1. Why Choose a Radial Filter
Most of the edits that you will make in Lightroom's Develop Module in the Basic Panel and other panels will apply adjustments to the full photo. However, like many of the other Adjustment Tools in Lightroom, a radial filter gives you the ability to target an adjustment to a specific area. This is super helpful in editing since there are many times that we need to be able to edit one specific area and leave the rest alone!
2. Where to Find the Radial Filter
The Radial Filter is an adjustment tool and can be found on the right-side of the Develop Module just below the histogram. It is located in between the brush tool and the graduated filter.
To apply a radial filter, with the radial filter selected, click and drag a circle over an area of your photo. You can always adjust the shape, size, and position of your radial filter by using the small pins on the outside of the circle or just dragging the full filter to a new location.
3. Access to All Pretty Preset Brushes and Filters
When using the radial filter, you have access to all the Pretty Preset brushes and filters you have in your collection. Any of the brushes can be applied as a filter or brush. Any of the filters can be applied as brushes. This is a great feature in Lightroom since it gives you the ultimate in flexibility when applying adjustments.
4. Invert Mask
As a part of the radial filter, there is an invert mask checkbox that allows you to decide whether the adjustments you are making are applied outside the circle or inside the circle you create. By default, the adjustments are applied OUTSIDE the circle you create. By checking the Invert Mask checkbox, the adjustments will be applied INSIDE the circle.
5. Removing Part of the Effect
In Lightroom 6 and Lightroom Creative Cloud, you have the ability to brush on MORE of the effect beyond the circle constraints of the filter. OR, you can remove some of the filter from where it is being applied. With your radial filter selected, choose the "Brush" option at the top of the tool panel. This will open a new brush section of the Radial Filter Panel. By default, the brush will be set to ADD more of the effect. However, if you want to remove the effect, just choose the "Erase" option next to the brush settings.
Jessica Foreman created this great video for Pretty Presets showcasing the Radial Filter and why you might choose to use it over the brush tool in some cases.