Pentax-DA 560mm F5.6 ED AW
Prime lenses with medium to long focal lengths are usually not very prone to distortion, and the Pentax-DA 560mm is no exception to this rule - there simply isn't any measurable distortion. This test was shot as F8. The camera was on a tripod, focusing was done with contrast-detect in live view. Live view was then turned off and the image taken with no further focus adjustment.
Verdict on Distortion
There isn't any distortion for all practical purposes.