Do CDS cells in Old light meters lose sesitivity?

Discussion in 'Miscellaneous Equipment' started by chip j, Jan 15, 2018.

  1. chip j

    chip j Subscriber

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    I have a Luna Pro I bought new in the early 80s. I traded it to a friend and he says it's underexposing B & W film compared to his cell phone meter.
     
  2. Sirius Glass

    Sirius Glass Subscriber

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    It was years ago that my CDS cells died. I do not remember whether they lost sensitivity or just died.
     
  3. paul ron

    paul ron Subscriber

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    How off is it? is the error linear?

    Ive got many luna pros and an assortment of other brands that are over 30 years old... they all work just fine.

    If I compared my meters to each other, Im sure many wont agree. I just compensate via ASA setting.

    My old Westons have all gone blind... but that is typical of that type of cell.

    If you feel your cell phone meter is a defacto last word... then why not use the phone all the time?



    .
     
    Last edited: Jan 15, 2018
  4. samcomet

    samcomet Subscriber

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    Doing a quick web search I found that CdS cells do in fact "fade" over time and, as they are essentially photo resistors there are usually small "pots" (potentiometers - variable resistors) in the circuit that can be adjusted to offset this loss or change in sensitivity. I am not familiar with your Luna Pro but this may be a cause.
    cheers,
    Sam
     
  5. Luckless

    Luckless Member

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    What specific model light meter is it?

    An important question to ask when dealing with older sensitive electronics is: "Am I feeding it the correct voltage as originally expected" - While some designs were well thought out and balanced against voltage variance even if they expected an older and more toxic chemistry for their cells than what can be readily found on the market now, others are far less forgiving.

    Also important in photosensitive cells is whether or not they have built up dust or other crud.
     
  6. OP
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    chip j

    chip j Subscriber

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    My friend finds that getting his phone out all the time is a hassle. I hardly used my Luna pro--it was in it's case in the dark almost all the time.
     
  7. E. von Hoegh

    E. von Hoegh Member

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    If it was losing sensitivity, it would be overexposing. If it has alkaline cells in, it would be undrexposing.
    Cds cells do age, losing sensitivity and linearity. Up to a point this can be compensated for. My 50 or so year old Lunasix is still accurate with zinc-air cells.
    See if your friend put alkaline cells in it, and check it against a real meter not a phone.
     
  8. BrianShaw

    BrianShaw Member

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    Mine is still accurate with the Gossen adapter and silver cells.

    But before blaming the equipment, are you sure your buddy knows the dial and how to properly meter? Often these are the bigger issue to resolve.
     
  9. Chan Tran

    Chan Tran Member

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    How does the cell age? How does it change? I would think if they age they would be less sensitive to light and thus causing overexposure and not under exposure.
     
  10. E. von Hoegh

    E. von Hoegh Member

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    Excellent point. Two meters with differing acceptance angles can give very different exposures for the same scene, as well.
     
  11. E. von Hoegh

    E. von Hoegh Member

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    How does it do with color film?:errm:
     
  12. BrianShaw

    BrianShaw Member

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    Not just that... not too long ago on some forum or another there was a similar question and the user did not know that the dome needed to be slid to the side for a reflected metering.
     
  13. jim10219

    jim10219 Member

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    What kind of cell phone does he have, and what app is he using to meter? The problem with light meter apps is that each camera is different and the app may not be written specifically for the camera used in that particular cell phone. So it may show close values, but not the correct ones. I would compare that Luna Pro to another known good meter first before assuming it's a problem with that meter.

    CdS cells tend to age pretty well provided they aren't exposed to excessive heat. Age and exposure to light doesn't affect them a great deal. However, this light meter was one of the first to use CdS cells, so I don't know how reliable the early ones were. In all honesty, I would probably check the rest of the circuit out for problems before assuming it's the CdS cell. They're not the most robust electrical component in the world, but they're not as fragile as selenium cells or many other components can be. There also may be a way to adjust it's sensitivity, by way of a potentiometer or replacing a resistor.
     
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  15. Jens Hallfeldt

    Jens Hallfeldt Member

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    Hi,

    so is my Gossen Lunarsix 3 (CdS cell), one stop off...
    My SBC cell Profisix works perfectly.

    Best
    Jens
     
  16. E. von Hoegh

    E. von Hoegh Member

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    They're extremely reliable. As for checking the rest of the circuit, there really is none, just a switch and some calibration potentiometers, no other active components whatever. What can happen regarding the potentiometers, is oxidation can build up on the moving contacts. Moving the potentiometer arm over it's travel, then returning it to the original position will cure this.
    But and however - it needs to first be established if it's the meter or the I-thingy.
     
  17. jim10219

    jim10219 Member

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    Good to know! I know sometimes early technology can have bugs that need to be worked out that no ones realizes until they've been on the market for a few years. Then again, some stuff comes out right the first time, and people tweak it over time (usually cost cutting reasons) and they become less reliable than the early ones.

    And by rest of the circuit, I was meaning wires, solder connections, switches, pots, and all of that stuff. I've made quite a bit of money buying and reselling once dead electronics by reflowing cold solder joints, spraying deoxit on potentiometers and switchs, or cleaning corroded battery connectors with vinegar. A lot of people will throw out anything electrical as soon as they stop working, even when often times the repair is simple. And even when the repair isn't so simple, I studied electrical engineering in college, so I'm pretty good with anything electrical that isn't software related.
     
  18. E. von Hoegh

    E. von Hoegh Member

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    Not all, I have a Grebe Mu1 Synchrophase TRF radio from 1925. It not only works, there's no evidence of any repairs being carried out. The period correct tubes are good too, but may not be originals.

    You're right about the cost cutting.
     
  19. Sirius Glass

    Sirius Glass Subscriber

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    I believe that the light meter has an adjustment screw on the bottom.
     
  20. E. von Hoegh

    E. von Hoegh Member

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    It does. That screw is to set the galvanometer to zero, it has nothing to do with calibration of the electrical circuits.
     
  21. Jens Hallfeldt

    Jens Hallfeldt Member

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    Yes, it is for the offset of the sero point iirc. I tried to use it to compensate the error, but it didn't give enorgh compensation...
     
  22. E. von Hoegh

    E. von Hoegh Member

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    It, per the meter's instructions, should be set to zero with the battery removed from the meter. It isn't intended to calibrate the meter, merely to set the galvanometer to zero.
     
  23. Jens Hallfeldt

    Jens Hallfeldt Member

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    That's why it didn't work. Now I offset the iso setting. ;-)
    I like the Lunasix 3 for its low wight, compared with my Profisix...
    Best
    Jens
     
  24. Sirius Glass

    Sirius Glass Subscriber

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    Well for around or under $100 you can get a Gossen Luna Pro SBC, case, telephoto attachment with case from KEH.com. I did. The instructions are not that clear for the attachment but after I understood the I put red and green dots on the light meter to correspond to the 7.5 degree and 15 degree settings.
     
  25. OP
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    chip j

    chip j Subscriber

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    He has the Gossen battery adapter & silver cells; also the tele attachment. @7.5 degrees, close-up of a face, it exposes 2 stops under compared to his cell phone.
     
  26. Sirius Glass

    Sirius Glass Subscriber

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    How does he know that the cell phone is correct? The tele attachment will work on newer Gossen meters.
     
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