[ATM] Refractor performance

[Tony.Lock at EC.GC.CA (Lock,Tony [Dartmouth])]

I have been reading Richard Snashall's recent refractor posts with
slightly interest than understanding and they have led me to a question
I cannot answer myself.  I hope this not a totally trivial question.

Assume I used some optimising program to design two telescopes using
identical glass:  

1. D=100mm; fl2000mm; f=20

And

2. D=150mm; fl= 2000mm; f=13.3

If I stopped down the second objective to 100mm making it effectively
f20, how would its performance compare with the first objective? In
other words, does the stopped down performance of an optimised short
focus refractor equal the performance of an optimised longer focus
refractor?

Tony Lock