Pentax SMC-K 135mm f/2.5 - Review / Lab Test Report - Analysis
Lens Reviews -
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Typical for most fix-focal tele lenses the SMC-K 135mm f/2.5 produces only minimal
distortion which are nothing to worry about in the field (0.5% pincushion distortion).
The chart above has a real-world size of about 120x80cm.
Thanks to the sweet spot behavior on the K10D vignetting is well controlled.
However, regarding the very large aperture vignetting is still a little stronger
at ~0.64V at wide-open aperture - at times this may be visible in very critical scenes.
From f/4 and up the problem is negligible.
The SMC-K 135mm f/2.5 may be old but the resolution figures are still quite impressive.
At f/2.5 the results are already very good and fairly even across the frame.
Stopping down improve the quality further till it peaks around f/5.6 with an excellent
(just) center and very good borders.
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
Chromatic Aberrations (CAs)
Low CAs were surely no design objective in the late 70s so it isn't really surprising
that this is a weakness of this old lens. CAs (color shadows at harsh contrast transitions)
can be fairly visible at times with an average pixel width around 1.3px at the image borders.
The Pentax SMC-K 135mm f/2.5 is surely not a mainstream lens on modern DSLRs. Stop-down
metering only, no aperture reading in the viewfinder and the lack of AF doesn't really
make such a lens a really desirable package. However, the cheap price
on the used market may do. The principal performance of the lens is very decent. The
resolution figures are very good, distortions are no problem and vignetting is well
controlled. Relatively high CAs are a weakness though. The build quality is
excellent and it's still fun to use such a Methuselah - occasionally at least.
PS: Please don't bet on many more non-DA/FA tests though - I would surely die testing the
zillion of old Pentax lenses ...