There are many aspects of photography that can take a minute to learn and a lifetime to master. This is true even when it comes to the applications we use to edit images, including Photoshop. I’ve used Photoshop daily for more than two decades now and I still stumble upon new tools and techniques with regularity. One tool that epitomizes this concept of having much more to offer than meets the eye is the simple paintbrush. You may wonder what’s so special about a Photoshop brush, but the reality is there are lots of things you can do with it—whether you’re painting or erasing, masking or dodging or blurring. Whatever brush tools you’re using, you need a better understanding of how those brushes work if you hope to make the most out of the tools. Namely, you need to know about two important modifications you can make to your brushes in Photoshop: Opacity and Flow.
At first glance, both of these settings may seem like they accomplish the same thing. After all, if you dial them down from 100 percent, they make a less than opaque mark, which you click to make a brush stroke on an image. But there’s one major difference between opacity and flow, and it affects how the settings are used.
Opacity is the maximum amount of paint, or density, that a Photoshop brush will apply to the image in a single click or stroke. Conversely, the flow dictates how fast that brush stroke will achieve the maximum opacity. Put simply, with a low opacity setting of, say, 50 percent, you can click and paint in the same spot for 30 seconds and you’ll never get beyond 50 percent density. Stopping and starting those brushstrokes (i.e. with new, individual clicks) you’ll add more and more paint, each time topping out at 50 percent opacity on a given stroke, but cumulatively adding up to nearly 100 percent. (Kind of like the math problem of the frog trying to jump out of a well and getting halfway each time. You’ll never fully get there, but you’ll eventually be very, very close.) Opacity sets the density of the effect being applied with each click.
Flow, on the other hand, dictates the rate at which the “ink” flows out of the virtual Photoshop brush. It’s almost like the pressure you might apply when putting actual pen to paper. In practice, it means if you set the opacity at 100 percent and flow at 50 percent, a single click won’t appear to have 100 percent opacity because of the low flow rate. But if you click and continue to paint with a low flow setting, passing over the same pixel again and again, eventually that brushstroke will accumulate enough density to top out at the opacity setting you established—in this example, that’s 100 percent.
Conversely, if you set the opacity to 50 percent and the flow to 100 percent, each click would apply the maximum amount of density to the pixel—in this case, 50 percent. No amount of subtle brushwork will change the nature of the stroke. Only releasing the click and starting again will allow you to reset and add more density to the pixels.
I like to think of opacity as the maximum density a single click will establish and flow as the rate at which that density will be achieved. A higher opacity and lower flow rate is a great way to achieve subtlety in your painting efforts, particularly if you combine it with a pen and tablet that can respond to pressure cues and increase or decrease the flow based on the pressure you apply with the pen.
Like so many things in Photoshop, building up an effect with multiple clicks and strokes is a better way to achieve a subtle, attractive result. Adjusting the opacity and flow of Photoshop brushes is a great way to gain control and achieve this.
Once you’ve reached a combination that works well for you, consider adding it to a Photoshop brush preset. By selecting the paintbrush tool and right-clicking (or control-clicking) anywhere on the image, you’ll bring up a window with brush settings, allowing you to change the size, hardness and shape of the brush. In the top right corner of this window is a gear icon that when clicked offers the option to create a new brush preset. You can save the size, shape and hardness of the new brush, as well as the tool settings, including the specific opacity and flow as reflected in the options bar atop the screen. Then simply name your new brush in a way that will make it easy for you to know exactly what effects the brush delivers.