Pentax SMC DA 21mm f/3.2 AL Limited - Review / Lab Test
Lens Reviews -
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Review by Klaus Schroiff, published July 2011
Special thanks to Janosch Kalateh for providing this lens!
We already tested the Pentax SMC-DA 21mm f/3.2 AL Limited a couple of years ago based on the Pentax K10D (@10mp) but for as another reference test we've just re-tested it again on the Pentax K5 (@16mp). Most of the findings remain valid on the K5 so please forgive us for having reused most of the old review here again.
The Pentax SMC-DA 21mm f/3.2 AL Limited is one of three so-called pancakes lenses in the current Pentax lens lineup. As you can observe below pancake lenses are pretty tiny beings - the DA 21mm has a length of merely 25mm. The species used to be fairly rare but we are currently experiencing a revival on the various micro-systems like Micro-Four-Thirds and Samsung NX - small is beautiful again. Pancakes have advantages and disadvantages. They're obviously as small and light-weight as it gets. Optically they're masters of back light due to the low number of lens elements (and therefore surfaces) and they tend to produce comparatively evenly sharp images. On the downside they only have a moderately fast max. aperture and the peak performance tends to be a little lower compared to conventional lenses - the theory tells that the degree of correction increases the more elements you add to the design ... assuming you do it right (there're limits to this specifically in terms of production quality). The DA 21mm f/3.2 is a dedicated APS-C lens so it can only be used on Pentax DSLRs. Its field-of-view is equivalent to about 32mm (full format) so it can be regarded as a perfect for street photography - a classic sweet spot of the Pentax system.
The DA 21mm is a Limited lens which means that Pentax uses quality material, a mainly manual assembly and an above average quality control (they say). The lens is certainly a very high quality item with all parts made of metal
and tight tolerances. The tiny focus ring operates smooth and slightly damped. As you can see below the lens extends a marginally when focusing towards closer distances. The front element does not rotate so using a polarizer is no problem. We received a user report that it is not possible to use a filter and the lens hood at the same time because the filter prevents the hood from locking at its intended position.
Typical for most Pentax lenses the DA 21mm f/3.2 has no internal AF motor and relies on a slotted drive screw operated by the camera. Consequently AF operations generate a moderate degree of noise. The AF speed is very fast on the K5 - finally the focus group is about as light-weight as it gets. Thanks to a "Quick-Shift Focus System" manual focusing is instantly possible once the AF has achieved focus - this works very well and noiseless.
|Equiv. focal length||31.5 mm (full format equivalent)|
|Equiv. aperture||f/4.8 (full format equivalent, in terms of depth-of-field)|
|Optical construction||8 elements in 5 groups inc. 1 aspherical element|
|Number of aperture blades||7|
|min. focus distance||0.2 m (max. magnification ratio 1:5.9)|
|Dimensions||63 x 25 mm|
|Filter size||49 mm (non-rotating)|
|Other features||floating element for close focus correction|