How does an extinction meter work?

Can anyone explain how an extinction meter works? I did an Alta Vista search and the best answer I came up with was:

 

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"A type of exposure meter that artificially reduces the light admitted in a sequence of known fractions, until a value is reached that is equivalent to the proper exposure."

 

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This didn't add much to my understanding.

 

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Peter Caplow

Peter, I have an old Universal Buccaneer 35mm that has a built in extinction meter (among other interesting features, such as a collapsing lens). The meter is in a separate viewfinder-like eyepiece and what you see is a circular neutral density step tablet that looks much like a Kodak Projection Print Scale, except with a number in each wedge. In use, you point the camera at the subject and look to see which is the first number you can just make out. This number is transferred to a scale on the back of the camera which along with film speed determines exposure and works quite well. Basically you are just determining a light value, ie. Zone V placement. I also have an infinitely much more sophisticated extinction meter, the S.E.I. photometer, beloved of Ansel Adams. Actually, Ansel's writeup of this meter would be a good source of information for you. It may be found in the old (pre 1980) Ansel Adams Photography Series. Hope this helps.

One of the drawbacks with an extinction meter is that once the eye has acclimatised to the view it becomes increasingly difficult to operate. I noticed in the responses that someone mentioned that they owned a photometer. I have never used them but Michael Langford in his book Basic Photography says they can be extremely accurate if not somewhat easily damaged. I also suspect that they are expensive. My own preference is to use a spot meter or a Weston light meter depending on the situation. It is ironic that when shooting 35mm I no longer use the camera's built in meter.

I remember seeing such a meter that was like a little box you peered through with a calculator dial on the side. It worked as described in the posting about the Bucaneer camera above. I think it dated from the 1930s or so.<p>

The good thing about extinction meters is that they never need batteries and have no moving parts to wear out.<p>

The bad thing is that in a lot of situations where one really needs a meter (such as in dim or contrasty lighting situations) they won't really work that well.<p>

They make interesting collectibles, but for photography I think you would be better off using "The sunny 16 rule" or the little chart that comes in the Kodak box.