Flashgun Guide Number (GN) and zoom-head setting relation

Started Feb 18, 2013 | Discussions thread
RobertMartinu
Contributing MemberPosts: 647
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Re: Flashgun Guide Number (GN) and zoom-head setting relation
In reply to Cyclic, Feb 20, 2013

There are two factors:

The easy one is the illuminated area: half the indicated focal length, 4 times the area/quarter the illumination density and thus half the guide number.

The hard one is the actual construction of the head. The flashtube produces a fixed amount of light. The closer to the fresnel lens the more of it actually hits it and is sent into the scene. Once you want the light more focussed the tube has to move back. More light misses the lens and is absorbed by the heads casing. Thats where the exact geometry of the flash comes to play. A large lens with a short actual focal length is better in collecting light, and the design of the reflector behind the tube is a tradeof between maximizing output and preventing a hotspot/uneven illumination patterns in the wide settings.

The hot light people get some more information from manuals regarding the design&adjustment of fresnel fixtures; the Dedolight system brochure is also worth a look. Sadly the flash tube isn
t nearly a point source - makes the practical application cumbersome.

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