Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF] (Sony) - Review / Test Report - Analysis
Lens Reviews - Sony Alpha/NEX (APS-C)
Sunday, 06 July 2008 14:09
Article Index
Introduction
Analysis

Distortion

The AF 17-50mm f/2.8 SP XR exhibits average distortion figures for a lens in this class. As such it produces strong barrel distortion (2.6%) at 17mm which evens out around the 24mm setting. Beyond the images show a moderate degree of pincushion distortion (0.9% @ 50mm).

Move the mouse cursor over the focal length text marks below to observe the respective distortion
17mm 24mm 35mm 50mm

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

Vignetting

As mentioned above the AF 17-50mm f/2.8 SP XR is a dedicated APS-C lens and these lenses tend to show fairly high vignetting figures at their max. aperture setting. At f/2.8 the lens shows very pronounced vignetting of ~1.4EV at 17mm and ~1.2EV at 50mm. Stopping down helps to reduce the problem but all-in-all it's not a characteristic to rave about.

MTF (resolution)

The Tamron 17-50mm f/2.8 produced pretty impressive performance figures in the MTF lab. At 17mm and 24mm the lens is able to reach some of the highest measured LW/PH values ever tested in the center of the image (around f/4). The center is already excellent at f/2.8 whereas the borders follow on a very good level. Stopping down increases the quality till reaching its peak around f/5.6. At 50mm there's only a really marginal decrease in resolution but the level of contrast is a bit reduced.

Unfortunately the lens has a less than desirable characteristic - at 17mm it suffers from very strong field curvature which can render the outer corners out-of-focus.

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 high towards the wide-end of the zoom range especially at large aperture settings with a peak around 1.8px on the average at the image borders at 17mm. 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 17mm. CAs are a far lesser problem towards the long end of the zoom range. Please note that CAs can be quite easily corrected in most modern RAW converters or via tool support.

Verdict

The Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF] is a very serious performer and an obvious alternative if you can't get warm with the Sony standard zoom lenses. In terms of resolution it is easily able to match the Zeiss 16-80mm f/3.5-4.5 ZA DT for instance - that is technically. Unfortunately the lens suffers from strong field curvature at 17mm which may push the corner portions out-of-focus. The level of distortions is about average in this class whereas vignetting and lateral CAs could be somewhat better. Mechanically the lens isn't top notch but it compares very well to other lenses in this price class. All-in-all it's an attractive offer and surely worth a deeper look when shopping for a quality standard zoom lens that does not require a 2nd mortgage.

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