of common optical distortions can be easily corrected in Photoshop (the
steps shown below use Photoshop 7 commands and menus, but many other image
editing programs have equivalent commands). At the wide angle focal length,
most Coolpix models exhibit barrel distortion, while pin cushion distortion
is sometimes found at the telephoto end. Add-on converters often increase
the visible distortion. Barrel and pincushion distortion make straight
lines turn into curved ones.
of optical distortion occurs with perspective. If you tilt a wide angle
lens up from horizontal to photograph a building, the resulting image
makes the walls appear to tilt back.
you use to correct all these distortions are similar:
the image you want to correct. Choose Open from the File
yourself some room to work. Choose Canvas Size from the Image
menu. Specify a square size that is 25% larger than the biggest dimension
(e.g., if the longest dimension is 2048 pixels, enter 2560 for both
horizontal and vertical size). Make sure that youre creating the
extra space on all sides of your original image (i.e., Anchor shows
that the center is chosen for the position of your original). If you
don't make the canvas square and the image centered, your subsequent
actions will be "lopsided." Dont worry about how large
this step makes your image. Well trim it back down to size in
at the image with a grid applied.
Pick Show Grid from the View menu. This applies a set
of light gray lines over your image that will help you evaluate how
much change you need to make. If straight lines bow outward from the
grid at the edges, youre seeing barrel distortion. If straight
lines bow inward from the grid at the edges, you have pin cushion distortion.
If horizontal lines are straight, but vertical lines converge towards
the center at the top, then youve got a tilt (perspective) distortion.
the appropriate transformation.
For barrel distortion, choose the Spherize filter from the Distort
submenu on the Filter menu and enter a negative value (usually
no more than 25; typically in the -5 to -10 range for Coolpix
lenses). Use lines that should be straight in the image to evaluate
the amount of distortion to use. For pin cushion distortion, choose
the Spherize filter from the Distort submenu on the Filter
menu and enter a positive value (usually no more than +25; again typically
less than +10 for Coolpix lenses). Use lines that should be straight
in the image to evaluate the amount of distortion to use. To correct
tilt, first select the pixel-portion of your image (i.e., not the extra
canvas space you made), and then choose Distort from the Transform
submenu on the Edit menu. Grab the corner handles that appear
to distort the image to form a quadrangle.
to taste. Use the rectangle selection tool to select the portion
of the image you want to retain. Choose Crop from the Image
menu to remove the extraneous sections.
your work. Select Save As from the File menu, and
save the file under a new name.
programs exist that provide more automated and sophisticated distortion
correction abilities: the two most often mentioned are a free tool written
Dersch, and while Andromeda Software
has a Photoshop filter called LensDoc that is also quite helpful.
my test chart taken with the Coolpix 995 zoomed all the way out to its
widest setting. You'll note the pronounced barrel distortion.
same image run quickly through the steps at left (I used a -8 value in
Spherize). Note that this still isn't perfect. The chart wasn't perfectly
parallel to the camera when the picture was taken (I must have grabbed
one of my depth of field test images). Still, as you can tell, this is
a lot closer, and it took all of one minute to perform start to finish!