TABLE OF CONTENTS Several types of image distortion can affect the appearance of your photographs. The two most common are barrel distortion from wide-angle lenses and perspective distortion from photographing an object at an oblique angle. Because these are so common, we often mentally correct for these when viewing photographs.
This chapter primarily addresses the issue of wide-angle lens distortion, with a discussion of the issues of perspective distortion at the end of the chapter.
Wide-angle lenses are quite common and are often the default focal length in point-and-shoot-style cameras. Barrel distortion appears as curved objects in the edges of the images and can make straight lines appear to be curved.
A number of third-party plug-ins for Photoshop have enabled the correction of lens distortion, and Photoshop has included this capability without the need for a plug-in beginning with CS2.
It should be noted that this process is destructive meaning that it changes the pixel values in the image. I generally work on a duplicate layer with destructive processes so that the original image data can be compared with the results of processing on two layers of the same image.
As with all processes used in image analysis, all work should be done on a copy of the original. It is also recommended that the History Log be set to record all adjustments made to the image. See Chapter 1 for more information about using a valid forensic workflow.
In the sample images in Figure 17.1,...
