Has anyone tried a tele convertor on this lens? If so witch one do you suggest. I see a 1.4x and a 2x listed, both are simailar in price. I dont want a ton of quality loss, so I am thinking the 1.4x maybe the way to go. Am I correct in my calculations? 1.4x would equal 280mm on long end and 2x would be 400mm. I think if my calcs are correct, the 1.4x is not even worth the price for the distance gained. I shoot mostly in daylight, and alot of air shows. I would assume as long as there is plenty of light the 2x would AF perfect.