Pentax SMC DA 21mm f/3.2 AL Limited - Review / Lab Test - Analysis
Lens Reviews - Pentax
Article Index
Introduction
Analysis

Distortion

The DA 21mm f/3.2 produces a relatively pronounced degree of barrel distortion (~2%) - this is not dramatic but a bit higher than expected/desirable for a newly designed fix-focal lens.

This aspect cannot change just by using another Pentax DSLR so we copied the old findings here.

Vignetting

The DA 21mm f/3.2 is a dedicated APS-C lens and as such it tends to produce a comparatively higher degree of vignetting. At f/3.2 the problem is indeed more pronounced at 1.16EV. That's not extreme for a 21mm lens although not stellar either. The situation is already much better by f/4 and no longer field relevant from f/5.6 onwards.

The vignetting figures are slightly higher than in our initial test. This related to the different tone curve of the K5 vs the K10D. The vignetting tests are performed based on JPEGs in default settings.

MTF (resolution)

The DA 21mm f/3.2 Limited produced very good although not superb resolution results in the MTF lab. The center performance is already exceptionally high straight from f/3.2 whereas the border quality isn't quite as high. At f/3.2-f/4 the borders show good results but the real boost towards very good quality occurs at f/5.6-f/8. At f/11 there's a general decrease in quality due to the usual diffraction effects.

Please note that the difference between f/3.2 and f/4 is only a 2/3 aperture so the gain in quality isn't overly pronounced between these two settings anyway.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) are well controlled for a lens in this focal length class. The average CA pixel width at the image borders stays around 0.7px - this may be visible here and there but usually not to an annoying degree.

Interestingly the CAs remain somewhat lower than in our initial test despite the higher megapixel count of the K5 - probably a sample variation effect.

Sample Images

You can find some sample images in the old review.

Verdict

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. The center performance is exceptionally high whereas the borders require f/5.6 to achieve very good levels. The amount of barrel distortions and vignetting are Ok but nothing to rave about for a new fix-focal lens. CAs are quite well controlled. The coolest aspects of the lens are naturally the extremely small size, the very low weight and a relatively moderate price tag of around 500€/US$. Combined with the "longer" pancakes lenses, the DA 40mm f/2.8 Limited and DA 70mm f/2.4 Limited, you'd have a complete yet very unobtrusive fix-focal lens setup for street photography.

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