Distortion for rectilinear lenses comes in three flavors: barrel distortion, pincushion distortion, and complex distortion. The three types of distortion are illustrated below. In barrel distortion the edges of the image curve toward the center. For pincushion distortion the edges curve away from the center.
Complex distortion is often a combination of barrel and pincushion distortion with barrel distortion near the center and pincushion distortion near the edges. This type of distortion is sometimes referred to as moustache distortion as its shape resembles a handlebar moustache such as the one on Sir Arthur Conan Doyle.
Moderately wide angle lenses often have barrel distortion while telephoto lenses have pincushion distortion. Wide angle lenses are prone to complex distortion. The following illustration corrects complex distortion with Photoshop's Lens Correction filter and PTLens.
Attempts to correct the image in Photoshop's Lens Correction filter are unsatisfactory. Correcting barrel distortion in the center increases pincushion distortion near the edges. This is because Photoshop has only one slider for distortion correction that controls the amount of curvature. To correct this image we need two sliders: one to correct barrel distortion near the center and another to correct pincushion distortion near the edges.
This is the approach taken by PTLens. However instead of two sliders there are two coefficients stored in a database. Camera make, model, lens, and focal length are sensed in EXIF information. These parameters are used to lookup the coefficients and automatically correct the image. All you do is press the OK button!
A photograph of a building taken with the sensor parallel to the target will yield an image with parallel vertical lines. In actual fact the vertical lines converge due to perspective. To maintain parallel lines in the image rectilinear lenses magnify the image near the edges.
While this is desirable when photographing buldings, it's less desirable when taking pictures of people. In this case you may find that individuals near the edge of an image in a group shot may have large odd-shaped heads! This is especially true when using extreme wide-angle lenses.
Sometimes referred to as volume anamorphosis, this type of distortion cannot be corrected by PTLens. However it is easily corrected in recent editions of Photoshop using the Free Transform tool. Notice the extra long hands in the following image.
To correct this distortion in Photoshop do the following:
- Select > All (Ctrl-A)
- Edit > Free Transform (Ctrl-T)
- Edit > Transform > Warp (choose the Warp button in Options bar)
Then drag the lines to compress/expanded regions as needed.