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Applying the Lens Blur Filter in Adobe Photoshop CS5

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What formerly required the use of multiple channels, gradients, and editing steps can now be accomplished with a single filter. In this excerpt from Photoshop CS5 for Windows and Macintosh: Visual QuickStart Guide, Elaine Weinmann and Peter Lourekas show you how to use the Lens Blur filter.
From the book

In a photograph, some parts of the scene are more in focus than others. If your camera lets you adjust the depth of field via the aperture, or f-stop setting (it’s not a “point-and-shoot” type of camera), you can control how much of your subject matter stays in focus. Objects that are outside the depth of field — either in front of it or behind it — will be blurred. Other factors affecting the focus are the zoom setting and the camera lens.

The Lens Blur filter in Photoshop attempts to replicate this type of blurring. What formerly required the use of multiple channels, gradients, and editing steps can now be accomplished with a single filter. All of this number crunching does have a price, however: The filter may process slowly on a large image.

To apply the Lens Blur filter

  1. Click an image layer (or duplicate the Background via Ctrl-J/Cmd-J), then click the Add Layer Mask button add_layer.jpg on the Layers panel.
  2. Keep the layer mask thumbnail selected. Choose the Gradient tool gradient.jpg (G or Shift-G), click the Gradient picker arrowhead on the Options bar, and click the “Black, White” preset. Shift-drag across the entire document window vertically or horizontally to apply the gradient. The Lens Blur filter is going to use this gradient “invisibly.” Keep this in mind when you choose a Blur Focal Distance setting in step 5. (Don’t concern yourself with where the white and black areas of the gradient land; you’ll be able to swap them in the Lens Blur dialog.)
  3. Shift-click the layer mask thumbnail to disable the layer mask (you don’t want to mask the imagery). Click the layer thumbnail, then choose Filter > Blur > Lens Blur. The Lens Blur dialog opens.A
  4. Check Preview and click a speed. For a large file (larger than 100 MB), click Faster, or for a smaller file, click More Accurate.

    You can also change the zoom level for the preview via the zoom buttons or menu in the lower left corner of the preview window. We like to use the Fit in View setting.

  5. To mimic the depth of field in a camera, you will specify that the grayscale values in the layer mask control where the blur is applied. In the Depth Map area, do the following:

    From the Source menu, choose Layer Mask as the source for the depth map. The grayscale values in the source will control which areas remain in focus. (A setting of None would allow the whole image to blur uniformly.)

    To set the Blur Focal Distance, either specify which grayscale value (from 0, black, to 255, white) in your depth map will remain in full focus via the scrubby slider or, in the preview, click the area that you want to keep in focus. In either case, you are choosing a grayscale value from the gradient in the layer mask. Shades lighter or darker than this value will become progressively more blurry. You’ll see the effect of this after moving the Radius slider, in the next step.

    Optional: Check Invert to swap the white and black areas in the depth map, and thereby swap the areas in focus with those that are not.

  6. In the Iris area, use the Radius value to control the intensity of the blur. This produces the most pronounced effect of all the controls in the dialog. (The other Iris sliders are used for creating “photographic” highlights.) Keep the Shape setting on the default setting of Hexagon (6).
    • At any time while choosing settings, you can uncheck, then recheck Preview to compare the original and blurred images.
  7. Blurring averages the values of neighboring pixels and tends to gray out white specular highlights. In the Specular Highlights area, you can use the Brightness slider to brighten highlight areas that have become blurred and move the Threshold slider (slightly) to control which tonal range the Brightness setting affects. At 255, only pure white pixels will be affected, whereas at a low setting, most of the blurred areas will be brightened.
  8. Optional: If the blurring smoothed out too much noise from the original photo, you can reintroduce noise by doing any of the following: Move the Noise: Amount slider slightly, click Distribution: Uniform or Gaussian, or check Monochromatic to limit the noise to just grayscale pixels instead of color pixels.
  9. Click OK.AB
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