It's pretty late, the moon was out so I decided to test out the Sigma 50-500mm. To get a bit further out, I threw it onto the Canon 7D and took advantage of the crop factor. These JPEG images have been resized and sharpened a bit in Lightroom. I'm still waiting to bring this out to catch some surf shots.
I was able to bring out the Canon 100-400mm F/4.5-5.6 on the Canon 7D and grabbed some really nice closeup shots of a turtle swimming in the bay - while I was on a cliff above. Yeah that's quite a bit of zoom, the lens is very sharp and very fast. Click any of these images to get a little bigger view of the image.
Also (below) here's more zoomed in and zoomed out comparisons shot from the Canon 7D + canon 100-400mm. The bottom image is not a crop. Taking advantage of the crop on the Canon 7D, that's how close you can get. All images were hand held and stayed pretty sharp with IS turned on.