Pentax SMC-FA 20mm f/2.8 - Review / Test Report - Analysis
Lens Reviews - Pentax

MTF (resolution)

Classic wide-angles don't tend to be overly impressive performers on DSLRs but actually the FA 20mm f/2.8 produced pretty good resolution figures in the MTF lab. At f/2.8 the center performance is already very good whereas the border quality is acceptable. At f/4 and more so at f/5.6 there's a boost in quality with an excellent center and very good borders. At f/5.6 and f/8 this does also include the extreme corners (just). Typical diffraction effects start to have a higher impact (reduced quality) from f/11 onwards.

At f/2.8 the contrast level is relatively low. The lens does also suffer a little from residual spherical aberrations (shifting focus point when stopping down).

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

Distortion

The FA 20mm f/2.8 exhibits quite pronounced barrel distortion (2.5%). While this is quite typical in the focal length class it is not overly impressive for a fix-focal lens.

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

Vignetting

As a full format lens the FA 20mm f/2.8 benefits from the usual sweet spot effect on APS-C DSLRs. Nonetheless the vignetting characteristic is far from being good. At f/2.8 the lens shows rather strong vignetting (~1.2EV) which is often noticeable in the field. Stopping down to f/4 helps a lot and from about f/5.6 vignetting is not longer an issue.

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) are not a significant issue at f/2.8 but the problem increases steadily towards smaller apertures. However, generally CAs are comparatively well controlled for an (ultra-)wide angle lens if you stay at and below f/8.

In extreme contrast conditions you may be able to spot a little bit of purple fringing at f/2.8 but this should be a rare problem.




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