Inside Photoshop

Tips for straightening things out with the Lens Correction tool.

Inside-Photoshop
Inside-Photoshop

None of us needs reminding that it's good practice to get a picture just the way you want it in the camera, rather than trying to make it right after the fact with software. In the real world, though, factors such as your position, the available lens, and the qualities of your subject often don't allow for such perfectionism. We hate it when that happens but fortunately, you can use Photoshop CS2's Lens Correction filter to fix the vignetting, chromatic aberration, and perspective distortion that may ruin an otherwise worthy photograph.

If you point a 35mm or digital SLR up to photograph a building, for example, the building looks as if it's falling backwards in the photograph. Our visual system "corrects" the perspective, of course, and a large-format photographer does likewise by using the camera's swings and tilts to straighten the building's vertical and horizontal lines. Digital photographers (at least those who can't afford a 35mm perspective-control lens) can use Photoshop's Lens Correction filter interface to remove this distortion, lighten the corners, and straighten the building to create a visually correct result.

Here's how to use it:

Once you have the image open, duplicate its Background layer, then turn off that layer's visibility on your screen.
Choose Distort > Lens Correction from the Filter menu. Check the Show Grid box to superimpose a grid on the image. Be sure to set the grid to a useful size and color; the defaults of 16 and gray are usually too small and subtle to be very helpful.Use the Move Grid tool to reposition the grid along important lines in the subject. For the image shown here I dragged the grid so that one of its lines followed the edge of the roof above the six windows.
Use the Straighten tool to make the subject's vertical lines parallel.
If needed, use the Vignette tool to lighten dark corners caused by falloff, which you can get with zoom and wide-angle lenses. (You can also use Vignette to darken corners for creative effect.) Here, I lightened the corners ever so slightly.
Use the Transform tool to correct the vertical and horizontal perspective. After bringing the building up -24 with Vertical Perspective, I noticed that the right side of the building was slightly smaller. I corrected this by adjusting the Horizontal Perspective slider with a +4 adjustment.
When the perspective was corrected, slight barrel distortion -- a bowing out of vertical and horizontal lines -- became apparent. I used the Remove Distortion tool set to a subtle -3 to fix it.

Here are some other things to consider when working with the Lens Correction tool. As the image is scaled and corrected, transparent areas are created. The options to conceal this are Transparency, Edge Extension (not usually recommended), and Background Color. I recommend either scaling to increase the image to hide the transparent edges or ignoring this feature and using the Crop tool afterwards.

It's a good idea, when you're photographing a scene with the intent of correcting it with the Lens Correction filter, to frame the scene in the viewfinder, then take a few steps back to allow for space needed to correct and scale the image. In any case, don't crop the image before entering Lens Correction; having extra image information on the sides of the image when correcting perspective is very useful.

The Set Lens Default command is a time-saver when you're shooting with prime lenses (fixed focal lengths, not zooms). After correcting an image, click Set
Lens Default to save the corrections for the next time you're working with an image that was photographed with that same lens.

Just as with a view camera, perspective correction in Photoshop can cause parts of the subject to appear unnaturally "stretched." Use it with care!

Artist and photographic educator Katrin Eismann is the author of several books on digital imaging, including Photoshop Masking & Compositing (Peachpit Press, $55). Her work can be seen at katrineismann.com, and you can e-mail her with your own Photoshop tips and techniques at katrin@photoshopdiva.com.

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