Pentax Prime Lens Series

Pentax was one of the first manufacturers to offer a coherent line of pancakes that is, fixed focal-length lenses at reduced size. Since the explosion of hybrid cameras, compactness has become a real selling point, and so pancake lenses are once again in style… giving us the chance to see what Pentax has to offer now that we’re several years further down the road.

To continue with our analysis of Pentax pancake lenses, and to folllow our analysis of the Pentax smc DA 15mm F4 ED AL Limited, we present here the Pentax smc 21mm F3.2 ED AL Limited. As with all lenses in the “limited” series, this lens has a high-end finish: aluminium, 8 glass element in 5 groups, and of course the classic characteristics of all pancake lenses:

• Very compact — only 25mm long.
• Very light — only 140g.

Its Full-Frame focal equivalent is 32mm, which makes it a practical choice for landscape and ordinary street photography. Its optical formula includes an aspherical element in an effort to limit chromatic aberrations.

Measurements

Pentax smc DA 21mm F3.2 AL Limited mounted on a Pentax K5

Regrettably, the DxOMark score for this lens is a disappointing 11 points.

The good points of the Pentax smc DA 21mm F3.2 ED AL Limited:

• Compact.
• Light-weight.

Points that could be improved:

• Strong vignetting in the corners.
• Too-apparent chromatic aberrations.
• Rather weak resolution and not particularly uniform over the field.

Comparisons

Pentax smc DA 15mm F4 ED AL Limited vs Pentax smc DA 21mm F3.2 AL Limited

Just a small, quick comparison between the 21mm f/3.2 and the 15 mm f/4 previously analyzed: the 21mm stays ahead of the 15mm in nearly every respect except for chromatic aberrations.

Those who don’t really need a very wide-angle lens shouldn’t hesitate to choose the 21mm.

Pentax smc DA 21mm F3.2 AL Limited vs Olympus M.ZUIKO DIGITAL 17mm f2.8

With Full-Frame focal equivalents of 32mm for the Pentax and 34 mm for the Olympus, these 2 lenses have quite similar results. Still, we prefer the Pentax model, given that the distortion is better controlled. Otherwise, the qualities and defects of these two lenses are very similar.

Pentax smc DA 21mm F3.2 AL Limited mounted on a Pentax K5 vs Carl Zeiss Distagon T 21mm f/2.8 ZF2 Nikon mounted on a Nikon D7000

The advantages of the Pentax smc DA 21mm F3.2 AL Limited:

• The most compact lens.
• The lightest lens.
• The least expensive lens.

The advantages of the Carl Zeiss Distagon T 21mm f/2.8 ZF2 Nikon:

• Better resolution.
• Brighter (around +0.17EV).
• Better vignetting behavior.
• Better correction of chromatic aberrations.

Here we see the limits that one can encounter with pancake lenses. Because the emphasis is on compactness, sometimes certain optical characteristics are not as well-controlled — such as, for example, the fact that there are many chromatic aberrations and the resolution is weaker at larger apertures.

Conclusion

The Pentax smc DA 21mm F3.2 AL Limited is rather disappointing in terms of its correction of chromatic aberrations, though it is still a good choice for reportage photography. As this lens is getting a bit long in the tooth, it would be nice to see a redesign of this equivalent of a 35mm lens — a focal length that is very popular with photographers.