The effective aperture of the lens is increased by one or two stops respectively. Autofocusing does not operate if the effective maximum aperture is greater than f/5.6 (greater than f/8 on the EOS 1V and EOS 3).
The EF 2x II doubles the focal length of any lens it's mounted to, and reduces its effective aperture by two stops
Successors to the EF2x this model offers valuable improvements for further enhanced image quality. This extender allows users to combine practicality and functionality with a new optical system allowing them to extend the master lens focal length by 2x, with minimal loss of lens performance or image quality. With a dust- and moisture-resistant construction this extender has been designed to reinforce the EOS system. Specially designed to work in conjunction with the EOS-1V and IS super telephoto L-series lenses, it makes a highly dust- and water-resistant combination.
The EF2x II features improved anti-reflection materials inside the lens-barrel as well as improved coating around the outer edges of the lenses to reduce flare. All of these improvements will allow professional photographers to capture higher quality images than ever before.