Kowa Prominar MFT 8.5mm f/2.8 - Review / Lens Test Report - Analysis
Lens Reviews - (Micro-)Four-Thirds

Distortion

The Kowa is proud of very low image distortion produced by their new lenses. Due to the lack of electronic coupling this is also needed because flaws cannot be auto-corrected without data. Anyway, the good news is that the distortions are indeed exceedingly low and absolutely negligible. Given the fact that we are talking about an ultra-wide lens makes this quite an achievement actually.

Vignetting

The amount of vigneting isn't quite as impressive unfortunately. At f/2.8 we measured a light falloff of 2EV (f-stops) which is beyond our usual scale for MFT actually. Stopping down eases the problem but the issue never goes below 1EV. This can be visible in some scenes.

MTF (resolution)

The Kowa Prominar MFT 8.5mm f/2.8 can deliver decent resolution figures but it's not perfect. At fully open aperture, the center is already sharp and the borders are also pretty good. However, the extreme corners are soft. Stopping down to f/4 boosts the quality across the image field with an outstanding center followed by a very good border and good corners. The peak performance is reached between f/5.6 and f/8. Diffraction has a higher impact from f/11 onward but then you rarely need such settings on Micro-Four-Thirds cameras.

The tested sample had a good centering quality.

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 CAs (color shadows at harsh contrast transitions) are just slightly visible with an average pixel width of around 1px at the image borders. This is not disturbing though.