You may notice that when you use the Adjustment brushes in Lightroom 4, there are two sliders at the bottom of the panel called Flow and Density. They affect the opacity of the brush stroke but in two different ways. How they work may be confusing at first, but they are surprisingly straightforward.

Brush Panel - Flow & Density

When they are combined, Flow and Density allow you to really dial in how sensitive your adjustment brush is going to be. So take a look at the video and article tutorial below on the difference between the Adjustment Brush’s Flow and Density sliders.

Video Tutorial

The Flow Slider

The Flow Slider affects how much the brush is going to be applied to your image for every stroke. It is like a water faucet, where the more you open up the faucet, the faster the water will flow. Similarly, the higher the Flow on your adjustment brush, the more you apply with each stroke.

So if you have the Flow at 100%, one stroke of the brush will paint at 100% for each stroke. If you have the Flow at 50%, the first stroke will paint at 50% strength. Now, if you go over the same area with a second stroke, the second stroke will add another 50% to equal 100%.

Now if we use 20% Flow, the first stroke will flow at 20%. You will then have to go over the area again with 4 more strokes to get 100%.

So the way Flow works is that it builds up in opacity strength every time you add another stroke over the same area until you reach 100%.

Flow at 100-50-20

The Density Slider

Density slider is the maximum opacity amount that the brush will paint on the image. It doesn’t matter how many times you stroke the brush, it will only apply up to that much opacity over the area. So if you are at 50% Density, you will only have 50% opacity with your brush.

Density at 100-50-20%

Flow and Density Slider Together

Although Flow and Density approach opacity differently, they can work together. For example, if your Flow is 100% and your Density is 50%, one stroke will give you 50% opacity.

If your Flow is 50% and your Density is 100%, it will take you two strokes to build up to 100% opacity.
But if you have both Flow and Density at 50%, the first stroke will give you 25% opacity, or half of the 50% Density. From there, you can build up the opacity with subsequent strokes, but only up to 50% max opacity. This is because the Density limits opacity to a maximum of 50%.

Flow-50% Density-50%

The ability to adjust Flow and Density independently allows you to really dial in how sensitive your adjustment brush is going to be. Sometimes, you just want to apply 100% of the brush all at once. Other times, you may want finer control and will end up applying the brush a little bit at a time to build up the opacity that you desire.
And that’s how Flow and Density works with Lightroom 4’s Adjustment brush.

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