No problem. Its just camera stuff. You defend our country from terrorists, communists and democrats (kinda sucking on that last one recently, btw - they are everywhere.) Pretty sure I own you the beer.
Quick definitions for lenses:
The aperture is the size of the opening in a lens that lets light in.
The number defining aperture is the ratio of the focal length to the opening size.
So a 50mm F/1.0 lens could also be written 50mm 1:1 lens (what they used to write back in the day.)
Here are full stops. A full stop lets in twice as much light as the following stop.
F/1
1.4
2.0
2.8
4.0
5.6
8
11
So if you get a 2.0 lens like the 135mm 2.0, it is going to let in EIGHT times as much light as a lens that has a 5.6 aperture.
Aperture is one of the three factors that controls DEPTH OF FIELD.
APERTURE (more light=smaller number= shallower DOF)
Focal length (longer focal length=shallower depth of field)
Focus distance = how close you are to a subject.
So you could have a 1200mm F5.6 lens, but if your subject is a mile away, you can get him in focus, but everything else is going to be in focus.
That shallow depth of field is what separates your subject from the background and ultimately makes your pictures stand out and look good. For ariel shots, it isn't that important b/c you won't be close to much. You will be focusing at infinity for most things.
This was shot at 200mm F/2.8 from about 100 feet away.

See how everyone is pretty much in focus?
This was shot at 200mm F/2.8 from about 30 feet away. See how everything is blurry. If I was at F/5.6, the background would look more like the first pic with the annoying chick.

Being able to isolate your subjects like that is what separates a great photo from a regular snapshot. And that's why you buy good lenses.
If you are shooting arial stuff during the day, you may be perfectly suited to using an average cost super zoom like a 18-200. You will be focused at infinity, so you dont have to worry about isolating your subject. I think lenses like that are just like turning your camera into a point and shoot.
If you wanted to spend more money, a Sigma 50-500mm lens might be a better choice, but again, only in the daytime. You can get some incredible zoom with it, and when focused at infinity, you will have decent image quality. I'm not sure you would really be able to tell the difference between a lot of lenses when you are a mile away.
[This message has been edited by Guitarsoup (edited 5/27/2010 2:50p).]