This is a 1.4× teleconverter for Canon's EOS-R mirrorless system. It is only compatible with some of Canon's RF mirrorless lenses; see their individual reviews to see if it's compatible. It cannot be stacked with any other teleconverter, and it cannot be used with any of the EF to RF adapter rings. This is because the lens (or EF to RF adapterI have the Kenko 1.4 TC and it fits several of my lenses. I find the results very good with the Canon 24-105/4 L IS, the Tamron 90 and 180 macros, and the Canon 70-200/4 L IS. I find the results not so good using the Sigma 150-500 and the Sigma 18-200, using the 5D2 and the 30D. And I've read some not-so-flattering reviews of the Tamron 18-270
Adding a tele extender does not change the focal length of that lens. The thing that does change is the angle of acceptance. The focal point of that lens does not change and can not change. Also any time you increase the length of the lens tube, you will sacrifice some f-stop value. Adding the 1.4x to your 400mm lens dose not turn it into a
Extender EF 1.4x II_This tele extender can be used with fixed focal length lenses 135mm and longer (except the 135mm f/2.8 Softfocus lens), and the EF 70-200 f/2.8L, 70-200 f/2.8L IS, 70-200 f/4.0L, 70-200 f/4.0L IS USM, and 100-400 f/4.5-5.6L IS zoom lenses. Superb optically, it preserves the image quality of the lens it's mounted to and
The Canon 1.4x lens extender extends the focal length of a lens by 40%, making (for example) a 200mm lens function as a 280mm lens. We used Canon's 200mm ƒ/2.8L II USM telephoto lens to test
I would not even consider the 2x tel-con on any lens. I currently own the 1.4 II tel-con and can say it works very well on the ef 70-200mm f2.8L lenses. I am not sue but I don't think I had the version II extender when I had my original ef 70-200mm f2.8L zoom but I don't see why it would not work on it.
I'd suggest getting the 1.4II. The difference between the II and III is quite small as other's have pointed out and you'll be saving a fair amount of money especially if you pickup one a used one. The 2xIII works nicely on the 70-200II however focusing speed is reduced by 50% so be aware of that. xUYp.