Users of old cameras with built in lightmeters face a bit of an issue when trying to find compatible button cell batteries to power the meter and give accurate readings.
There are some work around solutions to this;
1. Use a seperate hand held meter, useful and accurate across a wide range of lighting conditions
2, Use 'sunny sixteen' to calculate it in your head, accurate enough and useful to know in emergencies, but it can get a bit wayward when trying to figure out the correct exposure for 400 film pushed three stops to capture a candle lit portrait in a dark room.
3. Guess based on experience. This can work, but we are all fallible.
4. Use digital camera to meter the scens and transfer the readings, means carring two cameras and frankly, it's just a big bag of wrong.
The problem is, back when I was a nipper, camera manufacturers designed thier kit to use mercury cells which gave a voltage of about 1.3 volts and the inbuilt meters were calibrated accordingly, which was fine, until someone developed a conscience and realised that mercury is quite nasty stuff and something of an environmental disaster.
While modern Alkaline equivalents will fit, they give 1.5 volts which is close enough and we can figure out how much to adjust our readings to compensate for the extra votage, right?
Wrong.
Alkaline cells discharge in a non-linear way and older cameras do not have the electronic circuitry needed to compensate for this, so in effect your light meter will never be consistent and your exposures will be off as a result.
The discharge curve of alkaline batteries is non-linear, as we can see in this graph your 1.5volt battery will be accurate once at some point in its life, and for the rest of it your meter readings will be way off.
However, Zinc -Air cells provide a very flat discharge rate, across its life it will operate at around 1.3 volts - much closer to the behaviour of the Mercury cells older cameras were designed to use.
My research suggests that the small difference in voltage when using a Zinc -Air cell instead of the original Mercury cell typically results in meter readings that produce an under- exposure of about half a stop, but most importantly, the meter will provide a constant and reliable reading for most of its life.
1.4 Volt Zinc -Air cells are cheaply and widely available in the form of Hearing Aid batteries. I use 675, you can easily recognise the Zinc -Air type as they have a pull off tab which covers a small hole in the cell, this allows air in and activates it and after a minute or two it will start to work normally. When storing your camera for longer periods, remove the battery and re-cover the hole to preserve battery life.
You will notice that these batteries have a smaller diameter than the Mercury originals, I simply make a simple spacer ring to help keep it centered in the camera battery compartment. Rob