Home    Up Contact Legal Stuff  

          The Watkins exposure meter

Light metering, 19th century style
    This remarkable device would be a good fit for a site covering photography history; it sits in my history of computing collection because, in addition to measuring light, it calculates how to best capture it on film.
The Watkins Standard exposure meter
Click photo to enlarge
    The Watkins Standard exposure meter (“Standard” is the model name) was invented in 1890 by Alfred Watkins. It is what was called an actinometer: it measures the light intensity at the subject to be photographed by exposing a strip of light-sensitive paper to the incident light. The time required for the paper to darken to a predefined tint (shade of gray) would depend on the light intensity, and this time could then be used to calculate the required photographic exposure time.
    The predefined tint is present on a piece of paper located next to the light-sensitive strip, so the user can stop the timing once the two papers look identical.
    The photo below shows the components of the device. The top row has the brass body and caps, and below them are the blue glass window, the cardboard disc with the gray tint semicircle next to a semicircular opening below which the strip of sensitive paper is threaded, held up by the dark blue felt-lined plate.
The Watkins Standard exposure meter - disassembled components
Click photo to enlarge
    The device is 6.8 cm long and is made of brass. The hollow body is a thin-walled cylinder 3.1 cm in diameter, divided into two compartments. The cylinder serves three purposes:
  1. At one end is a removable cap with a round opening, which is where the light comes in through a blue glass plate. Removing the cap exposes a cavity storing the coiled strip of light sensitive paper; the end of the strip is held by a felt-lined plate against the glass window, and threaded out through a slot in the edge of the cap. This allows you to pull out a new stretch of paper before each use.
  2. The outside of the cylinder carries the rotating rings and scales of the calculator described below.
  3. At the other end there is a cap that, when removed, remains linked to the cylinder body by a chain stored inside this end of the cylinder. This chain, and the cap hanging at its end, form a one-second pendulum used to measure the “actinometer” time. Pretty neat!
The Watkins Standard exposure meter - top end  The Watkins Standard exposure meter - bottom end
Click a photo to enlarge
    Once you’ve measured the time, you’re ready to use the built-in calculator. This consists of a sequence of adjacent movable rings, each having a pointer that can be set to a number on the scale around the preceding ring. Basically what we have here is analogous to a slide rule with three annular slides, one cursor, and six scales. The number and order of these scales varied between models; mine has the following scales (left to right, when the glass window is at the right):
  • P = Plate sensitivity (the sensitivity of the emulsion on the glass plate used as a film). This value, for the plate types in common use the time, is given in a table in the instruction book. The greater the sensitivity, the higher the number.
  • S = Subject number. This is a function of the color or character of the subject being photographed: the more light-reflecting the subject, the lower this parameter. Values for common subjects (sky, portraits, dark wood, etc) are given in a table in the instructions.
  • D = Diaphragm aperture of the camera. This is given in a pair of adjacent scales, one for the f/stop notation still in use today, the other for corresponding numbers in the Uniform System (U.S.) adopted in the 1880s by the Photographic Society of Great Britain.
  • A = “Actinometer”, defined in the instructions as “the actinic force of light falling on the subject” – basically the light intensity. It is expressed as the number of seconds required for the light-sensitive paper to reach the standard tint.
  • E = Exposure – the time, in seconds or fractions of seconds, required to expose the plate for optimal results.

The values on scales P, S, and D are set, one by one, by rotating the pointers affixed to the next ring to their right. A double-sided cursor on a scale-less rotating ring is then set to point to a value on scale A to its left, and the result E is read under its other pointer on scale E to its right.
The Watkins Standard exposure meter calculating scales
Click photo to enlarge
    Watkins’ actinometer was well received; with electronic light metering still in the future, it was probably as good a solution as you could get. The instructions book quotes a review in “The Photographer”, which says: “The instrument, when first seen and handled, seems complex; but with a few instructions it is marvellous how easy the use of it becomes.”

    Alfred Watkins (27 January 1855 – 15 April 1935), the inventor of this ingenious light meter, was an English businessman, photographer, and amateur archaeologist. He invented the Standard meter shown here in 1890 and had it manufactured by R. Field & Co. in Birmingham until 1900, when the Watkins Meter Co. took over. Over the years a number of models were introduced, and the device was sold through the mid-1920s, although in 1902 it began to be replaced by the Watkins Bee meter, a more compact actinometer in the convenient form factor of a pocket watch.
 
Exhibit provenance:
    eBay.

More info:

Back Index Next

Home | HOC | Fractals | Miscellany | About | Contact

Copyright © 2024 N. Zeldes. All rights reserved.