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[photo-3d] RE: GN

  • From: "Brown, Fritz - LABS" <brownf@xxxxxxxxxxxxxxxx>
  • Subject: [photo-3d] RE: GN
  • Date: Fri, 7 Apr 2000 09:48:42 -0600 
Bob writes:

;>GN 50 ASA 25 DIN 15 in feet
;>
;>Am I correct in thinking this indicates a GN of 100 for
;>ISO 100 speed film?

George replys:

;50ft for ASA 25 -> 100ft for ASA 50 -> 200ft for ASA 100.

I say:
Nope.  Sorry George, but light decreases at the square of the distance.  The
real sequence is at a factor of 1.414.  So Bob had it right:  a GN of 50 for
EI 25 is a GN of 100 for EI 100.  Doubling the guide number is actually four
times the light output.


-Fritz Brown

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