Precision Digital 2X Teleconverter will effectively double the focal length of your lens, while maintaining the autofocus function--a 35 mm lens becomes a 70 mm lens. This 7-element teleconverter uses multi-coated optics to deliver improved image quality. 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, high-quality and versatile accessory that's well worth having in your camera bag. It's only compatible with lenses with a maximum aperture greater than f/4.0 in order to fully utilize its autofocus capabilities.