Reviewed By: Andrew Steve on 03/09/2007,
Customer Rating: 
For a compact lens, the Tamron 18-200mm should win an award for the most confusingly long name. The parts that really matter are '18-200mm' and 'Di-II'. This Tamron 18-200mm is a very wide range zoom lens designed for use only with DLSRs that have APS-C size sensors - i.e. those with a 1.5x or 1.6x crop factor compared with 35mm camera. The Canon version of Tamron 18-200mm has a standard EF mount and can therefore be fitted to the Canon EOS 10D and its predecessors (as well as the Canon 300D, Canon 350D and Canon 20D), unlike the Canon EF-s lenses.
After taking a few hundred shots with the Tamron 18-200mm lens on a EOS 10D, I'm fairly convinced about its good and bad points. The news is mostly good, provided you remember that any lens with this 18-200mm zoom range is bound to be something of a compromise. In particular, size constraints mean that the lens is relatively slow, especially at the 200mm end - f/6.3 - so it is not aimed at low-light photography unless you want to risk ISO 1600 or use flash.
The following comments are purely subjective about Tamron 18-200mm. I haven't carried out any scientific tests with resolution charts etc, although I do now have a reasonable idea how the Tamron 18-200 compares with my other lenses.
Build Quality of Tamron 18-200mm:
The Tamron 18-200mm lens has a good feel, and seems better built than some of the mid-range Canon lenses. The lens hood (supplied) is a sensible design, and fits on in reverse for storage without adding too much to the bulk of the Tamron 18-200mm lens. The 18-200mm zoom ring is a little stiff, but not unduly so, and that's better than being too slack and moving all by itself. The Tamron 18-200mm is an extremely compact lens considering its range - much smaller and lighter than the Canon 17-40mm and 28-135mm, although of course these are full-frame lenses.
Sharpness of Tamron 18-200mm:
I would rank it on a par with the Canon 28-135mm IS at most focal lengths - perhaps a little softer towards 18mm. The images respond well to a USM filter, and I would regard the overall performance as quite acceptable.
Focusing system in Tamron 18-200mm:
Despite its relatively small aperture at 200mm (f/6.3), the Tamron 18-200mm lens focuses quickly throughout its range, even in low light. The Tamron 18-200mm has a distance scale but not a ring USM motor, so there is no full-time manual focusing. (i.e. You cannot turn the focus ring in AF mode.) The motor is rather noisy compared with USM lenses. Depending on how you hold the camera/lens, you may find your left hand touching the focus ring as it is trying to rotate.
Chromatic Aberration
The Tamron 18-200mm lens can suffer from moderate CA - purple and red/green fringing - in high contrast situations, especially at the wider end of its range. This should not be too conspicuous at normal viewing sizes, but would show on large prints. Fortunately the CA filter on PSP9 works very well (if rather slowly) once you figure out how to use it, and it successfully removed almost all traces. This makes the CA issue a bit of a nuisance rather than critical, all things considered.
Distortion in Tamron 18-200mm:
There is a little barrel distortion at 18mm, but nothing obtrusive, and easy to correct in post-processing if you like.
I bought the Tamron 18-200mm as a 'travel' lens, so I could leave the 10kg camera bag at home and take just the camera and a single lens. The Tamron 18-200mm fits the bill perfectly, and I think it would also be ideal as a general purpose lens for someone setting out along the DSLR road. | 
