The effective aperture of the lens is increased by one or two stops respectively. Auto-focusing 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). 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 EF 2x II doubles the focal length of any lens it's mounted to, and reduces its effective aperture by two stops