P3D Re: FED Exposure / Battery question

[michaelk@xxxxxxxxxxx (Michael Kersenbrock)]

> One of the cameras I am currently running is a FED 'M'.
> Although very slightly underexposed the initial films run throught it were
> fine.
> Prior to going to the Stereoscopic Convention at Llandudno I replaced the
> battery. The shots I have had back since the swap have been severely
> underexposed - I adjusted the film speed setting to give me another stop and
> further films are still severely underexposed !
> 
> The initial battery which came with the camera is marked as an 
> 'AG13 Button Cell' (presumably from the factory) - it was not exhausted when
> I swapped it and it registers 1.15v on a multimeter.

That's a bit low for a mercury cell (1.35V nom rating) if that's what it is.

I'm not sure about the 'M', but my Fed (non-"Boy") expects to have a
Mercury cell (or a zinc-air cell... hearing aid types or Wein versions).

> 
> The battery I have replaced it with is an SR44 (also known as G13, 10L14 or
> EPX76) and is rated at 1.55v.

Which is high, and when it's in series with the variable resistance 
light sensor would have the equivalent effect of the camera thinking
it has more light than it does ("light" comes from the higher voltage
rather than from the actual light on the light sensor).  And as such
it'll under-expose (more).  Many FED's have been reported here to
underexpose a stop or two even with the proper Mercury cell (although
my personal one happens not to).  But with the higher voltage battery
it'll underexpose.  The circuit was talked about some years ago, and
it wasn't a "bridge" circuit which usually is voltage-immune.


> 
> The question is - Am I using the wrong battery therefore giving incorrect
> exposure settings. Anyone know the answer ? - preferably with a UK code number

Yes.  Wrong battery.

> (and presumably a non-mercury type as I don't think they are easily 
> available anymore).

Well, they are available in some places in the world... but alternatively
the easiest sources are either the Wein mercury replacement zinc-air batteries
or the zinc-air batteries sold for hearing aides (usually much cheaper but
you may be on your own to shim things to fit w/o cutting off the air hole
on the battery -- star lockwashers work nicely).

Mike K.

> 
> Thanks
> -- 
> Phil Palmer
> Oldham, Lancashire, UK
> 
> --------------------------------------------------------------------------
> The next Third Dimension Soc. (North West) meeting is at Bury, Lancs. on
> Tuesday, 16th June 1998.
> --------------------------------------------------------------------------
> 
> 


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