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Re: [OM] Formula for Converting Guide Number

Subject: Re: [OM] Formula for Converting Guide Number
From: Joel Wilcox <jowilcox@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 13 Feb 2002 07:58:18 -0600

At 22:32 2/12/02, Dave Dougherty, who sold his "Shipman" book, now asks the question:
Fellow Zuiks,

Guide numbers are usually expressed as a function of 100asa.  As light is, I
think, subject to the inverse square rule, does anyone remember the formula
to convert a GN from 100asa to say 400asa, etc.  I had it in a Shipman book
which I sold, having failed to write down the formula before I did so.

John Lind:
To convert a GN for one film speed to the equivalent GN for another film speed:

  GN2 = GN1 * SQRT(ISO2/ISO1)
    GN1 = known guide number rating at "ISO1" film speed
    ISO1 = film speed for which guide number is known (GN1)
    GN2 = desired guide number for "ISO2" film speed
    ISO2 = film speed for which guide number is desired
    SQRT = Square Root
<snipped application of above>

Joel's Poet's Method for Determining GN:

1. Take flash in hand.  Turn it over.
2. Look at panel with numbers on back.
3. Find a stop that doesn't have a decimal in it. F8 and f11 are a couple of my personal favorites. 4. Multiply your favorite stop number times the number of feet/meters indicated for that stop on the panel. Repeat process for any ASA you require. (Previous experience with a slide rule helpful but not necessary and should not be admitted under any circumstances.) 5. Write these numbers down. It's the GN (which stands for "Good for Nothing"). Take aspirin. Throw numbers away.
6. Switch flash to "TTL" and mount on TTL enabled camera.
7.  Set exposure comp to -2/3.  Shoot.

Alternative to Step 6:
6. On cameras requiring manual flash, remove flash and shoe. Load ASA3200 film. Shoot. Process (push two stops).

;-) ;-) ;-)

Joel W.


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