Designed for optimum performance with telephoto lenses of 100 mm and longer ( such as ATX300 AF PRO) . The Kenko PRO 300 2x AF converter doubles the focal lens of any lens that it is used with. The PRO 300 2x converter is 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.