Distortion

Lens distortion

Lens distortion is an aberration that causes straight lines to curve near the edges of images. It can be troublesome for architectural photography and metrology (photographic applications involving measurement). The simplest approximation is the equation, r_u = r_d + kr_d³ where r_u is the undistorted and r_d is the distorted radius. Depending on the sign of k, it can be either "barrel" (shown on the right) or "pincushion."

Lens distortion and coefficients for correcting it are calculated in Distortion, which also includes 5th order and tangent/arctangent distortion models. SFRplus measures distortion in less detail, along with sharpness and several other factors.

Distortion is worst in wide angle, telephoto, and zoom lenses. It often worse for close-up images than for images at a distance. It can be easily corrected in software. Picture Window Pro and PTLens have tools for removing it.