It depends on the price range of the lense, the cheap consumer ones that come free with the entry to pro-consumer slr suffer from many artefacts most notable Chromatic Aberration also they apertures are at most 3.5.
The nikkor 28-70 f2.8 for instance should be sharp at 2.8 at any mm, that lens is over a grand. I got a tokina of that a few years back, it does get noticeably softer the wider the aperture but for 1/3 of the price i cannot complain. Post processing can generally sharpen it out. It does not suffer from chromatic aberration, its a pretty heavy lense and quite pleased with it.
http://www.xbpro.com/u75/river.jpg,
I presume that is taken with Nikon AF-S 18-200mm f/3.5-5.6G Dx VR Lens ?
Personally that's a lot room in one lense, i can see chromatic aberation in that lense.