[MF3D.FORUM:1475] Re: NSA MF slide show

["Bill Glickman" <bglick@xxxxxxxx>]

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)

        This formula is based on a normal fl lens on a given format.   The
fl lens assumes all of the light will be utilized in the area the lens can
see and not outside that area.  With a flash bulb this may not be the case
unless you have some type of barn doors to direct the path of light.    If
you double the fl, you only increase the flash distance 1.25 x, this is
because more of the light is channeled forward vs. going wider..but like I
said, be careful using this formula with flash bulbs.   You can see from the
formula, doubling the GN will double the distance, using two flashes is
equal to one flash with a gn of 2x the smaller flashes.

        My earlier post did not consider the variance for MF fl's...I am not
sure what fl lens the sputnick uses...but you have to adjust accordingly.
Considering all the variables in this situation, a much simpler approach
would be a flash meter, however you may blow through a few bulbs to get
things just right...that is the beauty of the electronic flashes, I fire
them 10x till each one is accurate, then I fire the camera...it is best to
use a spot meter at the point where the camera will be located, so it sees
the same thing the film will see.  hope this helps

Bill G



----- Original Message -----
From: "Tom Hubin" <thubin@xxxxxxxxx>
To: "Medium Format 3D Photography" <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Saturday, February 10, 2001 12:52 PM
Subject: [MF3D.FORUM:1472] Re: NSA MF slide show


> Bill Glickman wrote:
> >
> > David
> >
> >        A Guide Number 300 flash will do the following...
> >
> > 300  Guide number of flash (in ft)
> > 16    f stop lens setting
> > 100   ISO
> >
> >      With these variables, you can properly expose to these distances
with
> > these fl lenses....  If you need more light, then fire two flashes, just
> > don't advance film if possible...or fire two flashes at once if you can,
you
> > will double the distances below for two firings...
> >
> > 19 feet max. for 50mm fl
> > 23 feet max. for 100mm fl
>
> Hello Bill,
>
> I am not familiar with these flash lamps but usually light intensity
> falls off as the sqaure of the distance. So I would expect that 4
> flashes would double the distance and that 2 flashes would only multiply
> the distance by the square root of two or 1.4142 .
>
> Tom Hubin
> thubin@xxxxxxxxx
>