Practos Exposure Meter.
Practos
Category
Photographic technology
Date
1950
Materials
Plastic, Metal
Measurements
100 mm (Height); 35 mm (Diameter)
Place of origin
Germany
Collection
Fox Talbot Museum, Wiltshire
NT 1525622
Summary
Practos Exposure Meter. It is one of the last of its kind, and probably ceased production as recently as the late 1950s. It looks like a small telescope, covered in numbers, with an eyecup at one end. You put it to your eye, almost automatically; and unless you know about visual extinction meters, you wonder what on earth is going on. All you can see are a few numbers, letters, and symbols, badly out of focus. The eyepiece pulls out telescope fashion. Adjust it to suit your eyesight. The tube is marked in millimeters so you can set it next time by scale. What Those Numbers Mean The numbers are arranged around the perimeter of the circular field of view. The brightest is 4M. Next, growing steadily dimmer as they go anticlockwise, are 2M, 1M, 30, 16, 8, 2, 4, 1, 1/2, 1/5. Then there's a little drawing of a cloud, outside a ring, and a little drawing of the sun inside the ring. There are now two scales, outside and inside, with pairs of numbers: 10-25, 25-50, 50-100, 100-200, 200-400, and 400-800. You will only be able to see all of these if you point the tube directly at a very bright light. Take the meter away from your eye. On the left is a fixed scale with a selection of film speeds. These run from 16-40 European Scheiner (Scheiner is logarithmic), 6-30 DIN, 3-800 ASA. Now read off the exposure against the fixed aperture scale on the right, from f/1 to f/32. The choices in this case are 1/400 at f/11, 1/200 at f/16, 1/100 at f/23 (yes, 23, not 22), and 1/50 at f/32. https://www.shutterbug.com/content/classic-accessoriesbrpractos-exposure-meterbran-accessory-history-lesson
Makers and roles
Practos, manufacturer