Canon "Extenders" are not compatible with many lenses because they have a protruding front element.
Yes, using an Extension Tube will allow you to use a Canon 1.4x or 2x "Extender" (teleconverter) but then you will not be able to focus on subjects very far away. Extension Tubes allow you to focus very close for large magnification(Macro photography), but you then lose infinty focus.
I would recommend you buy a 3rd party teleconverter that will directly mount to your your 70-300L and 24-70L. Kenko makes a good teleconverter tha does not have a protruding front element
Kenko Teleplus PRO 300 DGX 1.4x AF Teleconverter
Canon 1.4x "Extender" (teleconverter)
It is possible to put a 12mm Macro Extension tube on the front of the Canon Extenders, to allow them to physially mount on most lenses. However, after adding the extension tube the lens may no longer be able to focus to infinity.
The combo probably won't work on your 24-70 (not really sure why you'd want to put a teleconverter on that, since you have a 70-300mm).
But it might work okay on the 70-300mm.... Perhaps only at the long end of the zoom. But that's most likely where you would be using it, anyway And even if you can't still focus all the way to infinity, it might not matter. But, heck, most of the time with really long focal lengths we don't focus all the way to infinity.
The only time I've done this, I used the 12mm extension tube trick to be able to stack 1.4X and 2X teleconverters, then used them on a 500mm lens (and put that on a crop camera and a very sturdy tripod). Really a pretty ridiculous experiment just to see how it worked. But, as you can see, the lens still focused (manually) to infinity....
If you prefer to use the Canon Extenders, give it a try. But I'd recommend trying the 1.4X first, to see if that's enough. A stronger 2X is less likely to give acceptible image quality on a zoom lens.... not to mention the loss of AF and a dimmer viewfinder to try to manual focus.
Speaking of which, you do realize that an f5.6 lens plus a 1.4X becomes an effective f8.... and only a few Canon cameras can still autofocus: 5DIII (with ver 2.x.x firmware) and 1D series models. Other Canon models need f5.6 or larger to focus.
There are some workarounds.... For one, you can tape up a couple of the contacts on the teleconverter so the camera doesn't know it's there. That way it will still try to autofocus. It may be slower, hunt more and really struggle or fail in more challenging light, though.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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