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I have Canon EOS 650D and Tamron 70-200 f2.8 lens.
I would like to buy a teleconverter (1.4x or 2.0x) to increase the zoom, e.g., for shooting wildlife and the moon.
I took a look at several articles and understand that different pairings of lens-and-converter give different image quality.
So, how do I select a teleconverter?
Unfortunately, I can't recommend using a teleconverter with the Tamron 70-200mm f/2.8 at all (Update in 2020: The most recent Tamron SP 70-200mm f/2.8 Di VC G2 takes a TC better than the older Tamron 70-200mm lenses do - it's pretty much the equal of the Canon EF 70-200mm f/2.8 l IS II). The lens is pretty good for most of its range, but the weakest image quality is at 200mm. And the long end is where you're going to use a zoom the most when you have a teleconverter attached. The way a teleconverter works is to magnify the center of the image circle cast by the lens. Any flaws, softness, and other image defects will be magnified as well. Of course you will also give up one stop of maximum aperture for a 1.4X teleconverter and 2 stops for a 2X.
If you insist on using a teleconverter with that lens, I would suggest a 1.4X as they magnify the defects less than a 2X will. Many of the teleconverters on the market are made in the same factory and marketed under various brand names: Tamron and Kenko being two of them. There are two tiers under both brands. There's not much of a price difference between them so you should probably go for the Kenko C-AF Teleplus Pro 300 DGX/Tamron SP AF.
Here's an image quality comparison between the Tamron 70-200mm f/2.8 SP Di VC USM lens at 200mm f/2.8 and the same lens with the TC at 280mm f/4. Mouse over the image to switch from one result to the other.
I own the Kenko C-AF 2X Teleplus Pro 300 DGX and the Canon EF 70-200mm f/2.8 L IS II lens. And honestly, the only thing I've found the TC good for is taking photos of the moon. For most other uses, I find I get better image quality shooting with the lens alone and cropping the snot out of it when editing. And the Canon lens takes a converter a lot better than the Tamron does. From Bryan Carnathon's review of the Tamron 70-200mm f/2.8 Di VC USD at The-Digital-Picture
You buy a 70-200 f/2.8 to use at f/2.8. Otherwise, buy a Canon EF 70-200mm f/4 L IS USM Lens. It will give you modestly better image quality in a smaller and lighter package – with a lower price tag. Thus, I feel that for most, the Tamron VC is a better choice than the Sigma OS from an image sharpness perspective.
The Canon 70-200 L IS II is perhaps the best zoom lens I've ever used – and represents a very formidable competitor to any lens in this class. Surprising is that, at its lower price point, the Tamron 70-200 VC delivers image quality very close to the Canon IS II until the long end of the focal length range where the Canon easily bests the Tamron. Before deciding that you are going to give up that Canon advantage for the lower price, remember that we tend to most-use the full extents of the focal length range in our zoom lenses. And 200mm is perhaps the most important focal length in a 70-200mm f/2.8 stabilized lens. The better 200mm image quality is also translated into better with-extender image quality at the 200mm focal length setting.
I think that there is a general recommendation to use a teleconverter produced by the same manufaturer as the lens. It will probably have been designed to work with teles of the same brand.
You should probably avoid Canon teleconverters, because they have physical dimensions that may be incompatible with the lens.
As seen on this picture, the first element is "in front" of the lens mount. Thus a lens can only be mounted if there is room for this in the rear of that lens. Not even all of Canon's own lineup of tele lenses are compatible with their own converters. Most other lenses are not.
But apart from the Canon teleconverters, I believe that all other teleconverters should work.
