Hi Olly,

I do a lot of Wildlife work, and it takes big glass! That said, I use teleconvertors, one thing you want to make sure, is get the best quality convertor you can, you want APO glass and one thing to take into account on most AF camera systems, you will most likly have to manual focus due to the light loss, I use a 300 f/2.8 with a 2x Kenko pro convertor for most of my work, but also have a 600mm f/4 that sees it share of time on the tripod, a great many use 300mm f/4 lenses for their work, they are still small enough to hand hold, and not quite as expensive as the really big glass, I guess the most important thing is technique and patience, and lots of practice. But again, if you want bigger glass on a budget about the only way to do it is with a teleconvertor, and of couse anytime you introduce extra glass surfaces, you run the risk of lower quality images, but a great many of us, that do wildlife use them. A great place to practice technique is a local zoo, that way your shooting the subject matter and can develop what works for you, birds in the backyard are always good for practice..