Have gone full frame & got a Canon 5d Mk2.
Autofocus kicks in ok on my 50mm lens when pressing shutter 1/2 way.
But with Canon EF 70-200mm f/4.0 L IS USM, I can only auto focus by pressing button on back of camera body.
Shutter 1/2 way does nothing.
The 70-200 auto focus is fine on the 550d, so I dont think its the lens.
I am being stoopid?