Pentax SMC-FA 31mm f/1.8 AL Limited - Review / Test Report - Analysis
Lens Reviews - Pentax

Distortion

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.

Vignetting

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.

MTF (resolution)

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 Imatest Explanations

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.




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