Kenko Teleplus PRO 300 DG AF 2x Teleconverters 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.
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.