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Converting Guide Numbers from 4" to 6" reflector t

 
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Sirius Glass



Joined: 06 Jun 2010
Posts: 122
Location: Southern California & Virginia

PostPosted: Sat Sep 03, 2011 5:28 pm    Post subject: Converting Guide Numbers from 4" to 6" reflector t Reply with quote

I have a 7" flash reflector and the Guide Numbers for the 5, 5B, 25, and 25B bulbs with a 4" to 6" reflector. How do I convert the Guide Numbers from a 4" to 6" reflector to a 7" reflector?

Should I take the area for a 4" to 6" reflector of 19.63 square inches and divide it by the area of a 7" reflector 38.48 and multiply by the Guide Number?

7" Guide Number = 5" Guide Number*(19.63/38.48 )

Or should I use the 4" to 6" Guide Numbers?

Steve
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Last edited by Sirius Glass on Sun Sep 04, 2011 1:35 am; edited 1 time in total
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1banjo



Joined: 16 Nov 2008
Posts: 475
Location: kansas

PostPosted: Sun Sep 04, 2011 1:01 am    Post subject: Reply with quote

hey you can take a look at http://graflex.org/flash/navy/
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Sirius Glass



Joined: 06 Jun 2010
Posts: 122
Location: Southern California & Virginia

PostPosted: Sun Sep 04, 2011 1:34 am    Post subject: Reply with quote

1banjo wrote:
hey you can take a look at http://graflex.org/flash/navy/


I read that before I posted. It does not give an indication of the reflector size for each bulb or even all the bulbs.

The manufacturers' published material for #5, #5B, #25, and #25B all are rated for 4" to 6" reflectors. Should I blow off the difference with the 7" reflector or compensate for it? If I should compensate, how?

Steve
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mopar_guy



Joined: 07 Aug 2008
Posts: 126
Location: Washington, the State

PostPosted: Sun Sep 04, 2011 5:20 am    Post subject: Reply with quote

The difference should be roughly one stop. That is you will get less light and a wider angle with the larger reflector.
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Sirius Glass



Joined: 06 Jun 2010
Posts: 122
Location: Southern California & Virginia

PostPosted: Sun Sep 04, 2011 12:42 pm    Post subject: Reply with quote

mopar_guy wrote:
The difference should be roughly one stop. That is you will get less light and a wider angle with the larger reflector.


That agrees with a posting on Large Format Photography Forum and the calculations in my first post. No one on any of the forums disagree. I think I see a pattern here ...
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