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12/06/2005 02:55:23 PM · #1 |
I'm thinking about buying a Canon 580ex flash. And in the product description, it says:
High power flash output with maximum guide number of 58 at 105mm setting (ISO 100, metres)
What does "maximum guide number" mean?
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12/06/2005 03:02:32 PM · #2 |
Guide number can be in feet or metres (58 is metres is this case). that is a measure of distance with i think 100mm lens at ISO 100 (at some F/stop) that you can get a properly exposed image.
Basciall, the higher the guide number, the more light output. The Canon 430ex has a GN of 43 metres for example, so it is a less powerful flash.
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12/06/2005 03:08:31 PM · #3 |
Really simple. F-stop times distance to subject equals guide number. So with a guide number of 58 and a subject 10 metres away your aperature setting would be 5.8 (more likely 5.6, of course). This at ISO 100.
This is how we set flash exposure back in the old, pre-automated days. :-)
It's a "maximum guide number" because the flash will automatically throttle down if the aperature is too big or distance too short.
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