TAKO. INTERNET SEIT 1996.
Olympus-OM

Re: [OM] Formula for Converting Guide Number

Subject: Re: [OM] Formula for Converting Guide Number
From: "Ian A. Nichols" <I.A.Nichols@xxxxxxxxxxxxx>
Date: Wed, 13 Feb 2002 10:10:59 +0000 (GMT)

On Tue, 12 Feb 2002, Dave Dougherty wrote:

> 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.

Multiply by the square root of 2 for each stop increase in film speed.

e.g.  GN(400) = GN(100)xSQR(2)xSQR(2) = GN100 x 2

-- 
________________________________________________________________________
*             |                                                        |
|  /  | |/-\  |                      Ian A. Nichols                    |
| |   | |   | |             http://www.chm.bris.ac.uk/~cpian/          |
|  \-/| |  /  |                  i.a.nichols@xxxxxxxxxx                |
|             *                    iann@xxxxxxxxxxxxx                  |
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