I got to borrow a friend's 300mm lens yesterday..
While cutting the lawn this afternoon, I was waiting for a plane to fly overhead to see how close I could get to a jet cruising at 30,000 feet or so.
I was pretty pleased with how close I was able to get to the plane. This is the full image with no crop. I used the Canon 60Da, the 300mm & a 2X converter = 600mm x 1.6 crop = 960mm total.
Thanks to Adrian for the loan of the lens.