Pentax SMC-FA 20-35mm f/4 AL - Review / Lab Test - Analysis
Lens Reviews - Pentax
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Introduction
Analysis

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Distortion

The Pentax FA 20-35mm f/4 AL exhibits quite pronounced barrel distortion (~2.2%) at 20mm and very slight barrel distortion (~0.5%) at 35mm.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
20mm 35mm

The chart above has a real-world size of about 120x80cm.

Vignetting

Thanks to the sweet spot behavior on the K10D the vignetting is fairly well controlled for a wide-angle zoom lens. At 20mm @ f/4 you may see the slightly darker edges in critical scenes but at other settings it shouldn't be really field relevant anymore.

MTF (resolution)

The Pentax FA 20-35mm f/4 AL produced high resolution figures in the MTF lab. At 20mm the center resolution is excellent followed by very good quality borders and good to very good extreme corners. At 35mm the performance is a bit more even across the frame thanks to improved borders.

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) can be a problem at 20mm. The average CA width at the borders varies between 1.76px and 2.45px - this can be visible in field images. The problem is much better controlled at the 35mm setting.

Note: Lateral CAs can be reduced via imaging tools.

Verdict

The Pentax SMC-FA 20-35mm f/4 AL is a very good lens but it doesn't really offer much over its successor - the SMC DA 16-45mm f/4 ED AL. Similar to its cousin it has one primary problem: lateral CAs (color shadows at harsh contrast transitions) at 20mm. Other than that the lens is capable to produce a very good resolution and comparatively low distortions and vignetting. The build quality is about average for a consumer grade zoom. The lens had its days during the film era but it's time to move on now.

Optical Quality:
Mechanical Quality:
Price/Performance:discontinued

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