Help could some one explain flash and how it works underground.
I am especial interested in guide numbers and how that relates to the aperture and lens on my 350d.
I have a 19 -35 and a 50 mm lens.
I have been thinking about multiple smallish off camera flashes and how this compares to light painting/ action shots of people whist capturing the feel of a mine.

Ok, this isn't too difficult to get the hang of. All flashguns have a calculator on them somewhere, a guide for determining the aperture against the distance from the subject to give a well exposed image.
Guide number = Distance x Aperture.
The guide number of a flash can be worked out simply;
First set the calculator to 100 ASA (all flash guide numbers are calculated using 100ASA film speed)
see picture below
🔗Personal-Album-342-Image-38721[linkphoto]Personal-Album-342-Image-38721[/linkphoto][/link]
You can see at a glance, the 2 metre mark is opposite the f22 mark. 2x22=44 the guide number of the flash. you can transfer this up and down the scale to arrive at roughly the same figure. This particular gun has an advertised GN of 45 at f8.

Knowing the guide number you can then calculate the right aperture for the distance using; Aperture = Guide Number (GN) divided by Distance.

It's important to remember that the DISTANCE is the FLASH to SUBJECT distance and NOT the camera to subject distance.
So, if you have a gun with a GN of say 30 and your subject is 5 metres away, dividing 30 by 5 will give you 6 there is no f6 on your lens so set it to the closest stop, f5.6 or f8.
That is the basic calculation behind it but does not allow for reflective surfaces or highly light absobent surfaces.
Perran Iron Mine down here in Cornwall sucks light out of caplamps like a black hole and literelly halves a guide number at 20 feet. At 50 feet it's hopeless.

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Thanks that all makes sense.