some info i found
Kenko PRO 300 AF 2x Teleconverter are made with high quality multicoated optical glass supplied by Hoya corporation, the worlds largest manufacturer of optical glass. Kenko PRO 300 AF 2x Teleconverter 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 AF 2x Teleconverter is designed specifically to be use with telephoto lenses of 100mm or above, and work best with telephoto lenses of 200mm to 500mm. The Kenko PRO 300 AF 2x Teleconverter 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.
The Same goes for the 1.4x