Question about Aperature and Focal Length

Okay--I'm a bit confused right now after some reading. If I have a scope that has a 80mm objective and a focal length of 400mm, and I stop it down to 40mm, the actual focal length of the scope remains the same, correct? It will be the F# number that changes?

I know that this is a simple question--I just need a reality check.

Thanks--

Don

Thank you very much! For some reason, I couldn't get my mind around this

Don

Well Don, the focal ratio is defined as focal length / objective diameter. Thus.

400mm/80mm = f/5
400mm/40mm = f/10

thereby, by stopping down the objective, you have increase the focal ratio.

Ron B[ee]