Kenko 1.4X PRO 300 Teleconverter DGX for Canon EOS Digital SLRs
Kenko AF 1.4x Super High Quality Teleplus converter also has genuine Gated Array IC and is compact for light weight and easy handling. Full AF is capable with lenses having a maximum aperture of F/4.5 or brighter. (Manual focusing will be required with lenses of smaller aperture.)Placed between the camera body and lens, a teleconverter contains a set of optics that will effectively increase the focal length of a lens by a specific amount. Common magnifications are 1.4 times and 2 times, but the Kenko teleplus PRO 300 AF converter has a 3x magnification. For example, using a 2 times (2x) teleconverter will increase the focal of 300mm lens to 600mm. The Teleplus PRO 300 converters are made with high quality multi-coated optical glass supplied by Hoya Corporation, the worlds largest manufacturer of optical glass. These glass elements are  designed to match the optical quality of the prime lens (even at the edges) and telephoto zoom lenses. The optical design of the elements and light path is wide enough not to cause vignetting. The PRO 300 family is designed specifically to be used with prime telephoto lenses of 100mm or above, such as, a 300 mm f/2.8 lens, and work best with telephoto lenses of 200mm to 500mm. The PRO 300 can be used with telephoto zoom lenses as well as prime lenses. However, Kenko does not recommend them for zoom lenses that have a range starting under 50 mm.