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

Subject: Re: [OM] Better Flash recs & handle mounted flash Q
From: george <george@xxxxxxxxxxxxxxxxxx>
Date: Tue, 29 Feb 2000 20:38:22 -0800
John (and Wayne);

Yes, of course I should have said something like 'not enough light' will
fall on the 64 meter object. My mistake.  I can dig the math you
describe below, but something doesn't 'feel' right to me.  I think the
inverse square law applies to a 'point source' of light, which a single
flash approximates. But when one starts placing more flashes in the
picture, I'm just not sure the effect is additive.  To carry this a step
further, if 2 T32s will provide enuf light to properly expose an object
45 meters away, then it follows from the math that 4 T32s should have a
GN of appx 64.  When I think about the physicality of this, it says that
4 T32s will properly expose an object which is farther than *** 2
football fields *** from the camera.  I have a tough time believing
this. 

As I think about it, perhaps the relative size of the object being
illuminated has something to do with it?  Like, perhaps a child would be
properly exposed by 1 T32 at 32 meters, a basketball player by 2 at 45
and the Pillsbury Doughboy from the movie Ghosbusters at 64 meters?

Maybe I'll go find my college Physics text.

george



"John A. Lind" wrote:
> 
> At 23:40 2/29/00 , George wrote:
> >Barry;
> >
> >I don't know what the solution is (besides possibly a T45) but I'd like to
> >just caution about this statement:
> >
> >"Two T32's=GN64"
> >
> >This is not true. Guide Number is a measure of distance the flash is capable
> >of lighting up, assuming some fixed ASA/aperture/shutter speed. 2 T32s will
> >be able to throw more light on a subject at a distance of 32 meters, but
> >none of that light will reach a subject at 64 meters.
> 
> George is only partly right.  It's not that _none_ of the light will get
> there, it's not nearly enough light will get there.  Two T32's is not GN64
> (EI100, meters).  It is about GN45 (EI100, meters).  Zounds, that means it
> takes a pair of T-32's to equal a T-45.  It all follows the inverse-square
> law.  Twice the distance requires four times the light.  Think in square
> roots of two (approx. 1.1415926).  Two flash units of equal GN triggered
> together gives you approx. 1.4X the single unit GN.  Twice the film speed
> gives you 1.4X the GN.
> 
> Soooo, lesssseeee for 64 meter distance:
> 1.  A T-45 or a pair of T-32's (BG2 plus hot shoe) gets you to GN45
> 2.  Going from ISO100 to ISO200 film gets you to GN 64 (close, but _no_
> Zuikos open up to f/1   :-(   hint, you're short by at least an f-stop).
> 3.  Go from ISO200 to ISO400 and voila, you now have a GN90.
> 
> Use your 50/1.4 lens wide open and just barely eek out enough light for a
> proper exposure at 64 meters.  Just to be safe, make certain it's an MC
> with S/N > 1Meg; you wouldn't want to lose any of that precious light on
> the way through the glass.
> 
> Seriously, I'm not trying to make fun of you Barry.  You are pushing the
> edge of the envelope for flash though.  By bumping up to ISO800 you could
> use f/2 and at ISO1600 f/2.8.  Now you're in the range of one of the modest
> telephotos, say a 180mm (still expensive) or a 135mm.  Remember you _will_
> get some ambient light on a playing field, so you're not trying to photo
> the "black bear in a cave."  OTOH you don't have any reflectivity from
> walls or ceilings as you do indoors.
> 
> -- John
> 
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