An Instrument for Calculating Photographic Exposures (British Patent No. 1388, 1890)

An Instrument for Calculating Photographic Exposures (British Patent No. 1388, 1890)

Author: Alfred Watkins

Year: 1890

British Patent No. 1388 (1890), granted to Alfred Watkins of Hereford, describes an instrument for calculating photographic exposures by combining an actinometer, a mechanical exposure calculator, and a timing device into a single compact instrument. The invention was designed to simplify the determination of correct photographic exposures at a time when exposure calculations depended upon multiple interacting variables. Watkins identified four principal factors affecting exposure: the actinic intensity of the light, the sensitivity of the photographic plate, the character of the subject, and the aperture of the lens. To determine the actinic value of the light, the instrument employed an actinometer using bromide paper sensitized with potassium nitrite, which darkened to a standard tint when exposed to light. The measured actinometer time was then combined mechanically with the remaining exposure factors. The patent describes a cylindrical instrument containing six concentric rings, four of which carried logarithmically divided scales representing light intensity, plate sensitivity, subject character, and lens aperture. These movable scales were linked together by stops and pointe