I've heard that the Tamron 1.4x has excellent IQ for its price. Unfortunately, it is listed as unavailable on B&H. (but I saw it FS used a few months ago for $70!) I don't think the Sigma is regarded very highly. The Canon is very good when used with high quality glass. Few lenses with cheap optics should be combined with any TC, anyway. Haven't heard much about the Kenko, though.
Kenko is small, cheap, excellent and works with all lenses. Buy off ebay from HK (best deal). The Canon is much larger and *might* be *slightly* better on a few of the Canon lenses... TC's are lens dependant.
My only experience is with Canon TCs. I own the Canon 1.4 Mk II and 2.0 Mk II TCs. In my view, they are both excellent and provide high quality images. When I use the Canon 2.0 Mk II TC on my super telephoto (a Canon EF 600 F4L IS USM), it slows down the AF considerably. By way of contrast, the Canon 1.4 Mk II TC does not appear to slow down the AF on my super telephoto much, if at all. Further, you have to stop down about two steps from maximum aperture with the TCs on the super telephoto to obtain the best quality images. Finally, you will not maintain AF with these TCs on Canon's super telephotos unless you use a 1D or 1Ds camera body.
I've done head-to-head comparisons of the Canon EF 1.4x Extender, Kenko Teleplus Pro 300 DG 1.4x TC, and Tamron SP AF 1.4x TC on a variety of lenses. The Canon and Kekno are very much equal, and both are slightly better than the Tamron.
The Tamron SP and Kenko Pro DG are the same internals, just different bodies, so should perform similarly. I have a Kenko and Canon and they are both very good, with maybe Kenko having the edge in the centre, but Canon better in the corners.
I have the Canon and Kenko. The Canon is only marginally better than the Kenko, but ironically the Kenko will work on more Canon lenses than the Canon TC e.g. 100/2.8 lens. And yes, you can get a good deal if you buy from a Hong Kong store on eBay, as I did.
Pixel Perfect wrote:
The Tamron SP and Kenko Pro DG are the same internals, just different bodies, so should perform similarly.
a) For 1.4x TC:
Kenko MC4 DGX has 4 elements in 4 groups.
Kenko Pro300 DGX has 7 elements in 4 groups.
Tamron (standard non-SP) has 4 elements in 4 groups.
Tamron SP has 5 elements in 4 groups.
So, Kenko MC4 and Tamron (old non-SP) are probably the same internally.
Note Kenko's DG and DGX are only different in terms of EXIF reportage and AF compatibility. Optics are identical.
b) For 2x TC:
Kenko MC7 DGX has 7 elements in 5 groups.
Kenko Pro 300 DGX has 7 elements in 4 groups.
Tamron (standard non-SP) has 4 elements in 4 groups.
Tamron SP has 7 elements in 4 groups.
In this case, the Kenko Pro 300 and Tamron SP are the same, as proven here.
c) Compatibility
Kenko and Tamron TCs are compatible with many lenses whereas Canon and Sigma are more limited in this regard.
Note that EARLY Kenko's DGX TCs have compatibility issues with 100 f/2.8L HIS, 200 f/2L IS, 70-200 f/4L IS and 800 f/5.6L IS. For example, they completely fail to work with 70-200 f/4L IS. I know 'cos I have a 1.4x copy of this TC from the early batch. When you purchase from eBay, chances are very high that you'll end up with this problematic copy. According to Kenko's website, they stopped production, corrected the issue and are now slowly shipping out the corrected batches. You are therefore STRONGLY ADVISED to check the Kenko TC with your lens before purchasing. I tested the Kenko 2x Pro300 DGX TC from the latest batch and it's fine.
With the 100 f/2.8L HIS lens, you can only look for Kenko and Tamron TCs. On the Tamron 1.4x SP TC, the AF will hunt and hunt before settling down, even in good light. But AF is swift and precise with the new batch of Kenko 1.4x MC4 DGX TC.
d) Image quality
This is where it gets very tricky. Performance depends on the lens used and chosen format (APS-C or FF).
On this site, the reviewer believes the Sigma 2x, Kenko 2x Pro300 and Tamron 2x SP are equally good on APS-C format.
On another site, various TCs were fitted with the 70-200 f/4L IS lens and compared on FF format. According to their tests, the Kenko 1.4x MC4 DGX gives the best performance.
