How do I work a "guide number" with my flash and 1970's SLR or Rangefinder camera?

Madame W · Dec 13, 2009

I don't understand what a "guide number" is. Consequently, I don't know how to work it.

Perki88 · Dec 13, 2009

http://en.wikipedia.org/wiki/Guide_number
http://www.vividlight.com/Articles/1214.htm