Here are two images; the first one shot with my Canon EF200mm f/2.8L USM, and the second image shot with my Canon EF200mm f/2.8L USM + a Canon EF 2x Extender. My digital SLR the Canon 20D, has a 1.6x magnification factor due to the partial image sensor. Because of this, my Canon EF200mm f/2.8L USM becomes the equivalent of a 320mm f/2.8 lens, whereas with the 2x Extender connected, it becomes a 640mm f/5.6. Some image quality is sacrificed however, when using the 2x Extender. But when you compare the price of a 200mm lens + 2x extender to a true 600mm lens, well, it's worth the sacrifice (at least until I can afford an actual Canon 600mm L lens).
Here is the first image at the 200mm focal length:
And here is the same moon (a few seconds later) at the 400mm focal length:
Isn't it lovely? Now just imagine what I could do with a Canon EF 600mm f/4L IS USM?
Whoever wishes to donate such a lens, of course I would be forever grateful! ;)
For those of you who might have missed it, here's a link to a photo session I did of a Lunar Eclipse last year. It was an amazing event. Have a look if you haven't already seen these.
And here's one that I posted on Pixel Critic:
