Here is a test picture made at the extreme 18mm wide angle with Pentax SMC-DA 18-55 that comes in the most basic kit for Pentax K10D and some earlier Pentax cameras.
Distortions appear at the edges. Extreme flare, an important decrease in resolution. Probably there is also vignetting but that cannot be seen on this picture.
The picture is uploaded in the original size to Picasa.