Even expensive lenses have distortionLens distortion can be a combination of different factors, and nearly every lens has it. Cheap lenses can have some amount of Chromatic Abberation, which is a purple haze
that appears around very bright edges of a highlight. Zoom lenses are also prone to barell distortion, which is the buble or pincushion effect you see at different zoom lengths. Normally you don't notice any geometric distortion until you take a photo that has straight horizontal or vertical lines, such as a building, a doorway, or even a product box.
Even if you have an excellent lens with little distortion, you may take a photo where the perspective isn't quite right. You may have intended to get a forward shot of a sign a few feet above you, but unless you have a ladder you'll never be able to get a straight shot at it.
All of these situations can be corrected with one of Photoshop CS2's most powerful filters: the Lens Correction filter. Before this filter you had to use several transformation and pincushion filters, which required a bit of trial-and-error as the effects of one filter would mess up the straight lines you fixed with the other one.
Open the filter by clicking Filter -> Distort -> Lens Correction
To start up this powerful tool, click on Filter -> Distort -> Lens Correction. A new window will pop up with your selected photo under a grid.
With this grid, it will be very easy to identify horizontal and vertical lines, where the edges of a building bubble, and other geometric defects.
If your perspective is right and you just need to clear out a bulge or pincushion, simply use the Remove Distortion slider. Moving to the right will simulate a pincushion effect, and moving to the left simulates a bulge.
Chromatic abberation can be introduced by a cheap lens, or the coating on some lens filters, and usually shows up as a purple or magenta haze around highlights. This occurs most frequently on shiny objects taken in sunlight. Most of the time, you'll only need to play with the Fix Red/Cyan Fringe, but adjust the sliders until the haze blends into the background.
The Lens Correction Window and Vertical Slider
The Transform sliders are used to correct an unfortunate perspective. Perhaps you wanted a building to be completely vertical, but because of where you stood the lines are somewhat crooked. Use the Vertical Perspective slider to correct for this, and the Horizontal Perspective slider to correct for any left-to-right lines that aren't quite horizontal.
Use Guides and Free Transform to fine tune the lines
You'll notice that one slider in this tool affects the corrections made from another slider. Use the grid to size up your lines, and make adjustments accordingly.
Since you're probably bringing in the edges, you'll probably introduce some "negative space" into the photo, visualized by Photoshop's checkerboard pattern (transparency). Use the Scale slider to bring your edges back to the edge of the photo.
You may have to do some minor correction after using the Lens Correction tool, but the majority of your work is done. There may be some transparency left, which you can either paint in or crop out. You may also have to rotate the photo a few degrees before or after using this tool to correct some perspective that you just couldn't tweak.
Before and after correcting with the Lens Correction filter
If you just can't get a square perspective, then drag out some guidelines (click on View -> Rulers, and then click and drag from the side rulers) to "frame up" where the edges should be. You can also dispay a grid by pressing Ctrl + ' (semicolon).
Then select all (Ctrl + A) and do a Free Transform (Ctrl + T). You can then drag the corners of the object by clicking the corners (with the Control key held down) until your object is within the dimensions. This is a very handy technique if you're trying to fit a smaller picture in a defined area, like replacing a picture in a photo frame, window, or other border.
The majority of your work is handled by using Photoshop's built-in Lens Correction Filter, which will save you lots of time. This tool can help make that "almost perfect" photo absolutely perfect.
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