As mentioned, the III has a different chip or software... designed to slow down AF by about 25% and improve accuracy.
And the added "weather sealing" is a rubber o-ring on the bayonet mount.
The II is still quite good, but hadn't been made in some years now, so I'm pretty sure all new old stock will have sold out by now... you'll only be able to find it used.
ALL Canon TC's have a protruding front element that limits the lenses they can be used upon. The front protrusion has to fit inside the rear of the lens. For example, I don't think they can be fitted onto any of the 70-300mm (maybe the L).
They will fit on 70-200 and 100-400mm. However, in order to be able to autofocus the latter, you need an "f8 capable" camera when using a 1.4X on the 100-400 (because it's an f5.6 lens already... f8 cameras are 7DII, 5DIII, 5DS or 1DX among the current models).
Canon recommends using their TCs on 135mm and long telephotos and they work best on primes. I've occasionally used my 1.4X II on 70-200/2.8 and 70-200/4... but more often on 135/2, 300/4, 300/2.8 and 500/4. I won't use my 2X II on any zooms... too much loss of image quality. (The 70-200/2.8 Mark II and 2X Mark III supposedly work pretty well together). I mostly use the 2X with 300/2.8 and 500/4.
Some people have found 1.4X or 2X useful on 45/2.8 Tilt Shift and some of the macro lenses. I have no need for that, so really haven't tried it.
No TC's I'm aware of from Canon or anyone else can be used with any EF-S lenses (not that there are many EF-S it would make much sense to use with a TC).
rmorrison1116 wrote:
Canon teleconvertors are not universal, they are designed for Canon lenses only. Results are unpredictable when used with non-canon lenses, especially the version III units.....
I disagree... Canon
does optimize their TCs to work as well as possible with a number of their own lenses and are under no obligation to insure they work as well with third party lenses. However I think it goes too far to say they "aren't universal".
A TC is a TC. Not that big a deal. If it physically fits onto the lens... great. If the added optics happen to work well with the optical formula of a particular lens... great. A "weaker" 1.4X "costs" less light lost and in nearly all cases has less negative effect on image quality than a "stronger" 2X that costs twice as much light (not to mention 3X, which do exist but probably aren't worth the trouble).
Even the electronics are not really all that complex in modern ones... for the large part they just pass communication through to control AF and the aperture of the lens. The electronics also "register" with the camera, telling it that a TC is installed between the camera and the lens (in which case Canon designs their cameras to turn off AF when the combo exceeds the camera's rated capabilities). This is confirmed by the EXIF data, which reflects the effect of the teleconverter on both focal length and aperture... And that you can tape over a couple of the electronic contacts to fool the camera so that it doesn't "know" the TC is there and will still try to autofocus.
It's the physical design of Canon TCs... the protruding front element... that is the single biggest limiter of what lenses they can be used upon (and it's even possible to work around that... by putting a 12mm macro extension tube on the front of the TC... though some lenses might not be able to focus to infinity when that's done).
Same as Canon, no doubt Tamron and Sigma test their TCs with their own lenses and optimize them to work together. But they are generally usable on any other brand of lens, too. Kenko doesn't even make lenses... just a selection of TCs (although Kenko's parent company does own and make Tokina lenses and Hoya filters).
Besides the manufacturers' recommendations (i.e. Canon's suggested lenses to use with their TCs)... it's a bit of a guessing game. But it is possible to find combos that work well together... as well as combos that don't.
If considering getting a teleconverter and looking for info, be VERY specific what lens you want to use it upon and what teleconverter you've been looking at. Maybe you can find someone who has used that SPECIFIC combo and can tell you their impressions of the results it produces. Otherwise, you'll have to just hope for the best and might end up experimenting with different combos until you find one that lives up to your expectations.