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WHAT AM I MISSING ? GUIDE # FOR D200 FLASH

RABaker

Sunnyvale, US
731 posts

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"RE: WHAT AM I MISSING ? GUIDE # FOR D200 FLASH"

RABaker Registered since 01st Oct 2003
Fri 10-Aug-12 07:53 PM

I'm sorry to disagree with some others here, but flash guide numbers do not have units - it is a just a number. This point was drummed into us in my first formal photography class. The guide number uses distance to determine an aperture as a starting point for exposure. Because the guide number uses distance to compute aperture, it is necessary to have a different guide number for a different unit of measurement. Most of the time a flash will list two guide numbers - one for distance measured in meters and one for distance measured in feet. Unfortunately, many manuals/instructions list the numbers something like "12 meters/30 feet" and so it has become a common misunderstanding that the guide number itself is expressed in units.

So... When the manual shows "Guide Number 12/39 flash (ISO 100, m/ft)" they are saying:
1. If you measure the distance in meters the guide number is 12; or
2. If you measure the distance in feet the guide number is 39.

I hope this helps.

Richard

A general, generic topic WHAT AM I MISSING ? GUIDE # FOR D200 FLASH [View all] , largebore Silver Member , Fri 10-Aug-12 12:53 PM
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