Archived posting to the Leica Users Group, 2002/07/12

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Subject: Re: [Leica] OT Flash guide numbers and they work? OT
From: "Johan Doumont" <johan.doumont@pandora.be>
Date: Sat, 13 Jul 2002 00:53:49 +0200
References: <3.0.2.32.20020712104411.013a5488@roanoke.infi.net> <3.0.2.32.20020712180529.01588e14@roanoke.infi.net> <001b01c229f2$b6d60920$633f4d18@gv.shawcable.net>

Ted,

The guide number is usually given for a film speed of 100 ASA and is nothing
more than the the f stop, multiplied by the distance (in meters !) by which
the light of the flash will be able to light the subject.

My Metz CT60 has a guide number of 60 (surprise !) and can get as far as 30
meters with f2, or 15 with f4, and so on.
I suppose your friend's flashlight will list the guide number in feet, but
the principle should still be the same.

Using a manual flashlight goes like this :

- - focus on the subject
- - check the distance scale on the lens
- - divide the guide number by the distance, this should give the f stop
needed for a correct exposure.

I remember Nikon having a 45 mm GN (guide number) lens, with an extra ring
to set the guide number of the flashlight you used. The aperture was coupled
with the focussing ring, so that it would be adjusted automatically.

Johan Doumont

> Hi folks,
>
> This is right off the wall ancient need to learn it over again.
>
> A chap down the street called and asked me how to use the guide number on
> the flash he has?
>
> Well OK I thought I knew.... however that was 45 years ago when the flash
of
> the day had guide numbers. Even then I had trouble figuring that stuff
out,
> then I discovered photo life was much easier when you didn't use flash and
> go available light. ;-)
>
> Besides any flash I used over the years has always been something
automatic
> no brainer thing.  So please help me with a very simple way to explain
this
> guide number and how to make it work with whatever flash and camera is he
> has.
>
> thanks guys and gals.
> ted
>
>
>
>
>
>
> --
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Replies: Reply from Ted Grant <tedgrant@shaw.ca> (Re: [Leica] OT Flash guide numbers and they work? OT)
In reply to: Message from Marc James Small <msmall@infi.net> ([Leica] Cook & Perkins Adapter)
Message from Marc James Small <msmall@infi.net> (Re: [Leica] Cook & Perkins Adapter)
Message from Ted Grant <tedgrant@shaw.ca> ([Leica] OT Flash guide numbers and they work? OT)