bf calculation

[Jan Verweij - Reeleez]

Hi,
I'm trying to understand the exact calculation that takes place when using
edismax and the bf parameter.
When searching I get a product returned with a score of 0.625
Now, I have a field called productranking with a value of 0.5 for this specific
product. If I add &bf=field(productranking) to the request the score becomes 
0.7954515
How is this calculated?
Cheers,
Jan Verweij