Silly Question

Hello

Maybe a silly qiestion , what does a Canon EF 2X II Extender do.

If I place it on a 70-200 L lens does it lengthen the focal length?

Yes. It's a teleconverter - the 70-200 would become (effectively) a 140-400 with a max. aperture of f/8.

Jon, how can you say f/8? dpp didn't even say if (s)he has a 70-200 f/2.8L or f/4L.

Anyway... A 2x teleconverter costs 2 stops, so if you have an f/2.8, it will become an f/5.6; if you have an f/4, it will become an f/8.

Note that only Canon's 1-series cameras can auto focus at f/8 - that means, if you have a 300D, 10D, 20D, or any other non-1-series camera, auto focus will not work with a 70-200 f/4L and 2x teleconverter.

The auto focus on non-1-series cameras works up to f/5.6, so if you have a 70-200 f/2.8, it will be able to focus with a 2x teleconverter.

There's also an 1.4x teleconverter, which costs 1 stop instead of 2 stops (with a 70-200 f/4L + 1.4x TC you still have AF on a non-1-series body).

You mount a teleconverter between the body and the lens.