SharpShooter Loc: NorCal

Meister, Canon absolutely does not support taping the pins even though probably all their techs have actually done it.
The reason they disable it on the cameras that don't have the focus systems to support focus at f8 is because it's not 100% reliable and they don't want to have to put up with the reliability issues when it doesn't work, even though it would work probably most of the time as is evident by the fact that some of the 3rd party extenders will focus.
But the 3rd party companies have no vested interest in the reliability of the cameras and just the opposite, many will use those extenders to not have to hassle with taping the pins. ;-)
SS

juanderfulpics Loc: central jersey

I agree with the others who recommended the 5dmkiii, auto focus is so advanced and quick you'll be amazed. Once you get used to it it's great. Also the fps are not too bad. I find myself using it for sports more and more. Good luck and have fun

CHG_CANON Loc: the Windy City

I'd consider the 5DIII and 24-105 combo that was posted for sale yesterday (following the idea for a new body first).

johnfromsouthwell Loc: Southwell, Notts. UK

Thanks imagemaster but what are the right circumstances?

imagemeister Loc: mid east Florida

I cannot say for sure, but evidently in bright sunlight and your are using a Canon TC making the lens f8, and you tape the pins, you can get some kind of AF - or use a third party TC where you do not have to tape the pins. ALL this is pretty IFFY IMO ! ........

SharpShooter Loc: NorCal

Meister, what is iffy??
You are not referring to taping the pins??
I've done a LOT of shooting with the pins taped. It's not like shooting w/o the tape but it is definitely NOT iffy. It's pretty darn reliable if you ask me.
Though the new 100-400 is said to be as sharp, and in fact sharper(according to Tony Northrup), with no extender and cropping to the same view as the new Sigma sport lens.
If that is true, then use without an extender and crop a bit.
The biggest mistakes I see made with all those big zooms(600mm) is way over-cropping. Photogs seem to think that all they have to do is get a 600 zoom and they are now nature photographers and all the work is magically gone!! You still have to crawl on your belly or build a hide and sit there for hours to get GOOD shots that don't need to be over-cropped!
Anyway, that all depends on even if the new 100-400 can even be taped since I've heard no reports at all on taping the new lens!
just my 2cents!! ;-)
SS

imagemeister Loc: mid east Florida

All I can say is - using a 50D and 60D - trying to AF at f8 is non existent for me using third party TC's !

johnfromsouthwell Loc: Southwell, Notts. UK

imagemeister,
Currently I am using the mk2 camera with the 100/400 L series MK1 + 1.4 x converter and it will not AF at f8 unless I tape the pins. With the pins taped it works after a fasion, but not good enough for BIF.

Thanks again to all those who replied.

imagemeister Loc: mid east Florida

That is more than the 7D, 60D and 50d will do ! - it is the "after a fasion" that concerns me ....!