Lensbaby Burnside 35mm F2.8
Distortion refers to the capability of a lens to represent straight lines as...straight lines. Wide angle lenses often create barrel distortion, where lines look they've been put on a sphere. Telephoto lenses can create the reverse effect, named pincushion distortion. The following images illustrate the various types of distortion:
Barrel distortion, left; no distortion, center; pincushion distortion, right
It is very difficult to create a zoom without some distortion. It is also very hard to design a wide lens, even a prime, which is distortion-free. On the other hand, normal and short tele lenses can more easily avoid the problem.
The way to evaluate distortion is pretty straightforward: take a picture of straight regular lines, and look if they curve. Calculating the ratio of curvature yields an evaluation of distortion.
Cameras such as the K-3 and K-1 include built-in distortion correction, which can be automatically applied to files captured files. However, the camera must know which lens is mounted. Since the Burnside does not communicate with the camera, corrections will not be possible.
Distortion Test results
The following image shows how the distortion pattern looks when photographed with the Burnside 35mm for full frame sensors.
The amount of distortion is small, at 0.9%. It should have little effect in actual shooting conditions except in some very specific cases. Post-processing programs will take care of distortion without trouble. Using sensor shift when available will also help.
At 0.9%, distortion will have little impact with the Burnside 35mm.