To begin, exposure of silver-halide film is a function of energy--a product of power and time. It is also logarithmic, meaning the same percentage change in exposure energy results in the same change in exposure density, regardless of absolute light intensity. Thus a one F-stop change in exposure produces the same change in density for both low and bright light conditions. Remember that one F-stop is a doubling (or halving) of exposure. For example: increasing light power from 0.5 cd/m² to 1.0 cd/m² will cause the same absolute change in film density as will a change from 4000 cd/m² to  8000 cd/m². This is the phenomena portrayed by a fundamental darkroom tool; the "d log e" chart--whereby density change is a linear function when plotted against a logarithmic exposure scale.

Another concept useful to photographers is that of Exposure Value (EV). In it's purest form, EV is merely a convenient way of representing any combination of lens opening and shutter speed that will produce the same exposure on film. When used in conjunction with film sensitivity (ASA or ISO) and light gathering ability of a lens, it becomes an absolute measure of incident light power. Exposure measurement devices often present measurement latitude in terms of EV. As an example the Topcon RE Super/Super D exposure meter is stated to be accurate between 0.5 cd/m² (EV 2) and 8000 cd/m² (EV 16) when using ASA 100 film and the f/1.4 lens. This changes to between 2 cd/m² (EV 4) and 32000 cd/m² (EV 18) with a f/2.8 lens.

I have done considerable research into the exposure meter of the RE Super/Super D cameras--theoretical as well as empirical. One of the features of this exposure meter is it's simplicity; a series circuit consisting of battery, CdS cell, galvanometer, and some fixed scaling resistors. As might be expected, the resistance change of the CdS cell is logarithmic to match film characteristics and EV settings of lens aperture and shutter speed (and also ASA adjustment). This means that the same change in galvanometer current (e.g. going from 10µA to 15µA, or from 140µA to 145µA) represents a constant EV  and will produce the same change in film exposure.

The empirical expression I have derived for the relationship between EV and current, as measured by the galvanometer, is:

Where: E = battery voltage (1.35 volt, nominal)
R = resistance of the  CdS cell, in ohms
K= 9µA, the difference in galvanometer current produced by one F-stop change (one EV) in either shutter speed or lens aperture setting..

The task now is to relate the change in EV as a function of a change in E. To do this I calculate EV for two different values of voltage ( E₁ and E₂ ) as follows:

The change in EV, ?EV, will be the difference between these two, or

Reducing this equation gives

where: ?E = the change in battery voltage, (E₂ - E₁).

Here we can see that EV does change with a change in the battery voltage E. But also note that the amount of this change is dependent of the resistance of the CdS cell, which in turn is a function of the intensity of the light it sees. Thus, when the light level is low (high Cds cell resistance) the change in EV, as a function of battery voltage change, is much less than for the same change in battery voltage when the light level is high (low CdS cell resistance).

This seems counter-intuitive, since at first glance one would assume that battery voltage errors could be compensated for with a constant offset of either the shutter speed or the lens opening; or as more commonly recommended an offset in the ASA setting. The explication of this unexpected result follows from the fact the constant K, used above, remains the same over the exposure measurement range. Thus a one F-stop change of either shutter or lens always produces a change of 9µA regardless of the intensity of the incident light; bright or dim. But the change in current as battery voltage changes produces a greater change in current when the CdS cell resistance is low (bright light) than it does for high resistance (dim light). Therefore the exposure error due to incorrect battery voltage must be larger in bright light.

An example is in order. Consider two cases: one for dim light that produces a CdS cell resistance of 150 K ohm and one for bright light with a CdS cell resistance of 10 K ohm. Calculate the change in current for two voltages, one the correct 1.35 volt and the second that of a new alkaline cell, 1.5 volt.

The results are best shown in the following figure. Here the exposure error is plotted as a function of incident light intensity (EV s) and battery voltage The fact that a voltage of 1.35 volt produces no error at any light intensity is the result of the exposure measurement system being designed assuming the use of a mercury-oxide battery. The EV scale on the X axis represents a film ASA of 100 and use of an f/1.4 lens.

Conclusion--An alkaline battery installed in a RE Super/Super D camera will result in unacceptably large exposure errors when used under bright lighting conditions, and it is not possible to compensate for the variable voltage of this cell by a simple offset of the ASA setting. By knowing the absolute light intensity, and exact battery voltage, a complex schedule could be worked out, but such a procedure would be time consuming. My suggestion is to find a source of constant voltage (1.35 volt) batteries.