When blurring most people will revert to the Gaussian blur filter. As good as it does the job, it has some severe limitations, especially when one wants to create a narrow depth of field (DOF) effect.Let's take this wonderful landscape as an example and assume that the yellow circle is the foreground and the blue square is the background:
Now if I wanted to create less DOF and blur out the blue background I can select the background only and apply the Gaussian blur filter on the selection. This however causes some nasty side effects as part of the unselected foreground (the yellow circle) also gets blurred and spills into the blue:
This is the biggest shortcoming of Gaussian blur and you can overcome this by using the lens blur filter.Make sure to have your foreground image selected and save this selection (Select menu, Save Selection...). Give it a name, for example foreground.Now select the lens blur filter (Filter Menu, Filters, Lens Blur...). this opens a new window. On the right you will see five boxes with several settings: Preview, Depth Map, Iris, Specular Highlights and Noise. We're going to use only the Depth Map settings.From the Depth Map pull-down menu select the name you have given to your foreground selection as it was saved. Use the slider to set the amount of blur and click the invert radio button. In our colorful landscape the image will now look like this:
Notice that the yellow is not bleeding into the blue anymore. If for some reason you don't get the desired effect, deselect the invert radio button.Here is an example with a more appealing picture:Original, a nice image but the flower just doesn't 'pop':
Gaussian Blur, the method that most people use. Better than the bland original but the pink bleeds into the background:
Lens Blur, the flower is now clearly separated from the background and stands out much more:
Another thing that Lens Blur does well is to create a realistic looking bokeh (camera blur, simply said). Unlike the Gaussian filter the Lens Blur filter keeps most of the image contrast intact.To use the lens blur on the entire image select 'Layer Mask' as the source in the Depth Map' settings but make sure that the layer doesn't have a layer mask. The Lens Blur filter has several different settings. The shape sets the geometrical shape of the bokeh, this is most noticeable in highlights. I recommend you play around with the other settings to get a feel for them.Original image (almost, I put the head into he shot).
Gaussian blur. The background looks flat and not really with a realistic bokeh. Notice also the blur around the head.
Lens blur. Notice how much more realistic the blur is. Notice also the difference in contrast between this one and the Gaussian blur.
That's it and enjoy the Lens Blur.