Today’s Question: In many cases I would like to keep my foreground subject in focus while blurring the background. Sometimes my backgrounds have more detail than I’d like. I’d like to use Gaussian Blur [in Photoshop], but if I make a selection of my foreground to protect it from the blur, for some reason the blur causes a “bleed” of color from the edges of my selection. Any suggestions on how to work around this?
Tim’s Quick Answer: The simple solution here is to use the Lens Blur filter (instead of the Gaussian Blur filter) in conjunction with a saved selection. The Lens Blur filter can use the saved selection as a mask, which will protect the areas you don’t want to blur and prevent the “bleed” effect you’re seeing with Gaussian Blur.
More Detail: Note that this technique was covered in detail in the article “”Step by Step: Blurring a Background”, which appeared in the March 2015 issue of Pixology magazine (http://www.pixologymag.com). The basic process involves working on a copy of the Background image layer, creating and saving a selection of the area you want to blur, and then applying the Lens Blur filter with the saved selection set as the mask for the filter.
In the days before the Lens Blur filter was available (it was introduced with Photoshop CS), it was possible to work around the limitation of the Gaussian Blur filter. This involved creating a selection of the area you wanted to blur, then applying a one-pixel Gaussian Blur to the selected area. You could then contract the selection by one pixel using the Contract command (Select > Modify > Contract) with the value for “Contract By” set to one pixel.
By repeating the process of blurring and contracting, by one pixel at a time for each, you are effectively building up the blur by an increasing amount as you move the selection outward from the edge of the area you’re protecting. You can automate this process with an action, but it is still far less efficient than using the Lens Blur filter combined with a saved selection.