About 1950 a new type of battery became available. This cell used mercury-oxide chemistry and had several attributes that made it ideal for use in photographic exposure meters. First it could be made is a small package; but most important, it had a flat discharge characteristic-meaning that it produced a constant voltage throughout it's life. This cell, when used in conjunction with the newly developed Cadmium-sulfide photo-resistive material and a micro-ammeter, provided an instrument capable of accurately measuring the intensity of incident light over a wide dynamic range.

When Topcon decided to incorporate an exposure meter inside their new RE Super/Super D camera they naturally adopted this simple technique. With a constant 1.35 volts from the mercury cell there was no need to consider voltage insensitive circuits, e.g. the bridge configuration. Bridge circuits are not only more complex, but they usually require a variable resistor with it's associated unreliability. The simple series circuit that Topcon adopted is one reason that so many of the exposure meters in the RE Super/Super D cameras are not only still functioning after nearly 40 years, they are still accurate!

But trouble was brewing in paradise, in the form of environmental awareness. Mercury is a poison and there was great concern that the disposal of mercury-containing batteries would contaminate both ground and water. Thus the decision by the US Environmental Protection Agency to outlaw the manufacture, and sales of mercury containing batteries in the US.

This was a devastating blow to the users of older cameras that were designed to use the mercury cell--no other chemistry is available that provides constant voltage, long life and low cost. Because I still use my old Topcons, I was faced with the dilemma of what battery to use.

Most battery manufacturers now make a 'button' cell with exactly the same form factor  as the banned mercury cells--and approximately the same voltage. Approximate is the  operative word here. And while this cell fits nicely into the battery compartment of your RE Super/Super D, the alkaline chemistry produces a discharge characteristic that differs greatly from that of the mercury cell.  If you see an 'A" as part of the number then it probably is an alkaline cell (most mercury cells use the letters 'PX').

The graph below shows the difference, plotting voltage as a function discharge. Note that over the life of the battery, the mercury cell maintains an almost constant 1.35 volts, then suddenly dies. Whereas the alkaline cell starts high and then gradually tapers off .

The question, of course, is 'what effect does this varying voltage have on exposure measurements?'. And the more pertinent  question, 'is there a simple way to compensate for it?'.  If a change in ASA setting is all that is necessary, then alkaline cells would be a logical choice for battery replacement since they are inexpensive and readily available.

As with many things, the answers to these questions are not simple and the truth may not be what is expected. It is obvious from the simplified schematic diagram of the Topcon exposure circuit, shown at the right, that given a constant level of incident light, the current in the galvanometer must change with a change in battery voltage. And since current is

the metric for setting exposure, voltage induced exposure errors will occur. Because the alkaline cell voltage changes over such a wide range, during discharge, the amount of error will also change. After all, when the cell voltage is about 1.35 volts there will be no error.

After considerable analysis I have reached the conclusion that alkaline cells produce too many variables to be used for exposure measurement. And, most surprising, the error is dependent on how bright the lighting is--the brighter the light, the greater the error. The figure below illustrates not only the error as a function of battery voltage, but also the dependence on scene brightness. I have considered battery voltage between 1.20 volt and 1.5 volt.

In dim light exposure accuracy is about ± 0.5 F-stop, sufficiently accurate to properly expose slide film. But in very bright light, the exposure error will probably exceed that of print film. It is impossible to compensate for exposure errors due to voltage changes by simply adjusting the metering system ASA setting, when shooting in bright lighting conditions..

Click here for the technical analysis I used to arrive at this conclusion.