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Sigma AF 17-70mm f/2.8-4.5 DC (Pentax K) - Review / Test Report |
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Lens Reviews -
Pentax
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Page 1 of 3
Review by Klaus Schroiff, published December 2007
Special thanks to Gerald Otte for providing this lens for testing!
Introduction
The Sigma AF 17-70mm f/2.8-4.5 DC macro is one of the few third-party alternatives to
the native Pentax standard zooms. The Sigma is a DC ("Digital Camera") lens specifically
designed for the reduced image circle of today's APS-C DSLRs where its field-of-view is
equivalent to 26-105mm (full format) - an attractive range suitable for a variety of
applications.
Unlike its in-house cousin (Sigma AF 18-50mm f/2.8) the AF 17-70mm DC is not a designated
EX lens (Sigma's pro grade lineup) but the build quality is still pretty good with tight
controls and no wobbling. The lens has a slight crinkle ("ZEN") finish and broad
rubberized control rings. The zoom ring operates pretty smooth whereas the focus action
feels a little "lifeless". The tested sample showed a tendency for zoom creeping but
Sigma has provided a transport lock (17mm only). The front element does not rotate so
using a polarizer remains easily possible. As you can see in the product images below
the Sigma uses a duo-cam system to extend its physical length when zooming towards the
long end of the focal length range. FWIW the tested sample (a used one) has sucked in
some dust over time.
| Specifications |
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| Equiv. focal length | 25.5-105 mm (full format equivalent) |
| Equiv. aperture | f/4.2-f/6.8 (full format equivalent, in terms of depth-of-field) |
| Optical construction | 15 elements in 12 groups inc. 1 SLD and 2 aspherical elements |
| Number of aperture blades | 7 |
| min. focus distance | 0.20 m (max. magnification ratio 1:2.3) |
| Dimensions | 79 x 83 mm |
| Weight | 455 g |
| Filter size | 72 mm (non-rotating) |
| Hood | petal shaped (supplied) |
| Other features | - |
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