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walkaboutcamera
 Post subject: GNs for flash
PostPosted: Thu Mar 25, 2010 1:33 pm 
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The GN for a flash changes with ISO speed rating of film or sensor. Often they give a GN for a flash without the ISO the number is for. Can an ISO of 100 be safely assumed when the ISO is not given? It seems to me that when they do give an ISO to go with the GN it is usually 100.
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Seren Dipity
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PostPosted: Thu Mar 25, 2010 1:40 pm 
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Yes, I believe you are correct.
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fizbot
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PostPosted: Thu Mar 25, 2010 2:22 pm 
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Yes, Guide numbers are typically referenced at ISO100 unless otherwise specified.

http://en.wikipedia.org/wiki/Guide_number
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walkaboutcamera
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PostPosted: Thu Mar 25, 2010 4:24 pm 
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Thank you
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mikefellh
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PostPosted: Thu Mar 25, 2010 4:38 pm 
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Don't forget that when comparing GN's that you not only have to have the ISO the same, but also the focal length of the flash.

When a flash head has multiple zoom positions manufacturers usually quote the best scenario to give the highest possible GN.
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dant
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PostPosted: Thu Mar 25, 2010 4:39 pm 
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Remember too that guide numbers are just a guide. There are no standards of testing with this. So numbers are not always indicative of actual power.
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walkaboutcamera
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PostPosted: Thu Mar 25, 2010 7:08 pm 
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It is a ring light that I am looking at so no zooming of the flash but still a good reminder to put in the thread. In my case if it only puts out half the power claimed it will still meet my needs, and given that it is a cheap flash that may very well be the case. I have bought online from B&H an having delivered to a place in NY state where I will be staying the week after Easter. Buying a Bower SDF-14C, keeping my fingers crossed.
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