Kowa Prominar MFT 25mm f/1.8 - 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. The 25mm f/1.8 stays just below 1% (barrel-type) which is certainly good albeit not unworldly.

Vignetting

The vigneting is reasonably well corrected for such a lens. At f/1.8 we measured a light falloff of 1.1EV (f-stops) which will be visible in many scenes. Stopping down to f/2.8 reduces the issue to a pretty much negligible degree.

MTF (resolution)

The Kowa Prominar MFT 25mm f/1.8 is capable of delivering an unusually high resolution. At fully open aperture, the center is already sharp and the borders are good. Stopping down to f/2.8 lifts the center into excellent territory followed by a good to very good outer image region. The real party starts at f/4 and goes till f/8 where the border/corner quality is nothing short of stunning. Diffraction has a higher impact from f/11 onward.

The tested sample had an Okay 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 absolutely negligible and nothing to worry about.