You could buy a Tamron 150-600mm and that would probably be the most practical solution to the problem. But attempting to extend the range of your 300mm will probably not work at all ... much less work to your satisfaction.
The way to extend the range of a lens is with a tele-extender. It turns out you cannot use a tele-extender with just any lens... only certain lenses work with them. The tele-extender is an adapter that fits between the camera body and the lens and they come in 1.4x and 2x versions. A 1.4x means you multiply the old focal length of the lens by 1.4 and you get the new focal length... so 300 x 1.4 = 420. That sounds great... until you find out that it ALSO changed the focal ratio by 1.4. If you had a 300mm f/5.6 lens... now you have a 420mm f/8 lens (this is not good).
And you might be wondering why that matters... the auto-focus system uses a prism to split the light into two halves in order to do phase-detect auto-focusing. The higher the focal ratio, the more difficult it is to make that system work at all. VERY FEW cameras can auto-focus at f/8. The 1D X can, the 5D III can, and the 7D II can. That's it... nothing else in Canon's current lineup would be able to focus at f/8. (and the 2x tele-extender would turn it into a 600mm f/11 lens -- even worse.)
The reason they make these 1.4x and 2x tele-extenders is because low-focal ratio lenses can use them. I have a 300mm f/2.8 lens. I can use a 2x teleconvert and that turn it into a 600mm f/5.6 lens -- and every Canon EOS camera can focus at f/5.6 so that's no problem. The image quality isn't as good as using a real 600mm lens, so there's no free lunch in that regard, but it can be done in a pinch.
Tim Campbell
5D III, 5D IV, 60Da
