[MF3D.FORUM:1490] Re: NSA MF slide show

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

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 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. Only twice as far but five times the 
flash needed.

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.

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