"Placed between the camera body and lens, a teleconverter contains a set of optics that will effectively increase the focal length of any lens with which it is used. The Kenko TelePlus MC4 AF 2.0x DGX converter has the effect of multiplying the focal length of your lens by 2.0x, turning a 300mm lens into an equivalent 600mm lens. The MC4 AF 2.0x DGX converter has genuine Gate Array IC (Integrated Circuitry). It means the converter's unique circuitry maintains signal integrity between the camera body and lens.The 4-element design is made with high quality multi-coated optical glass supplied by Hoya Corporation, the world's largest manufacturer of optical glass. Full AF operation with the TelePlus MC4 AF 2.0x is possible using camera lenses with open apertures of F2.8 or brighter. AF will work properly only if there is enough light and contrast on the subject to activate the camera's AF sensors. (Manual focusing may be necessary when using lenses with smaller open f-stop values than that given above.)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 teleconverter. Optically and mechanically they are identical to the prior high-quality DG series converters."