Thank you all for the advice. I shall print it all out and experiment tomorrow, at a helicopter operator somewhere in the North of England.
"Reciprocal of the focal length of the lens", blimey, I may have to sit down a while to work all that out. Just goes to show how much planning goes into those crisp pics.
The good thing about digital is that it saves the wasted cost of processing the duff pics.
Mmmmm....Duff
