The Pentax DA 21mm Limited has an average review score of 84.5%

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Pentax SMC-DA 21mm f/3.2 AL Limited - Review
The Pentax SMC-DA 21mm f/3.2 AL Limited is a very good lens but it is not as special as its longer cousins (specifically the Pentax FA 31mm f/1.8 Limited) nor is this really possible because of the increasing design problems when pushing things towards wide and ultra-wide angles.
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77

Pentax 21mm f/3.2 Limited SMC P-DA
The lens is really well built, as are all Pentax limiteds. I'm not aware of any other lenses of this focal length that are even close to as small and light as this lens. It makes for a very compact SLR. The lens has quick shift so you can readjust the focal point after autofocus.
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92

Pentax SMC-DA 21mm f/3.2 AL Limited
Mechanically well engineered and giving good resolution results across the frame and aperture range, this lens makes a nice, but not exceptional wide angle addition to the kit with virtually no space or weight penalty. A pricey headline figure, but expect to find it well discounted.
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NA