Earlier today I played someone whose rating was around 690. I lost a game and my own rating went down by around 5 points.

This evening, I played someone whose rating was around 830. I lost again but my rating went down by 15 points.

How is this right? Surely if someone with a 690 rating beats me, my own rating should go down by a lot more than if I lose to an 830 rating?

My rating at the time of both games was around 910, btw.

Was the 690 player you lost to a newbie? you will lose less points against these than full members....

spot on rage, pad the 690 you lost to was a newbie which therefore means your score whether you win or lose is reduced by a particular amount depending on how many games the newbie has played.
The amount you lost when losing to an 830 rating was the normal amount for a virtuoso to lose.

http://www.funkysnooker.com/help.do?section=rankings

that explains how the rankings work.

Ah, OK.

Ta.