Demystifying Gaussian blur.

In product photography, you can direct the viewer’s eye to a certain part of the image by applying a Gaussian blur to every other part of the image. People’s eyes will naturally move to the sharpest area. You might also use this blur to hide the features of a person, license plate, or brand logo you don’t have permission to use.

Gaussian blur is also useful for reducing chromatic aberration, those colored fringes at high-contrast edges in an image. For example, if you’ve taken a landscape photo of faraway palm trees against a light-blue sky, you might find bright white or red lines along the edges of your palm fronds. Applying a Gaussian blur will reduce the extremely bright pixels around the edge of the fronds, eliminating those bright spots.

You can also take a more creative approach to this tool. For a portraiture project, photographer Andres Gonzalez recalls using a Gaussian filter to create a surreal effect. In Adobe Photoshop, he added a duplicate layer over the original image and applied a Gaussian blur to that. Then, he says, “I went in with an eraser and erased the blur in places that I wanted to be focused. It created this foggy, frosted look.” 

How to restore sharpness.

One thing to keep in mind when applying a Gaussian blur is that greater blur intensity results in decreased sharpness. In the case of the landscape photo above, when you apply the blur filter to the image, the white or red chromatic aberration disappears but the border between palm leaves and sky loses definition.

You can add sharpness back into the image by decreasing the blur radius. This is measured in pixels, and it determines just how many neighboring pixels the Gaussian function will take into account as it calculates the blur. You can also use an edge detection tool on the filtered image. As with the blur filters in Photoshop, the Refine Edge tool allows you to adjust the radius to create smooth, hard, or feathered edges.