Cosina AF 100mm f/3.5 macro (Pentax) - Review / Lab Test Report - Analysis
Lens Reviews -
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Macro lenses are designed to produce minimal distortion and the Cosina follows this tradition (0.09%).
The chart above has a real-world size of about 120x80cm.
The Cosina is a full format lens but it does still produce a moderate amount of vignetting
at f/3.5 and f/5.6. The problem is negligible from f/8 onwards. The marginal decrease
from f/3.5 to f/5.6 is a bit surprising. Regarding the exposure characteristic of the
lens it seems likely that we're probably more talking about f/4.5 rather than the f/5.6
The Cosina is able to produce very decent results throughout most of the range.
At f/3.5 the resolution is a bit reduced (still good) at the borders but the center
is already on a high level. At f/5.6 the borders have already improved to very
good quality. The sweet spot of the lens is reached at f/8 with an excellent center-
and very good border-resolution. At f/16 diffraction effects start to have a more
pronounced impact on the quality.
As already hinted in the vignetting chapter the aperture accuracy doesn't seem to
be overly high. The exposure data suggests an effective aperture ~2/3 f-stops
bigger than specified (from f/5.6 onwards).
Please note that this test has been performed at a conventional working distance (~6m).
Dedicated macro tests are NOT possible at this stage!
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)
Lateral chromatic aberrations (color shadows at harsh contrast transitions) are
very low and not really field relevant at all tested settings.
The Cosina performed according to its reputation - "cream in a yogurt cup".
The build quality is dismal but optically it is pretty capable lens when stopped
down a little. Vignetting and CAs are very well
controlled and distortions are absent. It is probably no real competition
for a more "serious" macro lens but if you're on a very tight budget it
is still worth a thought. At around 120€/US$ (or so) it should easily
be able to beat other budget macro solution such as macro diopters, extension
tubes or the popular "pseudo" macro zooms.