Masking a Refractor and Focal Length

Hi! A question which I am confused over: I was told that by masking the front of a 5" refractor, making it say 4", I would increase the focal LENGTH of the scope and have greater magnification possible. Is that correct? Or is it just changing the focal RATIO? Sorry if this is a "dumb" question!
Ed Abel
Silver Spring, MD

It is NOT a "dumb" question, Ed. Let's do some analysis. Masking down from 5" to 4" still yield the same tube length and thus the focal length stays the same. However, the focal ratio has now consequently increased! In an achromat, all of the sudden the chromatic aberrations is also less.

Ron B[ee]