The EF 2X II doubles the focal length of any lens it's mounted to, and reduces its effective aperture by two stops. Attaching this high-performance extender to a 135mm or longer lens, or to the EF 70-200mm f/2.8 L boosts the focal length by 2 times. With seven elements in five lens groups, it preserves the prime lens' image quality.

With the extender attached, the f-number decreases by two stops. Auto focusing is possible with lenses whose maximum aperture is f/2.8 or faster. This latest version (MKII) is dust and water resistant for increased system protection.