Re: [photo-3d] GN

[Oliver Dean <3d-image@xxxxxxxxxxxxxx>]

Hi,

I think you guys are forgetting the inverse square law of illumination
-- the GN is not multiplied by the same factor as the ASA for faster
films.  The easiest way to figure the new guide number for the faster
film is this:  For ASA 25, GN 50 for 8.92 feet gives f/5.6.  If you use
ASA 100 film at the same 8.92 foot distance, you have film that is 4
times faster than ASA 25 and you would have to stop down 2 stops to
F/11. Now you can recalculate your guide number for ASA 100 as 8.92 feet
x 11, which comes out at about 98, not 160.

Hope this helps!

Cordially,
Oliver Dean



Bob Wier wrote:
> 
> >>GN 50 ASA 25 DIN 15 in feet
> >>
> >>Am I correct in thinking this indicates a GN of 100 for
> >>ISO 100 speed film?
> >
> >50ft for ASA 25 -> 100ft for ASA 50 -> 200ft for ASA 100.
> >
> >WOW!  One powerful baby!  Where did you get that?  I have
> >a Sunpak 611 - it's here: :-)
> >http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=298934511
> >listed at 160ft for ASA 100.  I thought that was strong.
> >Yours is stronger!
> >
> 
> Nope - I must have goofed the calculation - I suspect (being
> a 283 knock off) that it's weaker than the Vivitar (GN 120 at
> ISO 100).
> 
> --BW
> 
>               Bob Wier
>      mailto:wier@xxxxxxxxxxxxxxxxx
>    10:21 PM Thursday, April 6, 2000
>         Unix/Internet Administrator
>    Rocky Mountain College, Billings MT.
> 
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