The effective aperture of the lens is increased by one or two stops respectively with the canon extender 2x. 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).

canon extender ef 2x ii features

Dust and moisture resistant construction
Extends the master lens focal length by 2x
Minimum loss of lens performance
Reduces inner reflection for higher image quality

The extender ef 2x ii doubles the focal length of any lens it's mounted to, and reduces its effective aperture by two stops

The Canon 2x includes:
Canon front & back caps and Canon soft Case