Pentax SMC-DA 21mm f/3.2 AL Limited - Review / Test Report - Analysis
Lens Reviews -
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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.
The chart above has a real-world size of about 120x80cm.
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 1.04EV. That's
not too bad for a 21mm lens although not stellar either. The situation is already much better
by f/4 and no longer field relevant by f/5.6.
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.
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
Chromatic Aberrations (CAs)
Lateral chromatic aberrations (color shadows at harsh contrast transitions) are fairly
well controlled for a lens in this focal length class. The average CA pixel width
at the image borders stays at and below 0.9px - this may be visible here and there
but usually not to an annoying degree.
(Note: CAs can be reduced via imaging tools)