I'm trying to learn a bit more about using my flash (Canon 430EX), and I've been reading Understanding Flash Photography, by Bryan Peterson. In the book, he often says something like "I just entered my aperture and the flash told me the appropriate flash-to-subject distance ... so I set up my light stand at that distance and was good to go ..." or "I set up my flash and entered the distance to my subject ... the flash told me f/5.6 for a proper flash exposure ... ."
I (think I) understand that the distance for a proper flash exposure is a function of aperture, flash power, and ISO (and any ambient light I guess). My question is, can I manually set values for aperture, ISO, and flash power on the flash itself when it's off camera as a slave? I can only seem to set the power. And, based on that, my flash doesn't tell me the flash-to-subject range. It sounds like Peterson is able set all the variables right on his flash with his Nikon SB-900. Do higher end flashes (like the 580EX II maybe) allow for this -- assuming it's a model more comparable to Peterson's? Or am I just misinterpreting what he's saying?
If your flashgun is like MOST modern guns, then proceed as follows....
* Put the flash in Manual mode, and the LCD readout should provide a whole scale showing which apertures are recommended for the respective distances from the flash to the subject.
* From the scale select a convenient flash to subject distance and position the flash accordingly. The aperture indicated is then transferred by you to the camera, which should also be in Manual exposure mode.
* Select a suitable shutter speed... (usually camera 'X' sync speed, whatever that is... typically 1/200th sec.)
* The scale on flash LCD should change automatically when you select select lower than full power output... that's if your flash allows reduced power settings... most do, but some don't.
* It should similarly change to accommodate different ISOs as they are selected.
As stated, this advice assumes your flash works in similar manner to the way most flashguns work these days.
"Ahh... But the thing is, these guys were no ORDINARY time travellers!"