[MF3D.FORUM:1491] flash formulas

["Bill Glickman" <bglick@xxxxxxxx>]

Dave

> 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.

       Then I guess you can do the same with Wien IR receivers on the flash,
very clever...  we learn something never every day!  Do you have any pix you
can share with us that utilized these flash set ups for caves?  Should be
very cool....

> 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.

       This is great, I need to add this to my cheat sheet!  Excellent, I
never saw this expressed this way, it seems to make sense...this is a faster
way to determine feasability without setting up a scene and testing with a
flash meter...very clever...


> 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.

           Is this all the tricky formulas you have, or is there more, if
so, can you send it along, I would love to look at it, and program them into
my TI89..

Thanks David
Bill G