Actually, "Vignetting". "Barrel distortion" is the mild fish-eye effect
that many cameras exhibit at full wide-angle. Vignetting means that the
lens is not big enough in diameter to allow the image to cover the full
sensor, so you get the corners clipped.

And, yes, it's an issue - the Olympus telephoto extender lenses such as
the TCON-1.7C and TCON-30 are not designed to be used except at full
telephoto; they cause severe vignetting at wider angles. I haven't
heard about any problems with vignetting otherwise, though.

ECM