mikoyan29 Wrote:My longest lens so far is the 300mm. I think I want to get a longer one at some point, especially since I want to go to a couple airshows.

In my opinion, a 300 is good enough. Any longer focal length and you have trouble keeping the aircraft in frame and spend more time just trying to spot a moving plane. You don't need to try to get shots when the aircraft is 2,000 feet above you; just wait a few seconds and it will be in range of the 300 mm. If you went all the way out on a 500mm for instance, you would be very lucky to get close shots of the cockpit or completely fill the frame and risk unflattering shots that cut out wings or tail sections. And then there is the blur you could get just holding the 500 and panning the action.
A satisying 300mm is just fine. If you doubt me, maybe try a 2 X teleconverter on that lens before spending however many hundred on a 500.
I have a manual 500mm and use it on taking shots of the moon from time to time but it is not useful for aircraft in flight. The last airshow I went to I just used my 200mm and have thrilled people with those images.

Taken with a 200mm and cropped some: