The Lightroom Paintbrush Tool
It's easy to think that Lightroom can only do global changes to an image and that for targeted adjustments you need Photoshop, but this isn't true. You can do quite a bit with Lightroom's adjustment brush.
First off, keep in mind that everything you do in Lightroom is nondestructive whether you are working on a raw image or an already processed jpeg. Everything you do with the adjustment brush is saved only as a list of instructions to be applied to an image on screen as well as whenever you finally do export that image in some other form (print, web, and so on). There's no way to ever paint with the Lightroom adjustment brush that will damage your original image pixels.
You'll find the adjustment brush in the Lightroom Develop module at the far right of the tool strip. It looks like a filled-in circle with long line extending out to the right. You can select it simply by clicking on it, but there's also a keyboard shortcut that's even easier. Just press the "K" key. This works regardless of whether you're already in the Develop module or merely looking at an image in the Library module. Press "K" again and the tool will be deselected. Once activated, the options for the brush will show up directly below the tool strip.
You can control Exposure, Brightness, Contrast, Saturation, Clarity, Sharpness or Color (tint) with the adjustment brush. There's a slider for each except for color which has its own selection control accessed by clicking on the color swatch where the slider should be. You can set any or all of these for a given adjustment. The color picker should present no difficulties but I do want to point out a cool feature that isn't obvious. To select a color from your active image, click with your mouse anywhere at all in the open the color picker, then drag out onto your image. The resulting color will be the one from wherever you release your mouse. The default color is no color as indicated by an "X" through the color swatch. Once you pick a color, you may be wondering how to get back to that "X." You can easily change colors, but there's no obvious way to get back to "no color." The answer is to click on the open color picker and drag out of the color area, but not onto your image. If you release your mouse either over the non-image area of Lightroom, you'll get back the that default "no color."
Below these sliders are additional sliders to control the Size, Feather, Flow and Density of the brush. There's also a checkbox for "Auto Mask" that sets the brush so that it only affects areas of a similar color. There are actually two brushes that live within the adjustment brush tool allowing you to toggle between the two as needed without resetting everything. Each brush has its own settings for this second set of sliders but shares the contrast through color values with the other brush. The brushes are designated as "A" and "B" and can be selected by clicking on those labels directly above the slider for Size. If you're a keyboard fan, you can also use the slash key ("/") to toggle between the A and B brushes.
To use a brush, set its options as desired and then simply stroke on the image as you would with the paintbrush tool in Photoshop. When you do, you'll notice that it's not entirely the same as Photoshop since Lightroom will place a "pin" marker where you first stroked. You'll also notice that the Mask mode shown at the top of the brush options will change from "New" to "Edit." So long as you continue painting with the brush, every stroke will be part of this same set of choices designated by that "pin." If you move any of the sliders while in Edit mode the effect of that brush adjustment will change everywhere you stroked. Everything changes together. To create an entirely new brush, click on "New." When you paint now, you'll get a new pin and thus a new set of brush options to work with. If you need to go back to the results of an existing adjustment brush, just click on its pin and you will be returned to Edit mode on that brush. The active brush pin will have a black center. Any other brush pins you may have will have a white center. You can also use the "H" key to toggle the display of adjustment brush pins on or off if seeing them gets in your way. Lightroom always hides the pins while you are actually painting regardless.
If you hover your mouse over the pin for a brush, Lightroom will superimpose a display of the mask used for that brush. This same display can also be seen by pressing the "O" key. Press it again to hide the mask overlay again. This mask is saved with your slider choices as part of the metadata for that brush so that it can be applied to your image as needed. You can think of it as being similar to the mask on an adjustment layer in Photoshop. By default, Lightroom colors the mask overlay bright red which should work for many images but obviously not all. To cycle through other mask overlay colors, use Shift-O to change to green, gray and black, in order, and then back to red.
If you click with your mouse on a pin and drag left, the effect of that brush will be progressively decreased over the entire area of its mask. Drag right to increase the effect. If you want to lessen the effect on only a section of where you painted, or remove the effect completely somewhere, click on the word "Erase" next to the A/B brush selection toggles and paint on your image. You can also use Control-Z (Command-Z on OS X) to undo brush actions based on your brush history. To undo all your brush actions, click on the word "Reset" at the bottom of the brush options area. Be aware that this will reset all your brushes, not just the one that is currently active.
The adjustment brush supports saved presets for common editing tasks. Directly above the Exposure slider you'll see the word "Effect:" followed most likely by the word "Custom." Custom means you aren't currently using a preset. Anytime you move a slider for one of the Exposure through Color options you will be switched to a custom brush. To use a preset, click on the word "Custom" and pick the desired brush preset from the resulting dropdown list. If you really like how you have a custom brush set, select "Save Current Settings As New Preset..." from this same dropdown list. You can even modify a built-in preset if you want and resave it under the same name. If you later change your mind and want the default presets back, go to Edit >> Preferences >> Presets and click on "Restore Local Adjustment Presets."
When Adobe released the initial version of Lightroom back in 2007, you only could adjust the entire image globally. But as early as the following year Adobe had added initial support for targeted adjustments. If you're a Lightroom user and not using the adjustment brush, hopefully this brief article has convinced you to give it a try.