by Jim Whitcomb
Because the Brush tool stroke, used with the Airbrush option, looks and reacts exactly like an airbrush, let’s take a moment to study how an airbrush works and how the spray stroke behaves. An airbrush, as shown in Figure A, is essentially a tube connected by a hose to a source of compressed air. Located either on the bottom, or in this case, the top of the tube is a small attachment that connects to a container holding a color medium such as a paint or dye. When you press the control button, the air travels through the tube, creating a vacuum. The vacuum picks up the color and, in turn, sprays it out the nozzle.
You can set the spray stroke to anything from a fine mist to a course splatter; and you can adjust the rate to anything from a slow trickle to a full wash, resulting in a wide range of spray strokes. As the color spray leaves the nozzle, it spreads out in a conical shape. The majority of the color is concentrated in the middle and begins to thin as it reaches the outer part of the cone.[…]