Re: [Leica] OT Flash guide numbers and they work? OT

["J. Gilbert Plantinga" <gilplant@hvc.rr.com>]

I don't know how to explain it simply, but I do know the formula:
Aperture = Guide Number / Distance

I also know that the most useful fact is that the guide number for iso 
100 is usually the model number of the flash, and the bigger that number 
is the more powerful, and more expensive (and for me more likely to give 
that blown-out look) the flash is.

I had to check the instructions on the SF 20, but an example of the 
formula in use is:

f2 = guide number 20 / 10 meters

so with 100 speed film the SF 20 has a range of 10 meters at f2, but 
only 5 meters at f4 etc. The really nice thing is that the SF 20 will do 
all the calculations for me and display the results (on the LCD, not 
unfortunately the picture! Available light still rules!).

On my bigger Metz 54 MZ3 the guide number is effectively different when 
the reflector is zoomed to cover diverent focal lengths. Makes my head 
spin. But this Metz is also pretty simple for a dufus like myself to 
operate, and I've even checked out the high speed sync with it on the M7 
(it works).

Gilbert

On Friday, July 12, 2002, at 06:23  PM, Ted Grant wrote:

> 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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