Originally posted by Lowell Goudge why would a 2x TC not autofocus on a F2.8 lens??? surely any camera can autofocus with a combined aperture of F5.6
Beat me! All I know is my Tamron 2X TC with power contacts does not work at all with any of the SDM zooms lenses I have. It works fine with lenses with screw-drive autofocus, however (provided the lens is fast enough).
My Tamron 1.4X TC with power contacts does not work
reliably with SDM zoom lenses, in particular at the long end of the focal length, despite the effective aperture of F/4 (e.g. with the DA* 50-135).