----- Original Message ----- From: "Peter Fairweather" Subject: Re: Stupid Flash question
> Wendy > > Another thing which I have read is that the guide numbers are based on > the flash being used in an averagely reflective room. Outdoors the > guide number is effectively reduced by one stop. So the 36 0f the 360 > Pentax flash becomes 25. > > The other dodge to watch out for is which focal length the guide > number is quoted at. Not many quote for a 35mm lens as once they did. > More likely 100. I suppose one ought to allow for the APSC sensor > factor as well. > > Guide number is independant of focal length of lens or size of "film". However, a flash with a zooming head will change GN as it zooms, since the fresnel is focusing the light to a smaller or larger beam pattern. I have heard about the GN being based on an "average reflectance" scene (the average room, if such a thing exists). Some say it is myth, others say it is true. A flash is definitely "more powerful" in a smaller room than a very large one or outdoors though because of all the light bouncing around. William Robb -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
