----- Original Message -----
From: "William Robb" <[EMAIL PROTECTED]>
> (snip) The Spotmatic meter cells put out a
> remarkable amount of current, nearly enough to run the meter
> circuit on it's own. The battery is there to act as a booster
> more than to run the meter circuit.
>
My belief is that the SP-F used Cadmium Sulphide cell(s) in its meter. If
so then it is photoresistive, and works by resisting the current flow (from
the battery to the match-needle) in proportion to the light falling onto it.
CdS cells do not contribute current of their own (or so I was taught). They
can be extremely sensitive but are slow responding and can easily be blinded
by an excessive quantity of light, rendering them innacurate for several
minutes afterwards.
Silicon cells as used by K2 and KX are photovoltaic, but produce too little
current to deflect a match-needle and need a (power hungry) amplification
circuit. They respond to light fluctuation extremely quickly and have no
lag. I think William is thinking of this type of meter cell or perhaps
GPDs.
Gallium Arsenide Phosphide cells (GPD or Gallium PhotoDiode) took over from
Silicon starting with the ME and MX. Their colour response is better
matched to the human eye (and film) and they are more economical WRT battery
consumption. I don't recall if they are photovoltaic or photoresistive.
Meter cells are no longer a sexy part of camera specification (as they were
in the '70s). Thus we are now told what the metering modes are, but not
what type of cell is used. Perhaps someone can tell me (and the list of
course) what type of cells were used from LX until now.
Regards,
Anthony Farr
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