2.1. Causes of distortion

In a lens-coupled camera or detector, there are two major types of distortion which may often be seen in an acquired image: radial distortion and perspective distortion (Fig. 1).

Fig. 1 (a) Non-distorted image. (b) Barrel type of radial distortion. (c) Pincushion type of radial distortion. (d) Perspective distortion.

Radial distortion is caused by the increase or decrease of the magnification of a lens with respect to the radius from the optical axis as shown in Fig. 2.

Fig. 2 Cause of radial distortion.

Perspective distortion can be caused by two sources: a lens-plane is not parallel to a camera-sensor plane, known as tangential distortion, (Fig. 3) and/or an object-plane is not parallel to a camera-sensor plane (Fig. 4).

Fig. 3 Lens-plane and sensor-plane are not parallel.

Fig. 4 Object-plane and sensor-plane are not parallel.