Kenko Teleplus Pro 300 DGX converters are made with precision quality multi-coated optical glass supplied by Hoya Corporation, the worlds largest manufacturer of optical glass. These glass elements were 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 are designed specifically to be used with prime telephoto lenses of 100mm or above, such as 300 mm f/2.8 lenses 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.
Kenko Pro 300 AF Teleplus converters have genuine Gate Array IC (Integrated Circuitry). It means that the converter's own unique circuitry maintains signal integrity between the camera body and lens. These converters are designed to electronically operate the same way as an original manufacturer's converter. With the Pro 300 1.4x, DGX full AF operation is possible with camera lenses having a maximum aperture of F4 or brighter.
The same light and contrast requirements apply. The DGX Teleplus converters have updated circuitry to record exif data more accurately. In the exif exposure data (meta-data recorded with a digital picture) DGX converters record the equivalent aperture and focal length of the lens setting plus tele-converter. Optically and mechanically they are the identical to the prior high-quality DG series converters.