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I have two APS-C (70D and 600D) bodies with Canon EF and EF-s lenses, and Sigmas, among them the Sigma 70-200 f2.8 HSM OS. I am also considering getting the new Tamron 150-600 lens, pending reviews.

I was just curious if a Canon teleconverter, specifically the canon 2x III, will work on the Tamron and/or Sigma lenses. Note that I currently own the sigma 1.4x teleconverter but I'm not very happy with its IQ

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The Tamron 150-600mm f/5-6.3 lens with a 2x convertor would become 300-1200mm f/10-13. That's awful slow for those focal lengths. No Canon body I am aware of could auto focus at f/10, much less f/13. The viewfinder would also be fairly dark, and to get shutter speeds on the 1/1000 sec. range on the long end would probably require an ISO higher than what most would consider acceptable. – Michael Clark Jan 8 '14 at 23:25

Hard to tell. Canon's 2x III teleconverter includes a rubber extension that fits in to the back side of a lens mounted on it. This is used to block out excess light (that would have been for the outside edges of the image) from making it in to the teleconverter.

It also likely would not support full electronic control of a third party lens. To maintain full electronic control, a micro-controller in the teleconverter adapts values for the camera and it relies on a degree of knowledge about the lens. It isn't guaranteed that a third party lens is designed in such a way as to be compatible with it.

If it physically fits, the teleconverter will still magnify the image (a teleconverter is, after all, effectively just a big magnifying glass that magnifies the center of the image circle), but you might have to manually set focus and possibly exposure.

Do note however, that short of a very high quality zoom lens or a decent quality prime, the IQ of any 2x teleconverter is going to be marginal.

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I would be very surprised if the EF 2X III will not physically mate with the Sigma 70-200 f/2.8 HSM lens. There is plenty of room inside the rear barrel for the extension.… – Michael Clark Jan 8 '14 at 23:17
The Sigma lens was released in February 2010, the Canon EF 2X III was released in August 2010, so there may well be some compatibility issue there. I would also only consider the combination if the body I plan to use was released at least several months before the Sigma lens. Then I would probably try both the EF 2X II and the EF 2X III with my body and the Sigma lens before buying. – Michael Clark Jan 8 '14 at 23:32

There are a few aspects to this question.

Physical connection: will they mate up and connect to the camera and operate? Almost definitely, sometimes with some limitations (and occasionally with some extras: the Kenko teleconverters connect in with Canon lenses and allow them to autofocus at F8 instead of F5.6, although I found the actual use in the field too unreliable to depend on. It did work).

Image quality: Vendor TCs are generally designed to work with specific lenses. The Canon TCs aren't supported for use on all Canon lenses and may not give quality results with unsupported lenses; many of the unsupported lenses for teleconverters are the zooms. You'll get loss of sharpness and contrast to varying degrees and in some cases some rather nasty chromatic aberration effects or corner warping.

The only real way to know for a specific combination of teleconverter and lens is to try it. Find a buddy with the piece you need, or rent it from a place like borrowlenses or lensrentals and go out in the field and take test shots. Then load them up and pixelpeep away to see if you think the quality is acceptable for your needs. There's no "vendor support page" or "in theory" answer that'll get you a definitive result here, ultimately it's going to require a field test.

The Canon 2X III TC is an incredibly sharp unit, FWIW. And it works very well on high end Canon zooms. That sigma lens is supposed to be good, so I think the combo has potential, but I haven't tried it. it'd be a worthwhile test, though.

The new Tamron 150-600 looks really interesting to me as a bird photographer. I've arranged a test unit that I expect to put through its paces and write up in February. At that price, I think it'll blow the Sigma 150-500 away in the market and I'm tempted to buy a unit myself. We'll see how it works in real life. I would not expect good results for the 2X TC attached to that lens, though. I'll probably test it to see, but I wouldn't expect to use that combo out in the field.

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I have been able to try the Canon 2x III extender and Sigma 70-200/2.8OS combination and they do mount and autofocus on the Canon 70D and T5i.

The resulting image quality, unfortunately, is beyond the scope of what I can test.

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thanks for your response, but I don't understand your last comment – dassouki Jan 9 '14 at 19:18
Simply that while I have checked for physical and operational compatibility, I wasn't able to look at any photos taken with that particular combination. – mpr Jan 9 '14 at 20:43

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