The Bower's 2x DGII Teleconverter (4 Element) for Canon will effectively double the focal length of your Canon lens, while maintaining the autofocus function. A 50 mm lens becomes a 100 mm lens. The teleconverter uses multi-coated optics to deliver high-quality images. With an 85 mm f/1.4 lens, for example, the focal length will become 170 mm, which effectively sets the maximum aperture at f/2.8 (cutting the amount of light captured by two f/stops). This teleconverter is a portable and versatile accessory that's well worth having in your camera bag. It's only compatible with EF mount lenses with a maximum aperture greater than f/4.0 in order to fully utilize its autofocus capabilities.