One of the creative techniques a photographer uses to focus attention on his subject is the depth of field. Using one or two or all three of the controls available for determining the depth of field, he can make the subject pop out against a blurred background or make everything sharp in the picture.
For instance, to take the picture of a woman seated on the steps of a Hindu temple in Singapore, use an opening of f/5.6 and a lens with focal length of 52mm (78mm in the 35mm format). Despite the moderately wide aperture, the busy background still comes off relatively sharp. Zooming to a telephoto setting would have blurred the background but you just wanted to capture the colorful dress of the woman and did not want to tax the patience of the model, so took only one shot, knowing that you could adjust the depth of field in the digital darkroom using Photoshop’s Lens Blur filter!
The Lens Blur filter, if my memory serves me right, was introduced in version 7. It is a more effective filter than the Gaussian Blur in mimicking the blurriness that the lens creates when one uses a wide opening.
Here are the steps to blur the background using the Lens Blur filter.
Opening the image in Photoshop, do some tonal and color adjustments (as you always should before doing any creative editing).
The key to using the Lens Blur filter is the alpha channel. In the Channels palette, create a mask (completely black) covering the woman and the pillar immediately behind her. Then using a magic wand, select the mask and then invert it so you could apply a gradient mask on the background. Using a gradient mask made the blurring of the background more gradual, with the far end of the picture the most blurred.
In the Lens Blur dialog box, load Alpha 1, as the Source in the Depth Map field. Automatically, the filter blurs the background.
Adjust the intensity of blurriness by moving the radius slider to 66.
Still in the Lens Blur dialog box, enter an Amount of 2 in the Noise field, set Distribution to Uniform and click on Monochromatic. These settings introduced the appropriate amount of noise to make the blurred part of the image look as rough as the untouched portion of the image. By doing this, you restore the texture that was smoothened out when applied the Lens Blur on the background.
To heighten the focus on the woman, increase the overall contrast of the image by darkening the background. So in the Channels palette, load Alpha 1 as a selection.
With the selection loaded, copy the Background Layer into another layer, and set the Blend Mode of the new layer to Multiply. Adjust the Opacity of the new layer to achieve the amount of darkening that you want. Check the Before and After images to see the effect of blurring and darkening on the background.