Chris Barker <cbarker@xxxxxxxxxxxxxxxx> wrote:
>Franz Pangerl, in his book "The World of OM Systems" reckons that it is
because of
>the Silicon Blue Cells which do the measuring for OTF exposures (in the OM2
at
>least). He reckons:...if the silicon photodiodes only receive weak
measuring
>currents [in low light exposures], then they react with successively
extended
>exposure. This produces an unintentional but useful side-effect - a
partial
>balance of the reciprocity law failure."
>
>NB my square brackets.
Interesting comment, but I'm not sure I buy that. SPD's reaction to low
light levels is simply a property of that particular sensor. I recall the
SPD's predecessor, the CdS cell, which had the property of "memory" where it
would give falsely high readings (resulting in severe underexposure) right
after it sensed a bright light source (such as inadvertently pointing the
camera skyward). It would sometimes take nearly a minute for it to
normalize. Simply a property of the sensor, and in this case one that
worked against the photographer.
It could be that the SPD's low-light property is precise and predictable.
It certainly is useful if the low-light properties exactly match the
reciprocity characteristics of a particular film. But the fact is there is
a very broad range of reciprocity characteristics among the many brands and
types of film available. The SPD's low-light properties do not match all
films, and I'd be quite surprised if it matched any film. It just reduces
the amount of error. Close, but no cigar.
Dealing with reciprocity failure is akin to dealing with color balance in
duplicating film - for critical applications testing with your specific
equipment and conditions is required and lots of mistakes are made. Ask any
astrophotographer.
John P
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