Distortion is one of the least problematic aberrations for telescopes. Distortion is typically only large over very wide fields such as with wide-angle camera lenses. Over the small fields of view associated with telescopes, distortion is rarely an issue. Very wide field binoculars may show some noticeable distortion, especially less expensive pairs.
Distortion arises from a difference in image scale across a field of view. If the image scale is larger at the edge of the field than at the center, the corners of an image appear to bend outward. This is called pincushion distortion. If the image scale decreases at the edge of the field, the center of the image appears to bulge outward. This is called barrel distortion.
Above: Pincushion (left) and barrel distortion
The effects of distortion are easily seen in many wide-angle camera lenses. A photograph which includes straight lines (such as a brick wall) will easily display the curving of lines associated with distortion.
For this reason, normal wide-angle lenses are often unsuited to architectural photography and specially designed lenses with low distortion must be used. Over the small fields used by telescopes, distortion is normally negligible. Even wide field telescopes cover small angles compared to photographic lenses. While a 2° or 3° field is considered wide for a telescope, a 24mm camera lens covers 84° and a 14mm lens covers an enormous 114°.
Distortion is normally measured as a percentage. An 8" f/10 SCT exhibits 0.04% pincushion distortion, while an 80mm f/7.5 doublet refractor has just 0.0016% barrel distortion. For comparison, a typical wide-angle camera lens has around 1% distortion. The greatly exaggerated diagrams above exhibit about 50% distortion.