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: Mark Rabiner <mark@markrabiner.com>
Date: Fri, 12 Jul 2002 20:33:41 -0700
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> <3D2F8508.FF320E94@domain.com.br>

><Snip> 
> the hot shoe (nearest place to the focal plane, where distance markings
> on the lens are measured from)and not from the front of the reflector.
> This difference is trivial at normal distances. This I found by trial
> and painful error...
> 
> Cheers,
> Amilcar
> 

The inverse square law i believe is the main inspiration behind the GN chart.
I've read charts from Kodak and they talk about a semi polished
parabolic reflector as the norm.
I'm not sure where that fits in.
I have a nice Gossen flash meter.
I take readings every foot and then work up my own chart. Which i stick
to the back of the reflector of my Norman 200 flash which is pure manual
and gives me F22 at 3000 feet with tech pan film at high noon.
The inverse square law only holds up so far when compared against reality.
By reality i mean the walls or lack of and the exact construction of the
flash head which will have a hot spot so many feet from it as it's
focused. Focased is the key word here.
I've got a piece of glass wool over mine to make it more inverse square
like; less focused; and otherwise more pleasant. 
I could check it outside too and i get all kinds of different readings
but on the long run i use the same F stops inside as outside. And it's
not what any chart would give me.
Although i do still believe in the inverse square law. And the circle of confusion.



Mark Rabiner
Portland, Oregon USA
http://www.markrabiner.com
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Replies: Reply from Amilcar de Oliveira <amilcar@domain.com.br> (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)
Message from Amilcar de Oliveira <amilcar@domain.com.br> (Re: [Leica] OT Flash guide numbers and they work? OT)