William Sommerwerck wrote:
> The question this raises is just how the camera can provide accurate
> (ingnoring reciprocity failure) four-MINUTE exposures in OTF mode. Does
> the OTF mode use a much-higher-quality photo diode? Or is there
> something else about the design of the camera that negates the problem
> of the dark current?
Hmm, maybe it's balanced with a matching
diode kept permanently in the dark.
Certainly, if the OTF metering involves an integration circuit,
it wouldn't of been hard to design in a summer to subtract the
"dark" photodiode.
Then again, the circuit given in
http://patent.womplex.ibm.com/details?pn=US04065777__&language=en
uses a photodiode with no bias across it (straight across the inputs
of an opamp). I guess it's using some weak voltage generating
characteristic
of photodiodes. Anyone know how they behave?
The other diagrams in the patent certainly look like an OM2/4.
quothe: "
US4065777: Photometric apparatus for single lens reflex camera
Inventor(s):
Maitani; Yoshihisa , Hachioji, JA
Shimoyama; Kunio , Hachioji, JA
Yoshida; Muneaki , Hachioji, JA
Hashimoto; Akihiko , Hachioji, JA
Kitagawa; Masahiro , Hachioji, JA
Applicant(s):
Olympus Optical Company, Ltd., Tokyo, JA
Issued/Filed Dates:
Dec. 27, 1977 / Aug. 24, 1976
Abstract:
A photometric apparatus for single lens reflex camera and of the type in
which an
exposure value is determined by photometry of light which is transmitted
through a
photographic optical system and is reflected by at least one of the
surfaces of a
shutter blind and a film, said apparatus comprising a photoelectric
transducer
element for receiving light reflected by at least one of the surfaces of
a shutter blind
and a film, and an operational amplifier for biasing the photoelectric
transducer
! element to maintain the voltage applied thereacross substantially null,
thereby
! enabling a photometric output of the photoelectric transducer element to
be
! effectively derived even when the amount of light received is minimal.
The shutter
blind is provided with a surface upon which at least two portions thereof
have different
coefficients of reflectivity to reduce adverse effects of reflected light
upon the desired
"
duncan
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