Canon EF-S 18-200mm F3.5-5.6 IS

Canon had a somewhat weird timing regarding the release of their EF-S 18-200mm f/3.5-5.6 IS. It is extremely difficult to design such a lens to start with and more so under low-cost requirements. Combine these side conditions with the rather extreme quality requirements of a high resolution APS-C DSLR a la EOS 50D (@ 15mp) and this sounds like trouble. The Canon lens has a couple of weak spots. At 18mm the border resolution is soft and heavy vignetting, extreme barrel distortion and CA problems don't help either. The situation improves significantly at 24mm and 50mm before deteriorating again towards the long end of the range. The situation would have been a bit different a couple of years ago when the sensor requirements were lower but combined with the EOS 50D the lens is somewhat outdated straight from the start. Better look elsewhere unless you still intend to use an older generation DSLR for a while.