Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF] (Canon) - Review / Test Report - Analysis
Lens Reviews - Canon EOS (APS-C)

Distortion

The AF 17-50mm f/2.8 SP XR exhibited average distortion figures for a lens in this class. As expected there is pronounced barrel distortion (2.5%) at 17mm which even out around the 24mm setting. Beyond the images show a moderate degree of pincushion distortion (0.84% @ 50mm).

17mm:

24mm:

35mm:

50mm:

The chart above has a real-world size of about 120x80cm.

Vignetting

The AF 17-50mm f/2.8 SP XR is a reduced image circle lens and these lenses tend to produce higher vignetting figures. The lens is no exception to the rule but the issue remains relatively well under control. At f/2.8 the lens shows pronounced vignetting of 1EV at 17mm, 0.9EV at 50mm and a little less in between. When looking towards the competition this is comparatively moderate. Stopping down helps to reduce the problem and from f/4 and up it shouldn't be field relevant anymore.

MTF (resolution)

Many dedicated APS-C lenses suffer from performance problems towards the corners of the image field but, surprisingly, the Tamron AF 17-50mm f/2.8 SP XR is one of the few lenses that which are capable to break this trend - at least within its focus plane (more on this later).
At 17mm the lens was even able to reach some of the highest measured LW/PH values ever tested in the center of the image (@ f/4). Even wide open the quality is already very good. At 24mm the quality remains basically on the same high level. At 35mm and 50mm there's a really marginal decrease in center performance whereas the border quality remains exceptionally high.

On the downside the extreme corners showed a very strong degree of field curvature at 17mm and a little less so at 24mm. This means that the focus plane isn't flat but it bends towards to corners.

As a consequence you will end with out-of-focus corners when shooting flat or very deep objects at large aperture settings (see the 17mm f/2.8 sample shot below). The problem will be reduced when stopping down thus increasing the depth-of-field around the curved focus plane.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are relatively well controlled except at 17mm at large aperture settings where the issue is very pronounced with a peak of 2px on the average at the image borders. Stopping down reduces the issue but you will need to stop down to f/8 in order to suppress the problem to uncritical level. At the other focal lengths CAs are a far lesser problem. Please note that CAs can be quite easily corrected in most modern RAW converters or via tool support.




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