have a Canon EOS Rebel XSI and have a Tamron 18-270mm lens for it. Do you know what extender is compatible with this arrangement? I purchased a Canon Extender EF 2X III but it does not fit the lens.
Using a teleconverter on a a Rebel body with a slow zoom lens is not likely to work well, unfortunately.
Autofocus will not work and image quality is going to be poor. You are much better off saving to buy a longer lens.
The Canon teleconverter (TC) has a lens element that extends out from the barrel and so prevents your current lens from mating to it. What that means is that you need a TC that doesn't extend like that. There are a few, but given that you have a Tamron lens, you might consider the Tamron 2x TC as an option. I couldn't find any documentation that said it wouldn't work, but a quick call to their support line should confirm.
1another brand to consider is Kenko. It's likely that autofocus either won't work or will work poorly and slowly, FWIW. And if you can borrow/rent the TC first, do so. you may find image sharpness isn't acceptable.– chuquiMar 10, 2015 at 4:25
The Tamron 2X SP AF Pro and the Kenko Teleplus Pro 300 DGX 2X are the same converter, just marketed separately by each company. They both have 7 elements in 4 groups and share the exact same dimensions and mass. The lower end Tamron F AF 2X (model 230FCA - 4 elements) and Kenko N-AF 2X Teleplus MC7 (7 elements in 4 groups) appear to be different designs. Mar 13, 2015 at 7:38