Digital Focus: Fix Perspective in Architectural Photos
Feature: Getting a Better Perspective on Architecture
Our eyes, working with our brain, deceive us all the time. You'd be surprised, in fact, how much they process the world around us.
Ordinarily, for instance, everything seems to be in sharp focus, despite the fact that our eyes can only focus on one thing at a time, and everything else should be correspondingly blurry. Likewise, we may not always realize just how different the light from the noon sun is from the sunset or from indoor fluorescent lights. Our brain adjusts automatically, and we see colors relative to the palette we're viewing at the time. And our eyes certainly don't make very tall buildings look like they're askew. Cameras, though, with their limited ability to capture perspective and no brain to make adjustments on the fly, do exactly that.
What am I talking about? Check out a picture that I took in Las Vegas. If you look closely, you can see that the Paris hotel behind the replica of the Eiffel Tower is not square and true. Instead, it leans in at the top, as if it's narrower there than it is at the base. That's a problem, because most real buildings aren't shaped that way. And to be sure, this one isn't either. It's a distortion caused by perspective--one that we usually don't think about in real life, but that leaps out of pictures in a pretty obvious way.
Avoiding Perspective Errors
I think you'll agree that it would be better to avoid this sort of thing. So how do we keep from getting perspective errors in our photos? In simple terms, perspective distortion creeps into your photos whenever you're shooting something very tall from a short distance. As you do, you need to tip the camera up to include all of it in the frame. It's that action--setting the lens at an angle to the subject--that creates the distortion.
One form of control over this problem comes from choosing your subjects. You'll avoid perspective distortion by shooting shorter buildings, since you can keep the camera level with the ground when you take the shot. Likewise, control your distance to the subject. If you want to shoot a tall object, try to do it from farther away. If you can get far enough away, there's no reason to angle the camera, and perspective errors won't haunt your shot. Check out this shot, for instance--the faux Arch de Triumph is small enough that I could shoot it without adding any perspective distortion.
Eliminating Distortion on the PC
That said, perspective distortion is sometimes unavoidable. Thankfully, it's easy to eliminate it afterwards using an image editor like Jasc's Paint Shop Pro.
The best thing about the Perspective Correction tool is that it does more than just straighten the lines of skewed buildings. It can automatically rotate those buildings as well, which is handy if you accidentally held the camera at a slight angle when you took the picture. Check the Vegas picture again and you see that the horizon isn't level. Let's fix both problems at once. Save my Paris Hotel picture to your hard drive and then open it in Paint Shop Pro.
The Perspective Correction tool is in the second cubby from the top in the toolbar on the left side of the screen. To access it, click the cubby and select it from the list of four tools that live there.
Once it's selected, you should see a frame appear on top of your image. The trick to using this tool is to drag the four corners of the frame over something in the picture that should be perfectly square, but is instead curved or skewed because of the camera's perspective. For this picture, align the frame with four symmetrical points in the hotel's architecture. Be sure to zoom way in to precisely position the perspective frame; if you don't you might make the picture worse. (The Zoom tool is listed under the View menu item at the top of the screen)
When you're ready, click the check mark for Apply that's on the left side of the Tool Options palette at the top of the screen (if it's not displayed, select View, Palettes, Tool Options); or you can just double-click on your picture. You should see the building snap to attention. That's a dramatic improvement. The beauty of this method is that when we applied the changes, the walls of the building were pulled true and the angled picture was rotated as well.
To finish it off, use the Crop tool (third cubby from the top of the toolbar) to trim away the unwanted background.