Excuse me for being dumb minded but...

[The Sailor]

Thank you Olly, I do understand the mechanics of it all it's just I don't bother with it all, I know what I need to take a photo, how to frame it and if I cannot get it into frame I change the lens without even thinking what f:1, 2 or 3, less to think about . I just worry that a lot of newbies will get bogged down in reading about focal lengths rather then getting on with looking at the sky, you have actually explained it quite well there, short and sweet without the "tech" , thank you .

Jim

[themos]

Get CCDCalc http://www.newastro.com/book_new/camera_app.php and try out different combinations of lens/scope/camera to get a feel for the effect on the image you get.