Watkins, Alfred

SUBJECT AREA: Photography, film and optics

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b. 1854 Hereford, England

d. 7 April 1935 Hereford, England

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English photographer who developed the first practical exposure-measuring system.

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His first patent was granted on 27 January 1890 and described a method of measuring the "actinic" value of light as a means of determining exposure. A strip of sensitized paper which darkened on exposure to light was used, and the time taken for it to darken to match a standard tint was measured. This time could be used to calculate the necessary exposure time, taking into account the speed of the plate, shutter speed and aperture. Watkins marketed a number of these actinometer designs, of which the most popular was the Watkins Bee Meter, which was in a pocket-watch form, introduced in 1903 and remaining on sale until 1939. Watkins was concerned that photographers recognize that exposure measurement had to take into account the effect of development time and temperature. In 1893 he devised the concept of the "Watkins Factor": he showed that when plates were developed by inspection, as was the practice at the time, a fixed relationship existed between the time of the first appearance of the image and the total time required to give a fully developed negative. The Watkins Factor was the figure that the first time must be multiplied by to give the second time. Watkins published tables of factors for different brands of plates and for different developers, and marketed various aids such as specially calibrated thermometers and clocks, as aids in using "Fac-torial Development" to give consistent negatives. After the early years of the twentieth century Watkins gave up direct participation in photography and devoted his time to a variety of interests, including the plotting of ley lines in England.

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