Collecting cameras is all very well, but we can often learn more about the history of photography--and about the difficulties under which our photographic forebears labored--by looking at accessories. The Practos exposure meter is a prime example. 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.
Let's say it's sunny and you can see 100-200. The 100 is on the outer (cloudy) scale and the 200 on the inner (sunny) scale, so use 200.
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. Let's choose 400 ASA. Set the 200 from the meter alongside 400, using the rotating scale in the middle of the tube. 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.
Know Your Limitations
Does it work? Moderately well, once you are aware of its limitations, which are fivefold.
First, it is strongly influenced by where you point it: it behaves more like a spot meter than a broad-area meter. This problem is compounded by an instinctive tendency to wave it about a bit until you can see more clearly, e.g., when it is pointing at the brightest part of the subject.
Second, there are quite wide individual variations in eye sensitivity. You can compensate for this by resetting the film speed, just as you would with any other meter.
Third, there is dark adaptation. If you have just come in to a dark building from a sunlit exterior, your eye will need time to adjust to its surroundings. Also, the longer you look through the meter, the more the eye adapts to the darkness, and the more numbers you see. Never look through it for more than a few seconds, especially in bright light.
Fourth, you have to decide which is the dimmest number you can see. By definition, this is on the edge of the eye's ability to pick it up. Can you really see 200? Or do you just think you can?
Fifth, this design dates from before the days when the one-stop safety factor was taken out of ASA speeds, so you need to set modern ISO speeds at 1/2 of the ASA scale. Hence, in the example earlier, instead of setting 200 (dimmest number visible) opposite a film speed of 400, you should set it against 200.
The Extinction Meter Is Extinct
The Practos is only one of many extinction meters, some of which were built into cameras. It's one of the most advanced and sophisticated, though I have to admire the ingenuity of the sort that are designed to be used well away from the eye so that there is less chance for dark adaptation as you are actually looking through the meter.
In a series of admittedly casual tests against a conventional exposure meter I found that the Practos got me within one or (at worst) two stops of the exposure indicated by the photoelectric meter. It invariably erred on the side of generosity, which is what you need with black and white film. I could consistently give 1/2 stop more than the meter indicated and rely on good accuracy.
All in all, I reckon I can guess exposures about as accurately as the Practos can indicate them, but then, that's the result of 37 years' experience, many of them using non-metered cameras. Unless you can guess with an equal degree of accuracy--and most people can't--then yes, an extinction meter such as this is better than no meter at all.