http://photo.yodobashi.com/gear/olympus/lens/kowa8528.html

http://dc.watch.impress.co.jp/docs/review/kowaworld/20150511_699381.html

Kowa lens has good optical performance. The image quality is decent at max. aperture but when stopping down it is about as good as it gets on the micro four thirds system. The low level of lateral chromatic aberrations contribute to the subjective quality perception. There's some vignetting at f/1.8 but not much beyond. Image distortions are also on the low side.

http://www.photozone.de/m43/950-kowa25f18

The Kowa Prominar MFT 12mm f/1.8 is the weakest offering in the Kowa lineup. The pure MTFs are actually quite decent. The center is always sharp at the mainstream settings and the outer image region is relatively decent at f/1.8, good at f/2.8 and very good between f/4 and f/8. However, field curvature is a major issue at large apertures so in the real life the results are less convincing here - at least in our sample which wasn't really well centered. Typical for the three initial Prominar lenses, both the distortion as well as the lateral CAs are low which is good because image auto-correction isn't possible. The amount of vignetting is generally on the high side though.

http://www.photozone.de/m43/954-kowa12f18

http://dc.watch.impress.co.jp/docs/review/kowareview/20160122_738791.html

KOWA Cine Prominar 20mm, 25mm, 32mm, 40mm, 50mm, 75mm and 100mm Lens Test

12mm F1.8 Kowa Prominar review

https://www.ephotozine.com/article/kowa-prominar-12mm-f-1-8-mft-lens-review-32027

25mm Samples

https://www.lenstip.com/2276-news-Kowa_Prominar_MFT_25_mm_f_1.8_T1.9_-_sample_images.html