The Canon EOS and Photography Forums

I am looking at a 1.4x teleconverter to use with my Sigma 70-200 f/2.8 and to a lesser extent with my Tamron 70-300.  I've got it narrowed to four models.

Sigma APO Teleconverter 1.4 EX DG
Kenko Teleplus PRO 300 DGX 1.4x
Tamron SP AF 1.4x Teleconverter
Tamron AF 1.4x Teleconverter[/li][/list]

I know the Sigma would be the best - but it would be limited to only being used with my 70-200 and its the most expensive.  The Kenko and Tamron SP are about the same price - are they that much better than the regular Tamron converter? 

It's really hard to say which TC will be best with a particular lens. Ideally the TC would be designed to work with a specific lens and be optically matched to it, however that is rarely done these days (though Canon are coming out with a lens which has a built in matched TC as part of the optical design!).

I suppose that in theory the Sigma APO 1.4x (http://www.amazon.com/exec/obidos/ASIN/B000CD8BGU/bobatkinsphotogr) should be the best match for the Sigma lens since if Sigma are going to optimize the design for anything, they'll optimize it for their own lenses.

I can tell you my experience with Tamron and Canon 1.4x TCs. The Tamron (non-SP) fives similar image quality with 300/4L and a 500/4.5L in the center of the frame. On a full frame camera the Tamron results is slight vignetting and corners that aren't as sharp. On a crop sensor camera they're pretty much equal.

In fact I use the Tamron 1.4x almost all the time when I need a TC on my 500/4.5L when shooting with a crop sensor body because AF is maintained and the image quality is almost identical to my Canon 1.4x.

The only Kenco I ever tested was an older Kenko 1.5x. It appeared to be absolutely identical to the Tamron 1.4x in terms of image quality and magnification. I suspect they were the same optics in a very slighly different housing. The new Kenco DGX 1.4x (http://www.amazon.com/exec/obidos/ASIN/B002C6QC3E/bobatkinsphotogr) gets good reviews and it is a little cheaper than the Sigma.

I'm going to have to ask Tamron for a loan of their 1.4x and SP 1.4x TCs so I can see how they behave. The Tamron 1.4x that I have is probably 15 years old! However don't expect to see any test reults here soon since I have a couple of cameras waiting to be reviewed and I'm pretty much tied up with other stuff next month!

Thanks Bob - that's helpful.  I went ahead and decided to start with the cheapest option - the regular Tamron after reading your comments and those of a few others, including one who found it on par with the Sigma at a fraction of the cost.  I am curious to see what I find out. 

So here's an update on my converter solution. 

I order both Tamron converters from keh.com - they were used, but in excellent condition.  Very pleased with keh by the way. 

• The 1.4x is great and works without a problem on my 17-50mm f/2.8, 50mm f/1.4, and 70-300 f/4-5.6
• It did not work well with my Sigma 70-200 f/2.8 - optically it was fine, but the autofocus would hunt and never settle in. 
• The image quality when paired with my 28-105 f/2.8 was all around awful - almost irrespective of focal length
• I was surprised it worked with my 17-50mm but it did - it's a little soft at the short end but nothing too bad.  The 24-70 isn't a super useful range though so it was more for kicks.
• I am super pleased that it worked with my 50mm f/1.4 - it creates a very nice 70mm f/2
• I can stack the two converters for a 2.8x effect - the image quality does degrade as one would expect though
• I tried to decide whether to get the Kenko, Tamron SP, or Sigma to go with my Sigma 70-200 and finally decided to bite the bullet and get the Sigma.  It was well worth - the AF speed is incredible and the image quality very impressive.  I decided to only get a 1.4x converter though, when I want to use my Sigma as a 400 f/5.6 I'll just use manual focus. 

Thanks for your help!