Wide angle distortions
When using wide angle lenses, perspective distortion is a common visual effect. For example, the corners may appear to ‘curl’ in to the image. Another common type of distortion is pincushion – where the image is stretched – which gets progressively worse as the lens angle gets wider. Sometimes, distortion is so bad that lenses can be rendered unusable.
This is especially true in infrared photography. The lens could be just fine in visible light but when used for IR the various distortions on the wide end become much, much worse. This is due to the fact that normal lenses are designed for use for normal, visible light photography. Any special corrections or coatings designed into the lens may not function in IR or even worse, have the opposite effect and make the problem worse not better. As a general rule the wider the angle the worse the distortions.