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Introduction

Pentax smc DA 21mm f/3.2 AL Limited is one of a series of Pentax pancake lenses the company introduced for its digital SLR cameras to fill in the gap that was left after the previous generation of K and M series lenses was discontinued. The lens was first announced at PMA2006 and is currently still in production (as of July 2009). The lens retails at ~US$450 and at this price is within budgets of amateurs and pros alike.

The optical construction of the lens consists of 8 elements in 5 groups - it is quite surprising that Pentax managed to cram so many elements into such a tiny lens - smc DA 21mm f/3.2 Limited measures only63 x 25mm (2.5 x 1in). And with its weight of  only 140g, the lens really pushes the definition of 'light and compact'.  The build quality of the lens is quite good, with aluminium barrel and focusing ring. Like all other DA and DA Limited lenses, the smc DA 21mm f/3.2 AL was designed specifically for Pentax  APS-C type digital cameras - the lens has a fully electronic aperture control  (so there is no dedicated aperture ring) and while it might be possible to mount the lens on older film Pentax bodies or another non-Pentax camera using an adapter, it would be impossible to use  the lens in any but fully open aperture mode. The lens supports auto-focusing, but there is no dedicated motor to rotate the lens elements, so the auto-focusing is enabled through an AF pin that extends from the camera body and locks into a slot on the bottom of the lens. Manual focusing is also possible using a dedicated focusing ring. Most Pentax lenses, including the smc DA 21mm f/3.2 AL, have 'Quick-Shift Focus System' - a Pentax marketing term for manual focus correction in AF mode. Once AF system locks on the subject in focus, you can use the focusing ring to finetune it manually, without the need to switch to a dedicated manual focusing mode. The minimum focusing distance of the lens is 20cm (7.92in) and the minimum aperture is f/22.

Since smc DA 21mm f/3.2 AL Limited is a dedicated APS-C lens, it cannot really be used on any full frame  cameras (not that Pentax has any at the moment). Nor can it be easily adapted to an alternative mount system due to the lack of a dedicated aperture ring. Hence the field of view of the lens will resemble that of a 31mm lens. Still a wide angle but not ultra wide anymore.

 

Summary
Lens Composition 8 elements in 5 groups
Angular Field 68 degrees
Minimum Focus 20cm/7.92in
Focusing Action AF/MF
f-stop Scale f/3.2-f/22, electronic
Filter Size 43mm or 49mm
Lens Hood MH-RBA 43mm (included)
Weight 140g/4.9oz
Dimensions 63x25mm/2.5x1"
Lens Case DA 21mm (included)