KENKO Teleplus Pro 300 converters are made with high quality multicoated optical glass supplied by Hoya corporation, the worlds largest manufacturer of optical glass. This glass will match the optical quality of the prime lens (even at the edges, unlike many teleconverters on the market today). The optical design of the elements and light path is wide enough not to cause any vignetting.
They are designed specifically to be use with telephoto lenses of 100mm or above, and work best with telephoto lenses of 200mm to 500mm. The PRO 300's can be used with telephoto zoom lenses as well as prime lenses, but, due to their design, Kenko does not recommend them for lenses that have a zoom range that starts under 50 mm.
Kenko PRO 300 converters are intended to be used with expensive telephoto lenses and larger camera bodies. They have all metal lens mounts in both front and rear, as well as a metal core, to support today's heavy professional camera bodies.
Kenko Teleplus Pro 300 2x converter Reviews