Imaging: Gaining a sense of perspective

Have you ever taken a photograph of a building or small object and found that the framing looks odd or off-centre? Even when you double-check everything and retake the shot it seems that gremlins are ruining your images. If the colour and focus are good, it's likely that the problem lies with perspective.

Our brains are remarkably adept at correcting our view of the 3D world. Every time you look at an object, your brain makes alterations to perceived focus, light balance and perspective. This reinterpretation of the real world is why photography can be tricky, and one of the strangest beasts is perspective. You can see this for yourself by looking at an object on the horizon and rotating your head left or right. Notice something odd? Probably not - that's the whole point. The horizon stayed level. It didn't tilt one way and then the other. Take a few pictures of the horizon by rotating a camera the same way and the image will show a correspondingly rotated view.

Correcting rotational errors

A photograph that is slightly skewed will be noticed by most people, as movement of just a few degrees tends to be picked up easily. Unless the effect is intentional, most times it will ruin an image. To avoid rotational errors - where the whole image is tilted to the left or right - look for natural lines in your image. Some obvious places to align your image are the horizon (outside), areas where the floor or ceiling meets the wall (inside), or people's eyes (close-up portraits). If the error is frequent, it may be your viewfinder or your camera technique. Perhaps you're pushing the button down too hard or holding the camera at an odd angle against your glasses? Occasionally, a camera may shift slightly as it rests on a surface or tripod. And let's not overlook the most basic issue - sometimes people just make mistakes.

Luckily the effect is easy to remove, provided you have enough space to trim the image afterwards. Most programs have a rotate option that will allow you to fix the problem (in Paint Shop Pro or Photoshop Elements, select Image-Rotate. For Photoshop, select Image-Rotate Canvas). As you rotate an image, the space left behind will be filled by the current background fill colour of your graphics program, so make sure that you have selected a suitable colour (black or white are the best options). The image will also need to be trimmed to discard the unwanted areas.

Scale perspective and deformation

Another common perspective issue relates to scale. Look up at a tall building, or a road disappearing into the distance, and you'll notice that the outside edges merge to a point in this distance. We equate scaled-down objects with distance. This gives us our sense of depth and many times it transfers well onto a photograph. It's also used to deceive us when creating some types of special effects. However, in some photographs it's not always welcome, and may create the wrong impression - a looming doorway may not be appealing for a family home, for example. Again, graphics software can help (look for the Perspective Correction Tool or similar), but it's important to remember that this operation will distort the entire image. As always, try to avoid the problem by facing the centre of the object. Obviously this is impossible if it's an 80-story building, but tends to be a little easier if it's a doorway or other human-scale object.

Photograph of New York's Chrysler building

Sometimes you can use perspective to create dramatic effects. There was no way I could accurately capture the beauty of New York's Chrysler building from its base on this overcast day. However, by choosing an odd framing (the corner) and deliberately rotating the camera to the left, it created a far more imposing view of the building.

Lens barrel and pincushion

Some lenses or camera settings can create slightly different perspective and distortion errors. Called lens barrel and pincushion distortion, they tend to appear in wide-angle or high-zoom photography. Generally the outer areas of the image are bent into a circular shape. Some graphics programs have a special tool for removing this problem, but it can be tricky. In a future column, I'll be returning to these types of distortions and other problems introduced by the lens, so stay tuned.

The picture on the left has two issues. It's leaning to the left by about 2.5 degrees, and perspective is causing the buildings to lean into each other - an effect that was worsened when the image was rotated to level the image. A light tweak of the perspective tool straightened the fake Arc de Triomphe and gave a more natural view of the surrounding buildings, as shown on the right.

It's the camera's fault!

Badly framed images are frequently blamed on the photographer. However, if you're having trouble framing an image, it may be your camera's viewfinder. Some viewfinders see a slightly different perspective to the lens, so even though you think the image is perfectly in frame, the lens may be aiming at a slightly different spot, and hence the photo is ruined.

This problem is usually due to "fixed" or "static" viewfinders on lower-end cameras. These are simply a piece of glass or plastic glued into a hole, and they don't move with the zoom. If positioned properly, what you see through the viewfinder should approximate what is captured by the lens. For longer distances this technique generally works, but as the object gets closer there are some problems that can occur.

Firstly, the object may be fully framed in the viewfinder, but the lens may be seeing a sightly different perspective. The result? Heads can be cropped out or the picture looks off-centre (see figure 1 and figure 2). This problem is greatly exacerbated when the viewfinder doesn't move in step with the zoom. To avoid this problem, use the LCD viewfinder - this is generally the exact image that your camera's sensor sees. If your digital camera doesn't have an LCD, there's not much you can do that doesn't involve buying one that does. The second viewfinder issue is something called parallax. In addition to a different frame, viewfinders also view the scene at a slightly different angle or perspective. This is particularly noticeable when the subject is close, and can be a real problem in macro photography as shown below.

Scott Mendham has been writing for PC World and producing the cover CDs for over six years. During this period, he has authored an alarming number of books on a wide range of IT-related topics, and will freely admit that his first computer was a TRS-80.

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