I have two of these discs. The 52mm one and a 72mm one. I believe that the 52mm one is now discontinued. As 52mm was the filter size for around 80% of Nikkor lenses, I believe that was the reason it was made then and now discontinued.
The daughter of the inventor figured out the ExpoDisc had uses for determining correct white light values for the new wave of electronic cameras. She was right, it is brilliant at doing that. As a consequence, manufacture has shifted from the analogue manual focus system to the electronic auto focus systems. The new ExpoDiscs do come with a neck lanyard which is heaps better than the waxed string that both of mine came with.
Now to the question originally asked. The Wallace Expos Disc, can be used under an enlarger once you have referenced your analyser to a neutral grey.
I had letters and telephone calls between Bob Mitchell in America about this, in conjunction with his COLORBRATOR for colour negatives.
Bob was kind enough to send me one of his own thin plastic diffusers which I cut to size and installed in my swing in/out red filter holder. It transmits considerably more light than the ExpoDisc does. Experiments told me that the ExpoDisc works brilliantly under the enlarger, but you will need a strong enlarging light coming through, otherwise your analyser won't have enough light to get a correct reading.
I myself run the Jobo Color Star analyser, which is the original and very basic version of these analysers.
The "mitchell COLORBRATOR" is pretty much the best system I have had the pleasure of using to get correct colour printing colour negatives.
It is the invention of the late Bob Mitchell and is designed to get your system colour correct so that you can then evaluate all of your other negatives.
It comes with a colour negative that you insert and make a print with. From that print you use the supplied neutral grey card with the hole in it, to scan the colour print you have just made. You locate the equal colour square on the colour print to the card with the hole in and use that exact square to adjust your analyser to neutral, (or whatever).
I then had a perfect neutral grey and I also knew what density (or time) was required to make a colour print.
When out shooting I used the Wallace ExpoDisc on one frame in the light I was shooting under. Basically I used my Nikon F3 by puting the camera into A for aperture priority and just hit the shutter button. It didn't matter whether it was a fast or slow shutter, the camera is just recording a neutral grey across the whole frame.
Back in the darkroom I would put a neg in I wished to enlarge. I would compose it as I wished then replace that neg with the frame which was exposed with the ExpoDisc. Put the analyser probe under the easel and adjust the enlarger filters until the analyser was satisfied.
Then I just replaced the original negative I wished to use and exposed. Invariably the very first print of a session was 99% correct. That system is that good.
Bob Mitchell also had a "MITCHELL GRAY KEY" which he also sold. The idea of the Gray key was that this was a piece of material with a key ring hole that could always be carried by the photographer. It is basically a piece of Laminex which is about the same reflectance as any grey card. It is brilliant and works as well as the Wallace ExpoDiscs.
I bought the Wallace ExpoDiscs as they were/are a way to turn your camera meter into an incident light meter and also allow you to test shutter accuracy etcetera.
If I was starting out I would be very tempted to purchase the "mitchell COLORBRATOR" as well as the "GRAY KEY" as a way to get a quite good consistent system in your colour darkroom.
Mick.
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