Over the years, I have tried to film wildlife in three ways: digiscoping; with a camcorder; and now with a video-enabled DSLR. For me, the third option is the best. Digiscoping has never produced a sufficiently stable image and using a camcorder the focal length of the lens is never long enough. The lenses on most camcorders seem to start with an equivalent of about 43mm, so a x25 zoom lens will provide the equivalent of a little over a 1,000mm. My favoured option currently is to use a fast aperture 300mm with two doublers, which provides me with an equivalent focal length of 1920mm at f11. Because my camera is high-definition (shooting with a resolution of 1920pixels wide) I can crop the final image to one third of the width and acheive a focal length of over 5,000mm. Providing a good compromise between quality and just obtaining a record of something.
The Canon 550D also shoots in crop mode, where only the central part of the chip records the images. This makes my 300mm into a 6,720mm lens when I use one doubler. However, I don't like having to switch between modes, so at the moment I'm sticking to full HD and cropping in post production if I need to. When I can afford (and when I'm fit enough!) it I'd like to get the sigmonster 300-800mm. Not really for the extra length but for the flexibility of easily achieving different framings. G