Real life lenses don't map an image geometrically exact to the film or sensor. In most cases the image is distorted one or the other way. There are three common types of distortion:
- Barrel distortion which causes straight lines outside the image center to be bent outwards (like a barrel).
- Pincushion distortion which causes straight lines outside the image center to be bent inwards (like a pincushion).
- Wavy distortion, which is a mixture of both - mostly a result of incomplete optical correction of one of the other ones. There exist two subtypes:
- with barrel distortion in the image center and pincushion distortion in the outside regions ("Bubble Distortion")
- with pincushion distortion in the image center and barrel distortion in the outside regions.
The lens correction model of panotools is able to deal with all of them.