70-200 f/4.0L + 2x - AF support on EOS 40D - Chuck?

[Doug_Kerr]

Hi, Chuck,

I have "traded in" my EF 70-200mm f/2.8L IS USM for an EF 70-200mm f/4L IS USM. (The f/2.8 was getting too heavy for an old guy like me.) It should arrive tomorrow.

I will mostly use the lens on my EOS 40D.

I have an Extender EF 2X II.

With the 70-200 f/4 and the 2X extender, what kind of AF support should I expect on the 40D?

Thanks.

Best regards,

Doug

[ChuckWestfall]

Hi, Doug:

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.

[Doug_Kerr]

Hi, Chuck,

Originally Posted by ChuckWestfall

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.

That was was I had understood from the literature. Still, I seem to have encountered rumors that the 40D would be willing to attempt AF with a reported aperture of up to f/8 (with perhaps not fully successful results).

Thanks for the info.

Best regards,

Doug

[Doug_Kerr]

Just to get closure on this matter:

I have my EF 70-200mm f/4L IS USM in hand. (It is lovely.)

When fitted with a Canon Extender EF 2X II and mounted to an EOS 40D body (maximum aperture f/8), the system does not attempt AF (regardless of AF point setting), nor does focus confirmation work in the MF mode.

As expected.

Best regards,

Doug