To be honest it's difficult to judge by eye how low they were and I do not know the distances and height aircraft use for their displays. It tends to change depending on the aircraft type and the airshow you are attending. For example jets tend to fly furthest away and higher and light piston powered types tend to fly a little closer. Also if you are at a smaller grass airfield, aircraft will fly closer than if you are at a show at a big military base.
What I can say however is that if you want to get shots like mine you are going to need at least a 300mm lens, I personally use a 120-400mm lens but 300mm will be fine. That lens will also need a decent Ulrasonic Motor/Hypersonic Motor, they are both the same but different brands of lens have a different name for it. Basically the motor allows for smooth autofocusing whilst panning. Instead of the autofocus constantly jerking and moving into focus. The motor will lock on and keep a very smooth focus as you pan.
All my aircraft photographs are taken at airshows. At all airshows, given that you have the equipment such as I described you should be able to get decent shots. The flying will be low enough at least some of the time to get these close shots. And though it looks difficult to get sharp shots of fast moving aircraft, it is nothing more than knowing your camera settings and practice. I certainly have no extraordinary skills with a camera, I just learned how to do it. Anyway I hope any of that helped and I thank you for your very kind words.
Whoah, I'm so thankful for all this information! It sure helps me alot. I bet that those lenses are expensive (from what I have seen they can go up to thousands. Not all, but yeah) so I think I'll have to save a bit before I go and buy them, hehe. But the information was very needed, atleast I know a bit about what lenses are good for this kind of shots!
My lens (Sigma 120-400mm OS) cost me around £600, which is just under 700 in euros, so expensive to a degree, but certainly it didn't cost me thousands. I'm sure there are cheaper lenses that will do the trick though and as stated before you can get away with a 300mm. You may struggle for distance when aircraft are further away with a 300mm but for close passes it will be plenty. Anyway I'm glad I at least was of some help to you. Hope someday you are able to take those pictures that you would like to.
Aah allright, even if I don't yet have the money and use for that expensive lenses, I will keep my eyes open when I go shopping. I want to start looking for some better lenses I probably could buy someday.
I'll see what Finland has to offer! And thanks, I hope that as well. ^^