Pentax SMC-FA 31mm f/1.8 AL Limited - Review / Test Report - Analysis
Lens Reviews -
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The Pentax SMC FA 31mm f/1.8 Limited exhibits only slight barrel distortion (~0.8%)
which shouldn't be really field relevant.
The chart above has a real-world size of about 120x80cm.
As a full format lens the 31mm f/1.8 enjoys a sweet spot advantage on the K10D.
Nonetheless vignetting can be quite obvious at wide-open aperture (1EV). The problem is
gone by f/2.8.
The FA 31mm f/1.8 Limited produced excellent resolution figures
in the MTF lab. At f/1.8 the center performance is already very good both in the
center as well at the borders. Typical for most ultra-large aperture lenses the
quality gets boosted quite a bit when stopping down to f/2.8 with the center
reaching outstanding levels followed by very good+ borders. The center resolution
peaks at f/4 whereas the borders reach their maximum quality at f/5.6. At f/8
diffraction begins to take its toll. All in all an impressive performance for an
ultra-large aperture lens.
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
very low for a wide angle lens. The issue shouldn't be really field relevant
with an average pixel width of 0.4px to 0.6px at the image borders. You can force
the lens to produce some mild purple fringing (color blooming) at large aperture settings
and in extreme contrast situations.