Why two CDS cells?

AgX · Oct 31, 2013

In the 70s Agfa released several cameras (35mm, type 126, type 110) that got two CdS cells, each with its own lens.

Selectronic S
http://upload.wikimedia.org/wikipedia/commons/7/70/Agfa_Selectronic-S.jpg

Agfamatic 300
http://upload.wikimedia.org/wikipedia/commons/7/71/Agfamatic_300_sensor.jpg

Agfamatic 5008
http://www.heritagecameras.co.uk/ekmps/shops/heritagecameras/images/agfamatic-5008-makro-sensor-pocket-camera-560-pocket-lux-flash-110-film-may-be-available-again--[3]-2550-p.jpg

I guess(!) one cell serves the autoexposure circuit, the other the mal-exposure indicator in the finder.
Could some electronics expert comment on that?

(I admit I have not yet taken apart any of them, but even if I did so, most probably I still not would not know WHY two cells were used.)

Steve Smith · Oct 31, 2013

Minolta used two in the SRT and XE cameras. I think they were arranged so that one was in the general sky area for both horizontal and vertical orientation.

They were wired in series and whichever one was pointing at the sky would have quite a low resistance compared with the other one which would provide the reading for the exposure.

They called it CLC which stood for Contrast Light Compensator.

Camera ads from the 1970s boast of its CLC (Contrast Light Compensator) metering, calling it "the brain". CLC is a form of TTL metering with two CdS cells. It compensates for over-exposure by assuming that the upper side of the picture is the sky (overcast) and that the lower part is the subject you want to photograph. ISO values can be set from 6 to 6400.

http://camerapedia.wikia.com/wiki/Minolta_SR-T_101

These differed though as they were gathering the light through the lens rather than being mounted on the body surface.

Steve.

AgX · Oct 31, 2013

Interesting, as I did not relate them to that compensating concept.
At the Agfamatic 300 photo you can see an aperture behind one lens. Depending on position this either can act as amplitude- or direction-control.

ath · Oct 31, 2013

AgX said:
I guess(!) one cell serves the autoexposure circuit, the other the mal-exposure indicator in the finder.
Could some electronics expert comment on that?

I think you are right. Automatic exposure with given aperture and adjusted time is quite easy to do with a CDS sensor. But then this sensor is blocked for other purposes - like displaying the time (Olympus XA) or driving a blur warning LED. CDS cells back then were cheap compared to more complex electronics. Today it's vice versa.

AgX · Oct 31, 2013

Yes, the Selectronic even has a digit visible in the viewer, indicating exposure time. I thought of that cost factor too, but was not at all sure about it.
But I don't know (or can't remember) any camera of an other manufacturer that employed two cells this way.

ath · Oct 31, 2013

That might not always be visible from outside. You can use double cells and only one "eye" is visible.

AgX · Oct 31, 2013

But that most probably mean custom made cells (two-in-one) which would have driven up costs.

Xmas · Oct 31, 2013

lots of SLRs had two cells in pentaprism cover void

e.g. Olympus OM1 had two cells

Pentax K1000 family had three - two to measure light like OM1 - third to switch off the metering when light was too poor for metering

AgX · Oct 31, 2013

Yes I know, typically used for yielding two metering angles, but also for compensating as Steve hinted at.
But I do not know other finder cameras that got two cells at the front.

(I somehow overlooked to say that in the title, and it is not really obvious in my first post, except when looking at the samples.)

Why two CDS cells?

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