Re: 70-200 f/4.0L + 2x - AF support on EOS 40D - Chuck?
AF operation on most EOS DSLRs other than the 1D and 1Ds series requires an effective maximum aperture of f/5.6 or larger. Since the effective maximum aperture of an f/4 lens becomes f/8 when a 2X extender is used, this means you will lose AF capability when using your EF70-200/4L IS with an EF 2X extender on your EOS 40D.
The 1D and 1Ds cameras are the only EOS models that support single-point AF, using the center AF point, at an effective maximum aperture of f/8. This is due to the designs of their various 45-point AF systems.
Professional Engineering & Solutions Division
ITC Group/Canon U.S.A., Inc.