>My intitial misunderstanding was the way Nikon expressed the >guide number rating of the pop up flash.
It's not just Nikon - everyone (that I know of) uses the same convention.
>However, another question comes up from the posts... > >With two stops of difference in the exposure is not a flash >with a quide number of 160, 4 times as powerful as a flash >with a guide number of 80 ?
That is correct. What you read (about GN 160 being 2x the power of GN 80) was wrong. Perhaps the confusion arises because doubling the GN doubles the effective distance, but increases the power by 4x. In my view, GN's are a means to an end - they allow you to work out how far away your subject can be for a given aperture and ISO.