Pentax-DA* 300mm F4 Review
Chromatic aberration occurs because the lens is does not always foucs all the different wavelengths onto the image sensor at the same point. The results are usually seen as a purple or green fringe on the image. Shooting wide-open or including high-contrast areas in the frame increases the likelihood of seeing chromatic aberrations. Stop down a few stops to keep aberrations to a minimum.
In the 200% cropped picture below, you will notice that the DA* 300mm exhibits very slight purple fringing on the palm tree.
If the DA* 300mm has distortion, then it can barely be discerned in real life.
The DA* 300mm does exhibit minimal chromatic aberration, but it is so slight that in most situations it won't show itself. Barrel distortion is negligible.