Hi,
Here is a link to a document entitled 'The mercury cell problem and its solutions'
Product Details | The Mercury Battery Problem | Other Repair Guides | Learn Camera Repair
A mercury cell holds a constant voltage over it's lifetime.
An alkaline battery slowly drops in voltage over it's lifetime.
Why does this matter?
If the circuit does not require a voltage reference, then it does not matter if a alkaline cell is used in place of the specified mercury cell.
So, in the case of cameras and light meters, if it uses a Whetstone bridge circuit, then a modern alkaline equivalent can be used without issue.
An example here is the Zenit TTL. It was originally specified to uses a mercury battery, but as the light meter is a whetstone bridge design, a battery that looses a bit of voltage will not affect the meter, nor will the extra 0.2volts.
Conversely, if the meter measures the voltage of the photocell against a known voltage (i.e a 1.3V constant voltage mercury cell), then fitting a 1.5v cell that slowly drops in voltage over it's lifetime will certainly mess things up.
Using, or making a cheap adaptor (as described in the document) using a schotty diode & 1.6V silver oxide battery is an option.
Wein make a Px625 size zinc-air 1.35V battery, but of course these slowly discharge from first use (when tab is pulled off & it is exposed to air), even if not used.
There is an (expensive) px625 adaptor available which claims to regulate the voltage of a 1.5V cell to 1.3V
I'm sure it works, would love to know how, as even with 'low dropout' voltage regulators, the difference between 1.5 & 1.3volts is so small.