Canon 2X II extenders...AF only with f2.8 or faster?

If this is a repeat, I do apologize. I've searched previous posts and have become more confused with each read.

I've heard that the 2XII extender will AF only with a 2.8 or faster lens. Is this correct?

Any issues with using it in conjunction with a 300 f4 IS?

What about stacking with a 1.4?

Thanks for the replies.

On a 40D you won't get AF with anything slower than f/5.6, so sticking a 2x on the 300 F4 makes it a 600mm f/8, so you'll struggle to get AF.

So yes if you start with a f/2.8 it will become a f/5.6 and AF still.

Think you may be able to tape the pins or something on the TC to trick the camera to AF? Am sure someone will chime in with details.. and stacking on a 1.4 should also work...

The Basics:

1.4X TC reduces aperture 1 stop
2X TC reduces aperture 2 stops

1D bodies will AF with f/8 lenses, everything else will AF with f/5.6 lenses.

So, a 1D will AF with a f/5.6 lens and a 1.4X TC or a f/4 lens and a 2X TC.

Everything else will AF with a f/4 lens and a 1.4X TC or a f/2.8 lens and a 2X TC.

The 300/4 you mentioned will only AF on your 40D with a 1.4X TC. Add a 2X and it becomes manual focus. Stack 2X and 1.4X and the IQ will be bad enough that focussing may become optional.

Interesting. Thanks for the replies.

I dont get how that works... because you can still AF at F/8 in AV ? o_0

zeva wrote in post #5077134
I dont get how that works... because you can still AF at F/8 in AV ? o_0

For AF purposes the camera only looks at the fastest speed of the lens - if you have a f4 lens it will see f4 even if you are shooting at f22.
If you put a 1.4tc on a f4 lens the camera now sees it as a f5.6 lens regardless of which AV you choose.

But all cameras seem to work fine with lenses that have f6.3 at the furthest zoom range...? Plus my 100-400 and 70-300 work fine at f5.6 plus a 1.4 TC....I guess the rules are made to be stretched?

Neilyb wrote in post #5077812
But all cameras seem to work fine with lenses that have f6.3 at the furthest zoom range...? Plus my 100-400 and 70-300 work fine at f5.6 plus a 1.4 TC....I guess the rules are made to be stretched?

There is a reason for all that you quote:
1) The f6.3 lenses work because they have electronics in the lens that only report f5.6 to the camera - these lenses have also not got the necessary pins to report any tc back to the camera, so whatever you stick on them they only report f5.6 so will attempt to AF.

2) The 70-300 has not got the necessary pins to report any tc so will always show f5.6.

3) The 100-400 will not AF on non 1 series Camera unless you have a non reporting tc (e.g. Kenko 1.5 and some other cheaper tc,s) or you tape the pins to fool the camera.

There are no rules that can be stretched as such. If you have a reporting tc and a lens with the necessary reporting pins - AF will be impossible on a f5.6 or slower lens on a non 1 series body.