Rather than investing in a $12,000 supertelephoto, I suggest investing a few hundred bucks in a 1.4X or a 2X teleconverter.
Teleconverters, which are also known by the name “tele-extenders,” extend the reach of a telephoto lens by optically multiplying the focal length. A 2X teleconverter doubles the focal length of your lens; a 1.4X teleconverter multiplies it by 1.4. The former makes a 300mm lens act like a 600mm. The latter makes a 300mm perform like a 420mm lens. The best of these teleconverters maintain the microcomputer connection between lens and camera, meaning autofocus and auto-exposure controls still work.
So, what’s the catch with teleconverters? They do tend to slow down the focusing of a lens, and they typically sacrifice at least one full stop of light. Longer converters may sacrifice two full stops of light. And, you’re putting another set of optics between your lens and your subject, so you may see a small impact on image quality. But, for amateur wildlife enthusiasts, this impact is likely inconsequential—and certainly not worth a $12,000 premium for a pro-caliber supertelephoto.
The challenge of losing a stop of light might seem like a bigger deal, but in practice it’s not so troubling. Instead of shooting at 1/250 at ISO 100 and ƒ/11 on a normal sunny day, you might open up to ƒ/8 or even ƒ/5.6. But, remember, this is the era of high performance at high ISOs, so it’s no problem to boost your ISO in order to compensate for the reduced amount of light delivered by the teleconverter. When you factor in the added benefits of crop sensors and higher resolution, there has never been a better time to use teleconverters to get you up close and personal to wildlife subjects.