<p>I was unaware that the 1.4x and 2x would work with the 70-300L when zoomed out. I tried them both on when I bought the 70-300 a few years ago, they didn't fit and I never thought about it again. But I just tried them now and they both fit when zoomed to 300. But not something I would do except in a pinch because, as pointed out, the natural reflex of zooming back would cause an expensive crunch. And I have a 400/5.6, so not something I really need to attempt.<br>
So, I just tried the autofocus. With a 60D, the 70-300L at 300 mm will autofocus with 1.4x (Mk1) in good outdoor light, but not particularly quickly. Indoors in poor light it can't autofocus. The 70-300L attempts to autofocus with the 2x (Mk1) but NEVER achieves focus. Just goes back and forth then stops. Ditto with a Tamron 2x. </p>
