Just as a counter point, I've never thought of nerd/geek as terms describing prowess in a field. I see 'geek' as a term to describe someone who is passionate about something that it isn't cool to be passionate about, or in a way that isn't concerned with being cool. Nerd would be basically the same, except it's taken on a stronger negative connotation, basically because it's used to imply lack of social skills, and perhaps more obsessive behaviour.