stroke, while using Opacity to control the maximum strength for a single brush stroke.

Even with the Opacity set to a reduced value, however, if you release the mouse and then paint over the same area again, the effect will build up. So again, the key difference between these settings relates to the behavior in the context of painting with a single brush stroke, but you can use the difference between the two to exercise greater control over your painting.