Pentax SMC-FA 20mm f/2.8 - Review / Test Report - Analysis
Lens Reviews -
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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 in line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
The chart is limited to the visually relevant LW/PH range of [850, 2350].
If you want to know more about the MTF50 figures you may check out the corresponding
The FA 20mm f/2.8 exhibits quite pronounced barrel distortions (2.5%). While this
is quite typical in the focal length class it is not overly impressive for a fix-focal
The chart above has a real-world size of about 120x80cm.
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.