As far as I know, it has only 3 bases.Base 10, base e(natural log) and base 2. Look for the "log" key. Just underneath it there is a small arrow. Click on that arrow and you will see the other two, ln, base 2.
+9466 It actually is possible.
http://web2.0calc.com/questions/how-do-you-enter-a-custom-base-for-a-logarithm
