# Tagged Questions

For questions about the metric describing flash power.

114 views

### Flash guide number f/stop calculation

I have been reading the B&H Speedlight Buyer's Guide. I am confused specifically about the guide number calculation. I understand the principle and how the inverse square law is used. Here is the ...
433 views

### Do flash guide numbers assume some amount of ambient light built-in?

My flash shows its potential range in meters, but is that indoors or outdoors? The question came up the other day when I was trying to work out whether fitting a diffusion dome would be suitable for ...
2k views

### 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 ...
236 views

### 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 "...
5k views

### What is the quantative relation between flash guide number and ISO?

I found that formula on the Internet: Guide Number = (Shooting Distance * Aperture) ÷ ISO Sensitivity Is it correct? If it is could someone please explain why ISO is related to guide number in this ...
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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 ...
187 views

### 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.