The 300mm f/4 L works very well with a 1.4x Extender gives you 420mm f/5.6. Image sharpness and focusing speed aren't quite up there with the 400mm f/5.6 L, but they are both more than adequate - and the IS can come in handy on dull days.... can't afford both the 300 F4L and this so was thinking this to start with
You could use a 1.4 extender to give you f8 which assuming you have a pro body will AF centre spot only, but a 2x converter will mean no AF even with a pro body. I consider a 5d3 a pro body in respect to AF at least.Thanks for all your feedback. I know no lens is perfect in all ways. Was just looking for something to do more nature shots. Even though I know you can use a 1.4 or even a x2 extender I'd prefer not to if I can help it.