I based my purchase on the above site and boy, was I disappointed. The Kenko 1.4x MC4 DGX performance with my 100 f/28L HIS lens on APS-C format was simply atrocious. Corners were blurry. AF was zippy though. My older Tamron 1.4x SP AF was much much better in comparison. However the Tamron SP has AF issues with my macro lens. So, I got another Kenko 1.4x Pro300 DGX. Optically, it's as good as the Tamron SP. BUT its AF was just as slow as the Tamron, and it refused to work with my 70-200 f/4L IS. I am now trying to get the Kenko 1.4x Pro300 DGX exchanged for a copy from the latest batch. Sigh...
IMHO, a new copy of the Kenko 1.4x Pro300 DGX from the latest batch should be your best bet in terms of compatibility, AF and image quality.
The electronic compatibiliity issues are many and complex depending on which lens and which of the many versions of the 3rd party adapters you get. Lots of the old versions are still being sold new.
With Canon, they all work on the specific Canon lenses they were designed for. I've had Tamron and Sigma TC's, as well as Quantaray that worked on some cameras with some lenses, and on different cameras, different lenses worked.
Just because someone has tested a TC on their camera with a few lenses does not mean it will work on your different camera model and your lenses. Its a mess.
thw2 wrote:
a) For 1.4x TC:
Kenko MC4 DGX has 4 elements in 4 groups.
Kenko Pro300 DGX has 7 elements in 4 groups.
Tamron (standard non-SP) has 4 elements in 4 groups.
Tamron SP has 5 elements in 4 groups.
So, Kenko MC4 and Tamron (old non-SP) are probably the same internally.
The Kenko Pro 300 DG 1.4x has 5 elements in 4 groups. it is too small for 7 elements and the Tamron SP 1.4x has 5 elements in 4 groups. Not sure where you got your specs. The optics are identical, not sure which one makes them though.
Pixel Perfect wrote:
The Kenko Pro 300 DG 1.4x has 5 elements in 4 groups. it is too small for 7 elements and the Tamron SP 1.4x has 5 elements in 4 groups. Not sure where you got your specs.
From THK site: Kenko Pro300 has 7 elements in 4 groups.
It is possible that the site has a typo.
In any case, my results show the Kenko Pro300 and Tamron SP have the same performance on the 100 f/2.8L HIS lens in APS-C format.
On the lenses Canon created their TCs to be used on (the super teles and 70-200s for example) Canon models are worth the extra $ in terms of IQ and AF speed. On everything else the Kenko is right up there. Just my experience FWIW.
thw2 wrote:
From THK site: Kenko Pro300 has 7 elements in 4 groups.
It is possible that the site has a typo.
In any case, my results show the Kenko Pro300 and Tamron SP have the same performance on the 100 f/2.8L HIS lens in APS-C format.
well thers a typo in those specs somewhere
both the 2x and 1.4 are speced as barrel length 27mm
there probably was a bit of cutting and pasting going on.
I use the Tamron 1.4x SP with my 100-400 and my Sigma 150--both combos produce excellent results. From what I've seen you cannot go wrong with either the Tamron SP or the Kenko pro 1.4x TCs. If you need complete weather sealing and plan to shoot with super tele's only the Canon version may be worth the additional cost, but I have seen no evidence that the Canon 1.4X II is any better than the aforementioned Tamron and Kenko counterparts in terms of IQ.
Sorry to dig this out...years late but I am still facing the same problem. While with EF mount I can use either the original Canon TCs...or the newer Sigma/Tamron ones, other mounts cannot.
Mark K wrote: Sorry to dig this out...years late but I am still facing the same problem. While with EF mount I can use either the original Canon TCs...or the newer Sigma/Tamron ones, other mounts cannot.
Well you win the prize for oldest thread resurrected this month. But what is your question? If I was shooting Sigma lenses I'd buy the Sigma. If I was shooting the newest Tamron lenses I'd buy the new Tamron. If I was shooting Canon lenses I'd buy the Canon. If I was trying to trick a non-f/8 capable camera into AF with a 5.6 lens/1.4TC I'd choose the Kenko (but only certain versions don't report).
If I was shooting another mount I'd buy that mounts TC. If I wanted to stack TCs I'd buy the older Canon ones.
) (at least most) EF lenses. 
Sorry to dig this out...years late but I am still facing the same problem. While with EF mount I can use either the original Canon TCs...or the newer Sigma/Tamron ones, other mounts cannot.