Messing With Perspective

Professional film photographers have specialised, expensive lenses that help them take pictures of buildings. These "tilt lenses" correct the inevitable perspective problems that crop up when you try to photograph architecture. Thankfully, in the world of digital photography, we don't need to spend a fortune on extra lenses, because we can fix the problem afterwards.

There's no doubt that tilt lenses are amazing. They take stunning photos and give you a lot of creative control over the perspective in a scene. But they're pretty expensive--as in, "I can get this lens or a new motorcycle"--and work only with certain high-end cameras. You might not get the same kind of results on the PC, but few people will be able to tell the difference.

Consider a picture like this one. The Houses of Parliament building looks badly skewed, as if it were ready to topple into the River Thames. Distortion is inevitable when you photograph a tall building: Vertical lines, such as the opposing walls, will appear to converge off in the distance. But that's okay, because it's something you can easily fix on your PC afterwards using the Perspective Correction tool in your image editing software. Open this photo--or another one of your own that suffers from similar problems--in your favorite image editing program. I'll demonstrate using Corel's Paint Shop Pro X.

Start by Straightening

Before we correct the perspective, we need to make sure the photo is level. This photo is clearly askew, but because of the way it's framed, it's not immediately obvious how to level it. There's no horizon, for example, to use as a reference point. The solution? Let's draw a line through the closest vertical structure of the building.

Click the Straighten tool, which lives in the sixth cubby from the top of the toolbar on the left side of the screen. Then grab one end of the line and position it high on the centre of the corner of the steeple that's closest to you. Position the other end of the line at the bottom of the wall, between me and my lovely wife. Zoom in if necessary to make sure the line is as close to centred through this part of the corner as possible. Click the Apply check mark (it has a green background) in the Tool Options palette atop the screen; if necessary, you can turn on Tool Options on by choosing View, Palettes, Tool Options. You should get something like this.

If your result doesn't look quite right, select Edit, Undo Straighten, and move the bottom of the line to the left or right and click Apply again. Within a try or two, you should get something that's as straight as possible.

Convergence Be Gone

So far, so good. Now we're ready to use the Perspective Correction tool to eliminate the converging lines. Conveniently, this tool lives in the same cubby as the Straighten tool; just switch tools and you'll see a box appear over your photo. The trick now is to position the four corners of the box over an element in the photo that you know should be square. Again, that's not really easy to do in a photo like this one, but feel free to zoom in to make sure you place the corners accurately in positions that would ordinarily look like a rectangle. Try to alter the lines of the rectangle so that they match the lines of something you know should be straight. You can see how I prepared my perspective box. After you click the check in the Tool Options palette, your picture should look like this.

Try the Opposite

This is a handy trick for correcting the look of your architectural photos, but don't forget that you can use the technique creatively as well. For every building that you fix, there's another that you can deliberately mangle. Consider this innocuous picture of the entrance to a building.

There's nothing wrong with the perspective in this scene--yet. Select the Perspective Correction tool and position it in the doorway, narrowing the bottom and making sure it's not level, like this.

Complete the change by clicking the check in the Tool Options palette, and you can get a vertigo-inducing effect (I cropped the picture).

That's just a taste of what you can do with the Perspective Correction tool. You'll be amazed at what you can achieve with a little experimentation.

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Dave Johnson

PC World (US online)

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