Today’s Question: In your answer about dodging and burning in Lightroom Classic you mentioned the Density setting for the brush, which reminds me that I’ve never understood what the Flow setting does. Can you clarify the difference between these two controls?
Tim’s Quick Answer: The Flow and Density controls determine the overall opacity of your brush strokes and whether that opacity builds up with more painting or whether it is fixed at a specific value.
More Detail: The Density setting is understandably a little easier to understand compared to the Flow setting. Put simply, the Density control determines the opacity of the brush stroke you’re painting at any given time.
To provide an analogy, let’s assume we’re painting with black on a white canvas. If I paint with Density at 100 then I’m painting with pure black, totally covering up the white canvas. If I paint with Density at 0 then I’m not actually painting at all. Painting with Density settings in between will yield what amounts to a shade of gray, such as middle gray with a Density value of 50.
If the Flow is set to 100, then you will be consistently painting at a single Density value. For example, if you are painting with Density set to 50 and Flow set to 100, you’re painting with middle gray. Even if you overlap the same area multiple times with a single brush stroke, you’re always painting with middle gray in this example.
If you reduce the value for Flow, you can think of the virtual paint as flowing from the brush at a slower rate. I’ll over-simplify the math here to make the concept easier to understand. Let’s assume the Density is set to 100 and the Flow is set to 20. When you initially paint, you’ll be painting at an effective Density of 20, since that is the setting for Flow. When you paint over the same portion of the brush stroke a second time, the Density goes up to 40 for that area. As you continue passing over the same area while painting, evenaully it will get up to the full value for Density.
So, for example, you could set the Density to 50 and the Flow to 10, and it would take multiple passes over the same area before the Density reaches 50, but no matter how many times you keep painting over that same area again within a single brush stroke the resulting Density won’t go above 50.
In the context of masking, the shades of gray used above for illustrative purposes relate to the strength of the adjustment. So, you can use a lower setting for Density to have a reduced adjustment effect in specific areas of the image, and you can use a reduced value for Flow if you want to be able to build that adjustment up within a single brush stroke based on how many times you overlap the same area.
Personally, I find the behavior of a reduced Flow setting has a tendency to cause obvious patterns of variability, such as striations of uneven adjustment. I therefore prefer to leave the Flow setting at the maximum value of 100, and instead vary the Density setting as needed from one brush stroke to the next to achieve the desired result.
