Hiya, Gregg,
The easiest way to remember guide numbers (for me, anyway) is to use ASA 100
and 10 feet as a reference point. Some flash units, like the Olympus PS200,
base their guide number scale on this. So if the GN is 45, at 10 feet from
the subject the correct aperture would be f/4.5 with ASA 100 film. (Sorry,
all, I still think in ASA, not ISO, but I'm trying to convert.)
Going up and down the scale, at 5 feet use f/9 and at 20 feet use f/2.25
(which isn't really practical with all-manual cameras so you either fudge
the distance a bit, or just don't worry about it 'cause modern print films
have lots of leeway).
Let's turn it around and say you're trying to calculate the unknown guide
number of a flash that has a scale for determining appropriate f/stops based
on ASA and distance.
Set the ASA marker to 100. Look at the 10 foot mark. The f/stop
corresponding to the 10 foot mark, multiplied by 10, tells you the
approximate GN. In the case of my Canon AB-46 flash, the 10 foot mark is
between f/8 and f/11. So the GN is approx. 95.
Help a bit?
Lex
(Who has had to relearn this stuff every few years.)
----Original Message Follows----
From: "Gregg" <giverson@xxxxxxxxxxxxx>
So, how does that guide number thingie work again; something divided by
something gives your something?
Gregg
Who only used a guide number except to compare flash output before 1975!
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