In order to achieve a natural-looking blur, click Filter/Blur/Lens Blur. Unlike Blur and Gaussian Blur, this filter imitates the blurring of a real optical lens, which helps maintaining the feeling of photographs.
On the top of the appearing dialog, you can first enable preview, then you can select whether it should be Faster (and less precise) or More Accurate. Next, you can see the Depth Map settings. As we have already created a quick mask, we wonít be needing them now. Anyway, Transparency and a previously created Layer Mask can also be used as depth maps. This setting enables you to control the focal distance of sharpness (that is, determine which areas will be sharp) with the Blur Focal Distance slider. In this example, we wonít be needing this either as we have already determined the sharpness plane (in the line of the bridge).
The Invert option is available, though. It inverts selection and thus makes the bridge blurred while leaves other areas sharp. Letís leave it untouched for now.
Iris settings are much more important for us. You can specify the Shape of the iris. The more blades it has, the softer the blurred areas will look. An iris with few blades (such as Triangle (3)) produces sharper blurs. Radius controls the extent of blurring. We used a value of 15. Blade Curvature also affects sharpness. More curved blades result in softer blurring, so the softest result is achieved by using an Octagon (8) Shape and 100% Blade Curvature. We wanted a bit sharper background, so we used Pentagon (5) with a 0% setting. Rotation of the iris affects the placement of blurred blots in a small extent.
The Specular Highlights region controls the strength of lighter blots. Brightness is an obvious setting. Threshold specifies how much of the lighter blurred areas will be affected by it. A setting of 255 will result in ignoring these sliders. Low values† make the lighter areas strongly stand out of the blurred background. We didnít need to use these options.
The last region adds Noise to the blurred areas. Amount specifies the extent of noise, and Distribution its type (Uniform or Gaussian). Select Monochromatic for single-colored noise. We didnít need any noise in this example, so we left Amount at 0.