Adobe Photoshop often has a few different tools that handle the same things in subtly different ways. Opacity, Flow, and Density all control the visibility of certain layer aspects, but each is a little different. We’ll explain.
What Is Layer Opacity?
Opacity pops up in two places: Layer Opacity, and Brush Opacity.
Layer Opacity is pretty simple: It’s a slider in the Layers Panel that sets how visible or invisible the selected layer is. At 0%, a layer is completely transparent; at 100%, it’s completely opaque. All the values in the middle represent a sliding scale.
It’s a convenient way of dialing back the effects of any adjustment layer you make.
What Is Brush Opacity and Flow?
Brush Opacity is best understood alongside Brush Flow. Both are controlled from the toolbar when you select the Brush tool (the keyboard shortcut is B).
For every brushstroke, Opacity controls the transparency of the paint you apply, while Flow controls the rate at which it is applied. No matter how many times you paint over the same area with the one brush stroke, you’ll never get more paint applied than the level of Opacity. However, when the Flow is less than 100%, the paint effect builds up the more times you paint over an area.
The best way to see this for yourself is to grab the Brush tool, set Opacity to 10% and Flow to 100% and then paint around. Then swap things: Paint with an Opacity of 100% and a Flow of 10%.
Both Opacity and Flow have their uses depending on what you want to do. Opacity is most useful for setting a hard limit on how much paint you want to transfer, while Flow is best for allowing you to build up effects gradually. For example, if you’re dodging and burning, it’s best to use Flow to control how much paint is applied as you can add more just by painting over the area again with the same brush stroke. If you use Opacity, you’re more likely to give your work unnatural hard edges.
Flow is also really useful when you’re working with a graphics tablet as it makes the whole experience more natural.
What Is Mask Density?
Mask Density is basically just Layer Opacity but for masks. With a layer mask selected, you’ll see the slider in the Properties panel.
At 100%, any black in the mask is opaque while any white is transparent.
As you reduce the density, the blacks in the mask become more transparent. At 50%, for example, all the blacks are reduced to a middle gray.
When editing photos, you might also want to know the difference between saturation and vibrance in Photoshop Lightroom.