How to Correct Lens Distortion with Photoshop Elements

Correcting lens distortion can be a chore for the uninitiated. This helpful excerpt from Photoshop Elements 8 for Windows: The Missing Manual will help guide you through the finer points of the process.

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If you ever photograph buildings, then you know that it can be tough getting good shots with a fixed-lens digital camera. When you get too close to the structure, your lens causes distortion, as shown in Figure 1. You can buy special perspective-correcting lenses, but they're expensive (and if you have a pocket camera, they aren't even an option). Fortunately, you can use Elements' Correct Camera Distortion filter to fix photos after you've taken them. It's another very popular Photoshop tool that Adobe transferred over to Elements, minus a couple of advanced options.

Correct Camera Distortion is a terrifically helpful filter, and not just for buildings. You can also use it to correct the slight balloon effect you sometimes see in closeups of people's faces (especially in shots taken with a wide-angle setting). You can even deploy the filter for creative purposes, like creating the effect of a fisheye lens by pushing the filter's settings to their extremes.

Here are some telltale signs that it's time to summon Correct Camera Distortion:

• You've used the Straighten tool, but things still don't look right.
  
• Your horizon is straight, but your photo has no true right angles. In other words, the objects in your photo lean in misleading ways. For instance, buildings lean in from the edges of the frame, or back away from you.
  
• Every time you straighten to a new reference line, something else gets out of whack. For example, say you keep choosing different lines in your photo that ought to be level, but no matter which one you choose, something else in the photo goes out of plumb.

Figure 1. Here's a classic example of a candidate for Elements' Correct Camera Distortion filter. This type of distortion is common when you're using a point-and-shoot camera in a narrow space that doesn't let you get far enough away from your subject. You can fix such problems in a jiffy with the help of this filter.

• If you have a problem with vignetting—a dark, shadowy effect in your photo's corners—you can fix that with Correct Camera Distortion, too. You can also create vignetting for special effects.

Adobe's made this filter extremely easy to use. Just follow these steps:

• Open a photo, and then go to Filter → Correct Camera Distortion.
  
  The large dialog box shown in Figure 2 appears.
  
  Figure 2. To use Correct Camera Distortion, look at the little icons next to each slider, which show you what happens when you move the slider toward the icon. For instance, if your photo suffers from barrel distortion (everything bows outward), then move the Distortion slider toward the pinched-in pincushion. The icon illustrates exactly what you want to do to your photo—slim it down.
  
  
  
  Note: Even though Correct Camera Distortion is in the Filter menu, you can't reapply it using the Ctrl+F keyboard shortcut the way you can with most other filters. You always have to select it from the Filter menu.
  
  
• If necessary, use the Hand tool to adjust your photo in the window.
  
  You want a clear view of a reference line—something you know you want to correct, like the edge of a building.
  
  If the distortion is really bad, this mission may be impossible, but try to find at least one line as closely aligned to the grid as you can, so you have a reference for changing the photo. You can also use the usual view adjustment controls (including zoom in and out buttons) in the dialog box's lower-left corner. The Hand tool adjusts both your photo and the grid. That means you can't use it to position your photo relative to the grid. Also, the Hand tool doesn't do anything unless you set the view to more than 100%.
  
  The Show Grid checkbox lets you turn the grid on and off, but since you're going to be aligning your image, you'll almost always want to keep it on. To change the color of the grid, click the Color box next to the Show Grid checkbox.
  
  
• Make your adjustments.
  
  This filter lets you fix three different kinds of problems: barrel/pincushion distortion, vignetting, and perspective problems. These errors are the ones you're most likely to run into, and correcting them is as easy as dragging sliders around. The small icons on each side of some of the sliders show you how your photo will change if you move in that direction. You may need to make only one adjustment, or you may need many (the bulleted list that follows helps you decide which controls to use).
  
  Watch the grid carefully to see how things are lining up. When you get everything straightened to your satisfaction, you're done. If you want to start over, Alt-click the Cancel button to change it to a Reset button and return your photo to the state it was in when you summoned this filter.
  
  
• Scale your photo, if you wish.
  
  As you make your adjustments, you'll probably notice some empty space appearing on either side of your canvas (the background area of your file). This often happens when Elements pinches and stretches your photo to correct the distortion. To make things right, you've got two options. You can click OK now, and then crop the photo yourself. Or, you can stay here and use the Edge Extension slider to enlarge your photo so that it fills up the window. If you use this second method, then Elements crops some of the photo.
  
  Edge Extension is handy, but gives you little control over how the photo gets cropped. After all the effort you put into using this filter, you may as well do your own cropping to get the best possible results.
  
