[MF3D.FORUM:1477] Re: NSA MF slide show

[Tom Hubin <thubin@xxxxxxxxx>]

Bill Glickman wrote:
> 
> Tom
> 
>      Yes, you are right...but that is already taken into account...here is
> formula for determining flash distance one can shoot at ...
> 
> GN (ft) / f stop * sqrt (ISO/100)
> 

Hello Bill,

I was addressing the case of multple flashes to compensate for a more
distant subject. That could be multiple lamps fired simultaneously or a
single lamp fired many times. 

At a given distance the light hitting the subject doubles when the light
source doubles. But if you double the distance to the subject then the
light hitting it is quartered. So the light source will need to be
quadrupled in order to get the same exposure. 

For the focal length I am just using your data and more-or-less fitting
it into the equation. It appears that the distance is proportionate to
the fourth root (square root of the square root) of the focal length.

To include N (the number of flashes) and fl (focal length), I would be
inclined to write this as:

MaxDist = GN (ft) / fStop * sqrt(ISO/100) * sqrt(N) *
sqrt(sqrt((fl/50mm)))

I like to do the messy stuff first and the division last so I would
arrange it this way: 

MaxDist = sqrt(sqrt(fl/50mm)) * sqrt(N*ISO/100) * GN / fStop

All of this assumes that the flash is located near the camera. That is,
the distance from flash to subject is the same as the distance from
camera to subject. I expect that this is often NOT true when slave
flashes are used. Then the math is certainly more complicated.

Tom Hubin
thubin@xxxxxxxxx