layer mask

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 in the layers panel

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.

layer opacity example
There are three black square layers here: one set to 100% opacity, one set to 50% opacity, and one set to 0% opacity (so it’s invisible).

It’s a convenient way of dialing back the effects of any adjustment layer you make.

What Is Brush Opacity and Flow?

brush flow and opacity options in the tool bar

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).

opacity vs flow example
Both of these were painted with a single brush stroke. Notice how the effect is built up in the areas of the squiggle on the right that have been painted over multiple times, while the squiggle on the left is a flat gray.

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.

opacity and flow second example
Opacity and flow can be used in combination to get different effects. The best way to get a feel for things is to grab the Brush tool and play around.

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?

density is in the properties panel

Mask Density is basically just Layer Opacity but for masks. With a layer mask selected, you’ll see the slider in the Properties panel.

mask density set to 100%

At 100%, any black in the mask is opaque while any white is transparent.

mask density set to 50%

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.

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Harry Guinness is a photography expert and writer with nearly a decade of experience. His work has been published in newspapers like The New York Times and on a variety of other websites, from Lifehacker to Popular Science and Medium's OneZero.
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