Guide numbers and Manual flash

The Darkroom

So, some of the expensive flashes you don't have to calculate with the guide number but with mine you do and I have no idea how to use guide numbers or calculate them. I've heard someone say ''Your guide number times your fstop over 100ISO. What does that even mean? Any help here or pointers to links with a short tutorial would be great.

patrickc68

gn divided by flash to subject distance =aperture
gn divided by aperture = fsd
ie.a gn of 60 divided by f8 =7.5
or gn x 7.5 =f8
guide numbers are only guides so its there or there abouts,,
as always i stand to be corrected

5uspect

Changing ISO has a similar effect on image brightness to changing exposure or aperture. So you can measure it in f-stops. So going from ISO 100 to 200 is one f-stop, just like doubling the length of your exposure or going from f/4 to f/5.6.

So the guide number tells your flash how much light to use for a given distance, f-number and ISO as measured in f-stops. The flash shoots out light in a cone, so the intensity decreases as a function of how the Pi times the square of radius of that cone grows with distance; what is called an inverse square law.

So the equation is distance in feet times the f-stop. To correct for ISO you divide by the ISO in f-stops. Where ISO is 1, 200 is 1.4 etc.

kelly1

Afaik, guide number is just an indicator of the max flash power and is not used for calculating exposure (generally). Use a flash meter or ETTL.

DaireQuinlan

kelly1 wrote: »

Afaik, guide number is just an indicator of the max flash power and is not used for calculating exposure (generally). Use a flash meter or ETTL.

The guide number originally was the ONLY way to calculate exposure, it's what it was intended for. True it's often better to use some kind of TTL flash metering, or a flash meter, or the built in metering that a lot of flashes have, but you can always default to using the GN if you're stuck for anything else.

kelly1

DaireQuinlan wrote: »

The guide number originally was the ONLY way to calculate exposure, it's what it was intended for. True it's often better to use some kind of TTL flash metering, or a flash meter, or the built in metering that a lot of flashes have, but you can always default to using the GN if you're stuck for anything else.

It's just too easy nowdays

DaireQuinlan

kelly1 wrote: »

It's just too easy nowdays

YOU PEOPLE WITH YOUR ELECTRONIC DOOHICKEYS AND THYRISTORS AND WHAT NOT ! GET OFF MY LAWN !