They are designed specifically to be use with telephoto lenses of 100mm or above, and work best with telephoto lenses of 200mm to 500mm. The PRO 300's 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.
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.
New DG Series have upgraded "Gated-Array" circuitry to work better with the digital SLR's and some of the the new digital only lenses. They still work the same with film SLR Cameras and lenses as well
Construction 4 Groups/ 7 Elements Dimensions (Length x Diameter) 2.6 x 1.7" Weight 184g (6.5oz) Notes MultiCoated, Depth of Field: 1/2 of prime lens/Exposure Magnification: Approx. 2 stops