Brushes in Aperture are handled quite differently than what you might be used to in any other application. Whenever you choose to brush in an effect, for example Dodge or Burn, you’re not actually dodging or burning the photo. You are in fact painting a layer mask between two versions (effectively two invisible layers) of the same picture—the original, and the dodged (lighter) or burned (darker) one.

This gives you two levels of control. First, while brushing, you can brush the effect in either more intensely or less intensely by adjusting the strength of the brush. What you’re really doing is choosing a black brush (0.0) or a white brush (1.0) or some level of grey in-between (0.2, 0.5, etc.). Second, you can adjust the intensity of the effect itself in the adjustment settings—at any time. For something like dodge or burn, this is one slider. For something like Curves, well, you have the entire Curves tool at your disposal. This lets you selectively brush the effect in and out of the photo, but then change the entire adjustment globally as well. Once you understand this, it makes brushes in Aperture arguably more powerful than any other software out there.

Here’s a detailed explanation, and something you can follow along with to really grok it yourself at home.

Applying an effect…

Let’s start with something without a brush at all, for example, Curves. I’ll make this really dramatic, so it’s easier to see.

Here’s the original photo:

And here’s the Curves applied:

…then brushing it in

Now if I choose to Brush Curves in, the effect disappears from the entire image, until I start brushing.

However the effect itself isn’t gone—we simply haven’t painted a mask between the versions, or the two invisible layers, yet. In this next shot, you can see that the photo goes back to the original, yet the Curves are still applied.

If I brush in a stroke at full strength of 1.0, you’ll see the Curves effect coming back through.

Then if I drop the strength to half, at 0.5, and brush again, the effect is there—but not as strongly.

Behind the scenes

So what is actually happening? The easiest way to see this is to switch to the Brush Strokes overlay. This shows only the brush strokes you’ve painted in. When you started brushing, there already existed a mask—but it was solid black. Black is a 0.0 strength brush. White is a 1.0 strength brush. And any shade of grey is anything between 0.01 and 0.99.

So you see, when you brush in an adjustment, you’re not actually brushing the adjustment at all—you’re in fact brushing a mask between the effected and the un-effected versions of the photo. Which means that you can continue to alter the effect, even after the brush strokes are applied.

First, let’s go back to the regular view (just close the brushes dialog).

Then, adjust the Curves, and notice that it’s only changing where the brush strokes are applied.

Starting with the brush

Some effects are designed to begin with the brush, not the adjustment itself, such as dodge and burn. In this next shot, I’ve cleared away the Curves and have grabbed the Burn tool.

You can see that the burn has been brushed in with a strength of 1.0, however the effect itself is quite low, at 0.4. That can be raised or lowered, just like the Curves can change, at any point in the workflow.

Brush Control

There are more controls over the brush, as well as over the brushed mask. You can alter the size and the softness of the brush as well as the strength. You can brush, erase, or using the feather tool, “feather” (or soften) the edges of an already brushed stroke. This is handy when trying to hide an edge you’ve painted in, or just soften the edge more than it was originally painted.

Be sure to look under the gear menu at the other options as well. You can apply the effect to the entire photo (all-white mask) or clear it (all-black), or invert what’s already there. You can view the overlays in a variety of ways, helping to see what’s being painted in. And you can limit the range of where the brush will be applied; to the Shadows, Midtones or just the Highlights.

Wrap it up…

Brushes in Aperture are intensely powerful. To do the equivalent in Photoshop, you would have to duplicate the layer, apply the adjustment to the top layer, add a layer mask, then brush between the two. In Aperture, all of that is done automatically the instant you click the brush icon on any Adjustment. And of course, you’re applying everything to the RAW file.