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

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Distortion

The Pentax SMC FA 31mm f/1.8 Limited exhibits only slight barrel distortion (~0.8%) which shouldn't be really field relevant.

Note: We didn't retest this aspect because it cannot change just by moving to a different body using the same sensor size.

Vignetting

As a full format lens the 31mm f/1.8 enjoys a sweet spot advantage on Pentax APS-C DSLRs. 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. The corners are a bit softer here. The quality increases a bit when stopping down to f/2.8 with a center reaching excellent levels followed by very good borders and corners. The quality remains exceptionally high all the way down to f/11.

Please note that the MTF results are not directly comparable across the different systems!

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 around 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.

Verdict

The Pentax SMC-FA 31mm f/1.8 AL Limited is an excellent lens. It is already sharp at f/1.8 and razor sharp from f/2.8 to f/11. Distortions and CAs are very well controlled. On the downside it vignettes quite a bit at f/1.8 so the ultra-large max. aperture takes its toll here. The bokeh (out-of-focus blur) is very smooth and buttery which is fairly unusual for a lens with aspherical elements. Typical for all Limited lenses the build quality is exceptional and it is a joy to use the lens. It also shares another, rather unfortunate, characteristic with its cousins - a high price tag. Quality comes at a price but 1200€/1000US$ seems a bit over the top but then it is probably just another example of diminishing returns upon higher investments. Nonetheless it is refreshing to see that Pentax focuses on quality rather than costs - there're enough plastic lenses out there after all.

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