Cosina AF 100mm f/3.5 macro (Pentax) - Review / Lab Test Report - Analysis
Lens Reviews - Pentax
Article Index
Introduction
Analysis

Distortion

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.

Vignetting

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 on paper.

MTF (resolution)

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 Imatest Explanations

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) are very low and not really field relevant at all tested settings.

Verdict

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.

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