Some time ago I was trying to find information about using my flash on full manual control; the goal being that I would have consistent output from the flash. However, during my research I found it very difficult to come across an easy explanation of how to manually use the flash.
Some would argue (and I would agree) that it would just be easier to meter the flash by remotely triggering it with a set of Pocket Wizards or eBay remote "poverty wizards". You simply adjust the output of the flash manually and meter it until you get what you want. While that is all well and good, I wanted to learn more about how the flash-to-subject-to-camera-settings worked in the whole scheme of things. That’s where guide numbers come into play.