  
• Click OK to apply your changes.
  
  If you don't like the way things are turning out, then you can reset your photo by Alt-clicking the Cancel button. If you just want a quick look at where you started from (without undoing your work), then toggle the Preview checkbox on and off.

The Correct Camera Distortion filter gives you a few different ways to adjust your image. Your choices are divided into sections according to the different kinds of distortion they fix:

• Remove Distortion. Use this slider to fix barrel distortion (objects in your photo balloon out, like the sides of a barrel, as shown in Figure 3), and its opposite, pincushion distortion (your photo has a pinched look, with the edges of objects pushing in toward the center). Move the slider to the right to fix barrel distortion, and to the left to fix pincushion distortion.
  
  Tip: Barrel distortion is usually worst when you use wide-angle lens settings, while pincushion distortion generally appears when a telephoto lens is fully extended. Barreling's more common than the pincushion effect, especially when you use a small point-and-shoot camera at a wide-angle lens setting. You can often reduce barrel distortion in a small camera by simply avoiding your lens's widest setting. For instance, if you go from f2.8 to f5.6, you may see significantly less distortion.
  
  
• Vignette. If you see dark corners in your photo (usually caused by shadows from the lens or lens hood), then you need to spend time with these sliders. Vignetting typically afflicts owners of digital single-lens reflex cameras, or people who use add-on lenses with fixed-lens cameras. Move the Amount slider to the right to lighten the corners, and to the left to darken them. The Midpoint slider controls how much of your photo is affected by the Amount slider. Move it to the left to increase the area (to bring it toward the center of the photo), or to the right to keep the vignette correction more toward the edges. Also consider turning off the Show Grid checkbox so that you have an unobstructed view of how you're changing the lightness values in your photo. Turn it back on again if you have other adjustments to make afterward.
  
  Figure 3. A classic case of barrel distortion. This photo has already been straightened with the Straighten tool, but things are still pretty out of plumb here. Notice how the columns and walls lean in toward the top of the photo. You can even see a bit of a curve in the pillar on the far right. Barrel distortion is the most common kind of lens distortion, but fortunately, you can easily fix it with the Correct Camera Distortion filter.
  
  
  
  
• Perspective Control. Use these sliders to correct objects like buildings that look like they're tilted or leaning backward. It's easiest to understand the sliders by looking at the icons at both ends; each icon shows you the effect you'll get by moving the slider in that direction. The Vertical Perspective slider spreads the top of your photo wider as you move the slider to the left, and makes the bottom wider as you move it to the right. (If buildings seem like they're leaning backward, move it to the left first.) The Horizontal Perspective slider is for when your subject doesn't seem to be straight on in relation to the lens (for example, if it appears rotated a few degrees to the right or left). Move the slider to the left to bring the left side of the photo toward you, and to the right to bring the right side closer.
  
  
• Angle. You can rotate your entire photo by moving the line in this circle to the angle you want, or by typing a number into the box. A very small change here has a huge effect. The circle tool is easy to work with, but if you prefer, you can type a precise angle, in degrees. Here's how it works: There are 360 degrees in a circle. Your photo's starting point is 0.00 degrees. To rotate your photo to the left (counterclockwise), start from 0.01, and then go up in small increments to increase the rotation. To go clockwise, start with 359.99, and then reduce the number. In other words, 350 is further to the right than 355.
  
  Tip: Each of the adjustment settings is accompanied by a box where you can type a number instead of using the sliders. If you want to make the same adjustments to several photos, take note of the numbers you used to fix the first photo, and then just plug them into the boxes for the other photos.
  
  
• Edge Extension. As explained in step 4 above, when you're done fixing your photo, you're likely to end up with some blank areas along the edge of your photo's canvas. Move the Scale slider to the right to enlarge your photo and get rid of the blank areas. Moving the slider to the left shrinks your photo and enlarges the blank areas, but you'll rarely need to do that.
  
  The Scale slider changes your actual photo, not just your view of it (as would be the case when using the Zoom tool). When you click OK, Elements resizes and crops your photo. So if you want the objects in your photo to stay the same size they were, then don't use this slider. Instead, just click OK, and then crop.

The most important thing to remember when using Correct Camera Distortion is that a little goes a long way. For most of the corrections, start small and work in small increments. These distortions can be very subtle, and you often need to make only subtle adjustments to correct them.

Tip: The Correct Camera Distortion filter isn't just for corrections. You can use it to make your sour-tempered boss look truly prune-y, for example, by pincushioning him (just make sure you do this at home). Or, you can add vignettes to photos for special effects. You can also use the filter on shapes, artwork, or anything else that strikes your fancy.