I have been thinking about getting an external flash unit and the more I have looked into it the more I realise that I need to learn. I started thinking that I could start with a cheap Chinese flash but have had cause to re-think.

Anyway, I found this very useful website that explains the differences between the range of flashes that accomodate the Pentax P-TTL flash system:

http://pttl.mattdm.org/