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08-23-2012, 07:10 PM   #1
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GPD metering cells, why only on M bodies?


In the M bodies camera manuals, GPD cells are promoted for being faster/more sensitive (better)...
But the next A bodies used SPD cells again.
The only other camera I know used GPD cell is Nikon FM, not on later FM2.

Why were GPD cells only used in M bodies, if it has better characteristic.
Expensive? Or may be it was only a trial?
Any technical explanations/from electrical viewpoint?
I'm curious.


08-23-2012, 07:36 PM   #2

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CDS, GPD and SPD identified in Pentax Camera-Body Terminology
GPD not sensitive to invisible light while an SPD needs blue filter. Since most all use SPD, it seems cost is better with the SPD even if a blue filter has to be used.
08-27-2012, 05:14 AM   #3
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QuoteOriginally posted by hiktaka Quote
Why were GPD cells only used in M bodies...?

The authoritative B. Dimitrov site says the later A-, P- and SF- series all use GPD cells too.


Last edited by ChrisPlatt; 08-27-2012 at 06:40 AM.
08-27-2012, 06:31 AM   #4
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It is interesting that, while the ME and MX introduced in 1976 had Gallium Photodiodes, the kit flash, AF16 from the time is listed as having a Silicon photodiode for its chop circuit.
I recall in 1971 experimenting with opto electronics in engineering school, devices were expensive, and Silicon devices were not as photo sensitive as Germanium . (we cut open industrial transistors and focussed biconvex lenses on them for audio transmission by light)
I don't recall when Gallium devices came out. The semiconductor properties of the Gallium family made it more photo sensitive than silicon.

Anyway while the earlier Ge semiconductors and selenium detectors were prone to deteriorate, by 1970's they had stable properties so lets hope the sensors in the MX etc last for many more years.


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