| Controlling depth-of-field before taking a picture has been standard practice since the invention of photography. The Lens Blur filter, introduced in Photoshop CS, provides a realistic way to simulate depth-of-field after a picture is taken.
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Click to enlarge
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Download the sundial.zip archive, open sundial.jpg, duplicate the
Background layer (Command/Ctrl-J)
to preserve the original, and save your
work as a PSD file. From the Filter menu,
select Blur > Lens Blur. You can see in the
preview the blur uniformly affects the
entire image as it would with any other
blur filter. However, depth-of-field is a
variable blur effect, so click Cancel.
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Add a new layer, select the Gradient
tool, and choose the Black, White
gradient option. Draw from the bottom
of the image (creating black) to the top
(white). Press Command/Ctrl-A to select
the entire canvas, and copy it. Now hide
the new gradient layer by clicking its
visibility icon.
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In the Channels palette, click the
Create New Channel icon to make an
alpha channel. Select it, and paste the
gradient from Step 2 into the alpha
channel. Rename the channel Grad
Only so later you’ll know which one
it is. Click the composite RGB channel
(the topmost one) to hide the Grad
Only channel and redisplay the original
image. Deselect the selection.
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Click to enlarge
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In the Layers palette, select the duplicate
background layer, and choose Lens
Blur again. It looks exactly as it did in
Step 1, but now you have a depth map to
control the blur. Select Grad Only from
the Source pull-down menu. Notice
the blur decreases from the top to the
bottom. A depth map works like a mask,
except it controls the amount of blur. The
whiter values receive more of the blur
and blacker values get less. This is better,
but the sundial is blurred even though
it’s the nearest object, which is a dead
giveaway that your depth-of-field isn’t
genuine, so cancel Lens Blur.
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You need to be able to keep the filter
from blurring the sundial. Make a new
layer, and paint with black over the sundial.
Click the visibility icon for the gradient
layer, and move your newly painted
sundial shape layer above it. Select All,
choose Edit > Copy Merged, and turn off
the visibility of the top two layers. In the
Channels palette, create a new channel,
select it, and paste the merged sundial
and gradation. Rename the new channel
Combined and deselect the selection.
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Click to enlarge
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Select the duplicate Background layer,
apply Lens Blur again, but this time use
the Combined channel as the depth map.
Now we’re getting somewhere! The path
gets increasingly blurred as it recedes into the distance, while the sundial remains
sharp. Adjust the Radius in the Iris
pane until you reach the amount of blur
you want; I used a Radius of 30. Now click
OK, and Photoshop will apply the blur
using your settings and the depth map.
Toggle the blurred layer off and on to
see the difference. The final effect is shown at left.
Tip: In the Lens Blur dialog, leave Preview
set to Faster during adjustments,
but always inspect the image using
More Accurate before clicking OK.
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Doug Nelson is a freelance writer, technical editor and founder of RetouchPRO.com.
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