Canon EF-S 18-200mm f/3.5-5.6 IS - £410
Physically, the Canon EF-S 18-200mm f/3.5-5.6 IS is only a single millimetre longer and three millimetres wider than its 18-135mm stable mate, but it's 140g heavier and £60 pricier. It also has a lot more telephoto potential, the long 200mm end being equivalent to a focal length of 320mm on crop-sensor DSLRs.
Build quality feels almost identical in both Canon lenses, although cosmetic differences include a silver-coloured rear edge to the zoom ring. Other similarities include a 4-stop image stabiliser, standard micro-motor autofocus system and the lack of a focus distance scale and lens hood.
As for differences, the 18-200mm has a narrower manual focus ring and features a zoom lock switch.
Sharpness could be a little better throughout the whole zoom range, but distortions are only slightly higher than with the Canon 18-135mm, which is pretty good considering the additional zoom range. Colour fringing proved quite noticeable, but, being a genuine Canon lens, it's fully compatible with in-camera corrections and DPP lens tuning.
The extra telephoto makes this lens a better buy than the cheaper EF-S 18-135mm IS.
18mm sample image from the Canon EF-S 18-200mm f/3.5-5.6 IS
200mm sample image from the Canon EF-S 18-200mm f/3.5-5.6 IS