Kowa YF Series Porro Prism Binoculars The Kowa YF Series Porro Prism Binoculars are a new range from the world-famous Japanese scope and binocular manufacturer. Kowa have infused over half a century of heritage in optics design and production into the new YF Series, which boasts top end features such as Kowa Repelling (KR) lens coatings and 100% waterproofing. The YF Series represents the perfect balance between retro design and modern functionality, with one of its most striking attributes being aesthetic beauty.