The history is simple. Mercury batteries have the most stable voltage
through their useful life, so many camera and light meter manufacturers
used them. Light metering with mercury batteries tend to be very
accurate and stable until they go and then the meter just quits. Thus no
bad exposures, but the need to carry a spare. I had the battery in my
OM-1 go out in Guanajuato, MX almost like a switch being pulled many
years ago. I sucessfully used the exposure guide from the film box until
I found a new battery a few hours later.
More recently, environmental concerns led to outlawing mercury battery
production in the US. I think all production in Europe has stopped now,
too, but there are still some Varta cells floating around. The problem
is that mercury cell voltage is 1.35v, while the 625 size cells used
with other technology are about 1.5v. This leads to a significant
exposure error (2 stops?). As long as your mercury battery lasts or you
can find others, you are fine. When you put in the spare, it's time to
start looking at alternative battery solutions.
Threre are several solutions. When you have your camera CLAed, the meter
is adjusted anyway and can simply be adjusted for the higher voltage.
This adjustment can also be done by itself. There is a thingy called an
MR9 adapter, which is the shell of a 625 battery with a shottsky diode
and plastic plug in it which holds a 357 silver oxide battery. The
diode forward voltage drop adjusts the voltage and the whole thing fits
where the mercury 625 battery went with no adjustment or modification of
the camera. There are available from John H at www.zuiko.com . Handy
tinkerers can just insert a shottsky diode in the meter circuit in the
camera.
I have heard mentioned a few times about this battery conversion,
I was wondering if someone could explain it to me (why it is done etc)
When I bought my om1 the man in my local shop put a new battery in
it saying the one in there was the wrong type.
Also bought a spare from him. Do i need to get mine converted?
The new battery is a mercury one i think
Thanks
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