Today’s Question: What is the difference between Opacity and Flow? I think I halfway understand this, but would sure like to hear your explanation.
Tim’s Quick Answer: Opacity is the degree to which a color you paint with will be opaque versus transparent, while Flow relates to whether there will be a fade-in and fade-out effect relative to opacity for each brush stroke. These options are available in various contexts (though all related to painting) within Photoshop, Lightroom, and other applications.
More Detail: I think it can be helpful to use airbrush painting as an analogy for understanding Opacity and Flow. With an airbrush you are spraying paint onto a surface. Think of Opacity as the degree to which the paint is “watered down”, which determines whether the paint will completely cover the surface you’re painting to or allow that surface to “show through” a relatively light coating of paint.
The Flow setting relates to how fast the paint is being sprayed out of the airbrush. If the paint is flowing quickly (a high setting for Flow), even if you move the airbrush extremely fast across the surface the paint will have a high effective opacity. If the paint is flowing slowly (a low setting for Flow), you are better able to vary the effective opacity along a paint stroke by varying the speed at which you move the airbrush across the surface.
So, Opacity is your primary control for determining whether your brush strokes will completely cover up underlying pixels or blend somewhat so the underlying pixels still show through. Flow is related, in that it affects the rate at which the color will be applied as you’re painting.
In other words, with Flow set below 100%, when you paint with the brush the effective opacity will initially be lower than what the Opacity is actually set to, transitioning to the full Opacity setting. Similarly, when you stop a brush stroke there will be a transition from the set Opacity to a lower value.
This explanation, of course, probably doesn’t leave the impression that the Flow setting is especially helpful. But that’s because there are two additional considerations you’ll want to take into account when making use of (or evaluating the value of) the Flow setting.
To me the real value of the Flow setting isn’t the Flow setting all by itself, but rather the Flow setting in the context of painting with a pressure-sensitive device such as a tablet and stylus. When using a pressure-sensitive device (with the Fade Jitter setting set to “Pen Pressure” on the Brushes panel in Photoshop, for example), the amount of pressure you apply with the stylus while painting determines the behavior of the brush strokes based on the Flow setting. In other words, you can use the pressure you apply when painting with the stylus to control the Flow behavior. That means instead of a fixed transition from a reduced opacity to the value based on the current Opacity setting, you can vary the transition so your stroke Opacity (and the transition between opacity effects) varies.
Another option in Photoshop is the Airbrush feature. This allows the opacity of your brush stroke to build up, based on the Flow setting. So, with a low Flow setting and a relatively low Opacity setting you can paint a quick brush stroke to paint with a low effective opacity. If you slow down (or stop) the movement of the brush the paint will “build up”, creating greater effective opacity for that portion of the brush stroke.
If you think about the Opacity and Flow settings in the context of a real-world airbrush (or can of spray paint), I think it will be easier to understand these settings in software such as Photoshop and Lightroom.