Tamron 17-50mm f/2.8 SP AF XR Di II VC - £345/$560
Lab test results look quite good on the whole, but while sharpness is good at the centre of the frame, it drops away alarmingly towards the corners. This is especially true at f/2.8, at both ends of the zoom range. Even when you're shooting at f/8, any fine detail is largely lost around the edges of images.
It also seems to cause a problem with overexposure, which often happens by about two-thirds of a stop. This results in overly bright images and blown highlights. The effect is most noticeable at the wide-angle end of the zoom range, where you always have to be ready to dial in some negative exposure compensation.
Sharpness test
Sharpness is very respectable at the centre of the frame, but it's poor towards the corners at some apertures.
Lab test at f/8
Sharpness at wide angle: 2383
Sharpness at mid range: 2404
Sharpness at telephoto: 2343
Fringing test
At maximum zoom
Fringing isn't too bad at wide angles but, unlike most lenses, this lens doesn't improve much at longer focal lengths.
Lab test at f/8
Fringing at wide angle: 0.33
Fringing at mid range: 0.32
Fringing at telephoto: 0.27
Distortion test
At minimum zoom
Barrel distortion is quite pronounced at 17mm, but there's practically no distortion at the 50mm end.
Lab test at f/8
Distortion at wide angle: -3.66
Distortion at mid range: -0.18
Distortion at telephoto: 0.09
Image quality verdict
Taken at 28mm
Edge and corner sharpness are quite poor, especially at the f/2.8 focal length, and you often need to stop down to f/5.6 for good results.
Read the full Tamron 17-50mm f/2.8 SP AF XR Di II VC review