Distortion Display

This shows how a rectangular grid is projected by the lens, and therefore how lines will deviate from being rendered as perfectly straight. We also calculate the degree of distortion along both axes of the frame. Again you can select any of the marked focal lengths on the lens, but as distortion is essentially independent of aperture, we only show data recorded at F8.

The display is a direct representation of the grid pattern captured by the lens (some tests only show you a mathematical curve fit using over-simplified equations). Our data therefore allows you to see any complexity in the distortion, and you can even use screen shots to help find appropriate correction parameters in your preferred image manipulation software.

Data panel
Along with the grid representation, we present three results in the data panel:
Short edge: defined as the percentage difference in length between the central vertical grid line and the left/right ‘short edge’. Describes the degree of bowing of the upper and lower horizontal lines, which are normally the most distorted.
Long edge: defined as the percentage difference in length between the central horizontal grid line and the top/bottom ‘long edge’. Describes the degree of bowing of the outermost left and right vertical lines.
Distortion type: Barrel or pincushion

Source : http://www.dpreview.com