Correcting extreme distortion

We photographers are used to seeing distortion in pictures. This is most commonly seen in architecture. The picture below showing The Bean (aka The Cloud Gate) in Chicgo illustrates this. Distortion in which perfectly vertical buildings appear to be leaning inward is such a common way to show cityscapes today that it’s become accepted as perfectly normal. I have to assume architects cringe when they see images like this, but the general public, as well as photographers, are mostly unphased.

This type of distortion is called keystoning, and it occurs when the camera is angled upward such that the plane of the digital sensor is oblique, or non-parallel, to the planes of the architecture. This happens with all lenses, but it is particularly prominent and obvious when using a wide angle lens. The photo of The Bean was taken with a 16mm lens.

There are two ways to correct this distortion. First, you can use a perspective control lens at the time of shooting. This is a specialized optic that has internal movements to eliminate the angled lines. Second, you can use Photoshop’s tools.

The two-step procedure in Photoshop is:

1. Choose Select > all 2. Then choose Edit > transform > distort. A box forms around the picture area.

At this point, expand the area around the photo by grabbing the lower right or left corner of the frame around the image (not the image itself) and dragging it down and away from the center of the image. You will end up with the picture area surrounded by black; see the above screen capture. This is important so you can see the boundaries of the transform tool.

The box that formed around the photo has handles, and you can pull these in any direction. To straighten keystoned buildings, you’ll want to pull the upper right handle up and to the right, and the upper left handle up and to the left. This is what I did in straightening the buildings behind The Bean, above. When you are happy with the results, hit enter or return.

With extreme distortion, parts of the image may seem depressed or flattened as the buildings are aligned vertically. Notice the shape of The Bean on the previous page compared to its shape on this page. You can see in the above image that the curvature of the artwork seems flattened. That’s why I grabbed the top middle handle and raised it. I was trying to restore the correct curvature. However, I wasn’t able to restore it completely because if I stretched the picture vertically too much, the top of the building on the left would be cropped off.

This is important: When you know you’ll be using this technique to correct keystoning, compose the image with enough space on either side of the image as well as top and bottom. If you don’t, there will be elements in

the photograph that will be eliminated as the picture is stretched and distorted.

If you shoot a building such that the camera is angled downward, the vertical lines of the architecture appear to converge at or below the bottom of the frame as opposed to the top of the frame. This is illustrated in the interior of Salisbury Cathedral in England, right. Notice how the columns are angling inward and pointing to the bottom center. I corrected this the same way, but instead of dragging the upper corners up and away from the center, I grabbed the lower right and lower left corners and dragged them down and away from the center.

If the distortion is too extreme, it’s necessary to go through this operation twice. In other words, I first apply the command, Edit > transform > distort, hit enter or return, and then apply the same command again. This will fix almost any type of architectural distortion. Just remember to leave enough space around the angled buildings when taking the picture so when the picture is stretched, important elements aren’t cropped out. §