Yes Lex,
That is how I remember having to do it "once upon a time". Now a sliding
scale is on the back of every flash I remember having looked at since I
don't remember when (maybe late '60s). Maybe this is intuitive to me
because I think visually and used to use a slide rule in school.
Lex Jenkins instructs:
> 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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