The 2x mark ii extender has been replaced with the Canon 2x mark III Extender.
The Canon EF 2x ii Lens Extender extends the focal length by a factor of 2x, in exchange for 2 stops of light. For example use of this converter on a 70-200 f/2.8 will give the lens an effective focal length of 140mm – 400 with a maximum aperture of f/5.6. Please note that not all lenses are compatible with teleconverters, and an image quality loss may be apparent. depending on shooting conditions.
The Canon EF 2x ii extender works by increasing the distance between the film plane (or CCD) and the rear element of the lens.
The extra distance doubles the effective focal length of the lens to bring distant subjects closer.
Canon EF Extenders are specially designed for many of Canon’s large aperture EF lenses and retain all metering and AF functions with most of these.