[MF3D.FORUM:1493] Flash and GN

["David W. Kesner" <drdave@xxxxxxxxxx>]

This didn't seem to make it the first time so here it is again 
- sorry if it posts twice:

Bill Glickman writes,

> Wien and Quantum also make the IR system here in
> USA...they
> require additional purchase of the transmitter,

The FireFly that I mentioned is not the transmitter/receiver 
infrared unit like you describe. It simply is triggered by the 
IR spectrum of any light. This means that you can put exposed 
film over a flash so no visible light gets through and it 
still triggers the flash. Cavers who shoot flat images like 
this because they like to use off axis lighting to emphasize 
depths.

> I never heard of the ones you are using,
> but will look out for them....

You won't find them anywhere. The sole US distributor is a 
caver named Peter Jones. Contact him at SITDCP, 80 Mountain 
Street, Camden, Maine 04843 (207)236-6112 <pjcaver@xxxxxxxx>.

> I will have to research this again.....you could be
> right...what formula do you suggest?

The simple formula to find out how many flashes you need is 
desired GN squared divided by actual GN squared. Say you 
wanted to use f16 to shoot a subject 10 feet away. Well fstop 
times distance equals GN so 16*10=160. If your flash has a GN 
of 100 then 160^2/100^2=2.6 so you would need to flash two and 
a half times. If the subject moved out to 20 feet then the 
required GN would be 320 and the number of flashes would be 
10.2.

I made a simple spreadsheet that I can carry with me in the 
cave so I don't have to do the math. A nice PDA or programable 
calculator would work as well, just not in a harsh cave 
environment *{;-)

David W. Kesner
Boise, Idaho, USA
drdave@xxxxxxxxxx