A lot of photographers get confused when it comes to using brushes in Photoshop, particularly when having to choose between flow and opacity. Some don't even see the difference, while others settle for working with just one setting.
There is an obvious key difference between flow and opacity and understanding it will lead you to more productive and advanced techniques. The best way to realize the difference between the two is to take out a piece of paper and a marker.
The flow of a brush is a lot more natural and closer to the actual physical process of painting. If you take a marker and paint on a piece of paper, going over the same area over and over again, you'll notice how more paint is applied every time. That's what happens on paper and that's how things are going to be on screen. Each time you paint over and over on a section of a photo, it will add color.
This is where opacity is different. No matter how many times you're going to paint over a certain area, it's not going to add any more color. If you're opacity is set to 30%, that's how it's going to stay. The only way to add more color by using opacity is to release the mouse and click again. That will add another 30% color, no more and no less.
Combining them both will give you greater control over your brush, which ultimately will help you with dodging and burning as well as coloring parts of the frame.
Aaron Nace from Phlearn has the perfect tutorial that explains the difference between flow and opacity.
Learn more about using Photoshop from these recommended books:
- The Adobe Photoshop Lightroom CC / Lightroom 6 Book: The Complete Guide for Photographers
- The Adobe Photoshop CC Book for Digital Photographers
- Adobe Photoshop CC Classroom in a Book