I do not think that light meters and lens f/ stop detents are in thirds by accident as your question suggests. There is a mathematical reason for these 1/3 divisions. Specifically, a 1/3 division is handier than any other because many photographic values are logarithmic. For example, the H&D curve is in logarithmic values; the x axis, (exposure) is the log of exposure (log E) and the y axis, (density) is the log of opacity. A one stop change is a doubling or halving of exposure and represents a relationship of 1:2 (or 2:1). The log of 2 is 0.30. A one third stop is 0.10, a nice even value. Any other change (e.g. a half stop) works out to an odd log value. Thus one third stop detents are widely used because they are very handy to work with. -----Original Message----- From: owner-photoforum@listserver.isc.rit.edu [mailto:owner-photoforum@listserver.isc.rit.edu] On Behalf Of Gregory Fraser Sent: 03 October, 2003 11:13 AM To: List for Photo/Imaging Educators - Professionals - Students Subject: Lighting Ratio Why is the illumination difference in a 3:1 lighting ration 1 1/3 stops and not 1 1/2 stops? Is it because the f-stops on cameras and meters tend to be divided into thirds or is there a mathematical reason? Greg
