I’m often asked what’s the difference between Flow and Opacity in the tool Options Bar for brushes. The short answer is that it’s like comparing an airbrush to a pencil.
Flow controls how much comes out of your brush at once. Imagine a can of spray paint. The longer you hold down the nozzle, the more paint builds up. It’s the same in Photoshop; but your mouse button or pen tip is the nozzle.
In contrast, Opacity is the maximum darkness you’ll get no matter how long you hold the mouse button or paint over the same area; you have to release the mouse button or pen tip, and then paint the same area again to make it darker, more like a pencil.
In other words, with Flow, the paint will build up as you hold the mouse button and you move your cursor back and forth; with Opacity, you have to make separate brushstrokes to build up the paint. So try both and use the setting that feels right for the way you work.
This tip previously published in Colin Smith’s Photoshop Tips column, in the June, 2020 issue of Photoshop User magazine.