change to purchase a new LENSE
 

I have a Canon DSLR EOS Rebel 450D Xsi, will all the camera's functions work with the Tamron lense SP 70-300mm F/4-5.6 Di VC USD (Canon mount) and a Canon Extender either the EF 1.4x II or the EF 2x II?
Or work better with the Tamron AF 2X Teleconverter?

The Canon teleconverters won't work with that lens.

[url]http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/lenses/extender_ef_1_4x_ii#Overview[/url]

Thanks
 

Thank you for your quick reply.

I used to have that zoom lens with a compatable 2x converter.
The auto-focus did not work when the 2x converter was attached.

A Tamron 2x or a Kenko 2x or 1.4x will work with the lens as far as I can remember, however you lose 1 stop of light with a 1.4 and 2 stops with a 2x, so the autofocus wont work with any converter with that lens. To get AF on a 2x you need an f2.8 lens and an f4 with a 1.4x.