Improvements in Apparatus for Calculating Photographic Exposures (British Patent No. 10,617, 1893)

Improvements in Apparatus for Calculating Photographic Exposures (British Patent No. 10,617, 1893)

Author: George Frederick Wynne

Year: 1893

British Patent No. 10,617 (1893), granted to George Frederick Wynne of Minera, near Wrexham, describes improvements to photographic exposure meters of the actinometer type. The invention was designed to simplify the calculation of photographic exposures by combining several exposure factors into a single mechanical computing device. Wynne observed that photographic exposure depends upon four principal factors: the actinic value of the light, the nature of the subject, the aperture of the lens, and the sensitivity of the photographic plate. Earlier exposure calculators treated these factors separately, requiring multiple scales, pointers, or slide-rule arrangements. Wynne's invention reduced this complexity by employing only two geometrically graduated scales: a diaphragm (aperture) scale and a time scale. Through a novel arrangement, the diaphragm scale simultaneously represented aperture, plate speed, and subject correction, while the time scale represented both actinometer time and photographic exposure. The patent describes two forms of the instrument: a circular watch-case style exposure meter incorporating an actinometer and rotating scales, and a flat slide-rule version with