This quick little tutorial will show you how you can use the Graduated Filter, a tool which is available in ACR (Adobe Camera Raw) and Lightroom. This tool can help you create a quick vignette with maximum control, or as shown here, it can be used to lighten a specific area of an image. And this is only the beginning of what it can be used to achieve!
Here is the example image, edited in Lightroom 5 for exposure, white balance and vibrance. As you can see, the marked area is quite dark and draws the eye away from the scene in the foreground. There are a few ways that I could approach this issue; the adjustment brush, reducing shadows, etc. But none of those is a fast as the graduated filter!
First, activate the graduated filter by clicking on the box icon in the tools area above the Basic panel (vertical red rectangle). Now you will see a selection of sliders available, all of which can be used to customize how the filter will be applied. For this particuar image, since I want to lighten the target area, I increased the exposure and decreased shadows (sliders move to the right). Then, you simply click on the spot where you would like the gradient to begin and drag it out until it covers the desired image area. I clicked near the little guy’s head and dragged toward the upper left corner. The center of the graduated area is marked with a circle (white arrow), so you can see where the gradient transition has landed. You can grab this to move the entire gradient or you can grab one of the outer lines to increase or decrease the filter area. Once you have applied the filter, you can tweak any of the customization sliders to achieve the desired effect.
Here is the final image, with the graduated filter applied and the area lightened significantly.