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

Subject: Re: [OM] Better Flash recs & handle mounted flash Q
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Tue, 29 Feb 2000 21:51:53 +0000
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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