# Tagged Questions

For questions about the metric describing flash power.

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 ...
2k views

I've been reading articles that talk about using multiple strobes next to each other, all pointing in the same direction: as a means to add more light over what one flash can do. Does the addition of ...
434 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 ...
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 ...
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.
820 views

### 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 ...