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Stopping down to f5.6 improves central sharpness a lot, but corners are still slightly soft. Very noticeable at 18mm is the high level of barrel distortion - any straight lines away from the middle of the frame become curves. Most wide angle zooms have some distortion like this but it’s usually not that obvious - Unfortunately with the Tamron 18-200 f3.5-6.3 XR Di II it is fairly obvious.Over the rest of the zoom range, image quality follows much the same pattern, with fairly good central sharpness but a big drop in performance in the corners. Distortion is only really obvious at the 18 - 24mm end of the zoom range but it is there throughout the range - changing to pincushion distortion at the long end of the range. Another interesting characteristic of the test images is the high contrast on hard edges but low levels of detail in other areas. Some images look as if they have already been digitally sharpened - edges of background figures look sharp but hair and faces are just colored blobs. All of this is designed to produce ready to go images, good enough for small prints, but less desirable if you want the absolute best quality your DSLR can produce. That really is the choice here - if you want convenience and don’t plan on having an exhibition of your photos any time soon, this is a good well made lens that will help you photograph anything that comes your way, but it’s a compromise. If image quality really matters then my choice would still be two lenses - something like an 18-70 plus a 70-300 - or if you really want just one lens have a look at the newly released Tamron 18-250 f3.5-6.3 Di II LD which has better performance overall and a bigger zoom range.
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