# Tagged Questions

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### What flash guide number do I need to underexpose a “sunny” background?

I'm familiar with the maths behind flash usage (guide numbers, etc.). Here's a more practical question that doesn't yet seem to have an answer on photo.se: Consider a subject, outdoors, on a regular ...
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### Guide number and exposure [duplicate]

I am having a bit of a difficulty calculating this. Guide#= 32 feet at ISO 100 X sync speed= 1/250 Min ISO= 100 Max ISO= 800 What exposure values to expose to capture subject that's far away? What ...
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### How can I calculate the effect of non-TTL flash on exposure?

I recently got an old Canon AE-1 film camera back in working order. I'd like to use my modern 430EXII Speedlight flash with it, however since there is no TTL metering, I have no idea how much the ...
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### Old Flash from the 70's

I have an old Consul 400 flash. It was distributed by Braun North America. I would like to figure what is the GN number at ISO 100, f/4, 10 feet.
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### What is the relationship between Guide Number and flash power level?

From this post, I understand that the flash guide number (GN) is given by the following formula: Guide Number = Shooting Distance × f-number ÷ ISO factor I'm assuming that the GN calculated via the ...