Let’s create the perception of a shallower depth of field in this image, with the police cars sharp, and the background more out of focus.
Duplicate the background layer (Ctl/Cmd-J), and add a layer mask to it ( circle-in-a-square symbol at the bottom of the layers palette.) We will blur the duplicate layer, and then mask off the foreground that we want to keep sharp.
This particular image has alot of noise, as the close up below shows. It was shot at ISO 800.
If we blur the background, the blur will eliminate the noise and look unrealistic against the foreground unless we bring the noise back. Notice also in the full size image that there are blown out highlights in the background neon lights. Because blurring is done by averaging pixels with neighboring pixels, a simple blur would dull these areas as highlights are averaged with shadows. The lens blur filter will allow us to retain the blown out highlights and also add back noise.
Click on the image thumbnail on the duplicate background layer.
Go to Filter>Lens Blur.
Set the Radius to the amount of blur that you want, or slightly more than you will likely want, planning to reduce the layer opacity to fine tune it.
The Specular Highlight section allows you to retain specular highlights that otherwise would be end up dulled to light grey. Your goal is to retain the blown out highlights without making them bigger. Brightness values in an image range from 0 for pure black to 255 for pure white. In this section, all brightness values brighter than the Threshold will be brightened, by the number of values specified in Brightness. It is best to start with the Threshold at 255 and Brightness of zero, and adjust from there. Set the Brightness somewhere around 5, and watch the areas that you know should be blown out as you slide the Threshold slider down. Stop when you are on the edge of making the specular highlights bigger than they were. Then fine tune the brightness slider. Turn the preview on and off to compare. In this case I settled on brightening all values over 238 by 5 points.
Add noise back in with the Noise Amount slider if needed. Leave the distribution on Gaussian, and check Monochromatic. Hit OK.
This blurs our entire picture. Now we will mask off the blurred layer where we want the sharp image below to show through. Click on the blurred-layer layer mask, and paint in black over the foreground cars. If the blur is too much, reduce the opacity of the layer.
Of course I recommend spending more time painting your mask than I did for the example — my edges still need some work.
Admittedly, this was a straightforward example, with only two planes — a foreground, and a background. In other situations you may need different amounts of blur for objects at different distances from the foreground. For this you will need to create a depth map. The depth map tells the Lens Blur filter how sharp each part of the image should be. Perhaps this will be a topic for a future post — leave me a comment if you have an interest in this.