Hrmm...well, i'll try my best to explain how logarithms work. They are like the inverses of exponents. Let's say you have 2^x=8. To find x, you would do log base 2 of 8 = x. (I can't express it in decent mathematical form here really). For all purposes though, since most times when you use log, it's assumed that the base is 10, you can use the change base formula, which states that:

log base 2 of 8 = log base 10 of 8 / log base 10 of 2

I don't have a proof handy for this, and even if I did, it'd be ugly in text. You don't have to express base of 10 however in most cases, so, saying log 8 / log 2 is sufficient.

Note: This is a terrible overview of logarithms, it's just what came to my mind and what i remember about them. Go to wolfram's to get a better idea of it. http://mathworld.wolfram.com/Logarithm.html

Oh yeah, after looking there myself, i found there's a notation for base 2 defined as lg. Hrmm..interesting.