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Re: [OM] Better Flash recs & handle mounted flash Q

Subject: Re: [OM] Better Flash recs & handle mounted flash Q
From: Joey Richards <bigjoe@xxxxxxx>
Date: Tue, 29 Feb 2000 22:56:49 -0500
Garth Wood <garth@xxxxxxxxxxxxxxx> wrote:
> >If you add a second t32, you will get twice the flash power at 64
> >meters, so you'd only need to pick up 1 stop through aperture choice
> >or film speed.  Four t32s would get you 64 meters at iso100 and f/1.
> 
> I believe you'd get approx. 1.4 times the flash power (in 
> terms of illumination) due to inverse square law.  Not that
> my addled middle-aged brain remembers all that well...
>

No, at any fixed distance, 2x the power at the source gives
2x the power at that distance.

  Po = k * Pf / r^2

where Po is power incident on the object, Pf is total power of
the flash (assumed uniform over some solid angle), and r is
distance between them.  k is a constant that's proportional
to the solid angle of the object from the flash's point of
view.

So if Pf' = 2*Pf, Po' = 2*Po.

If you solve for radius instead,

  r = sqrt (k*Pf/Po)

then double Pf, r only increases by sqrt(2)

joey

(who is currently physics-starved because it's been more than
a year since his last physics course... damn eecs degrees)

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