Huge can of worms you're opening here. Ask three people this question and you'll get six different answers. See these links for interesting discussions:
http://www.stanmooreastro.com/SamplingFratioMyth.htm
http://www.stark-labs.com/help/blog/...ioAperture.php
The second one is the easiest to understand.
Also here is a long thread on Cloudy Nights on this topic
http://www.cloudynights.com/ubbthrea...2/Main/3085977
If you google "focal ratio myth" you'll get a lot of hits.
Geoff