Do CdS light meters need to be woken up?

Discussion in 'Exposure Discussion' started by Sirius Glass, Sep 8, 2009.

  1. Sirius Glass

    Sirius Glass Subscriber

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    Do CdS light meters need to be woken up after long periods of non-use?

    I have a Sekonic L-98 light meter that sat for years before I got it. I used it some and then it sat for about a year. The mercury battery still tests "good" but in bright light the meter did not register in the High range. I took it to Bel Air camera and after they fiddled with it for a while, i suggested that they take it outside to see if they could get the High range to work. It was set for ISO 400 so it should read ~f/16 at 1/500 second. At first they could not get it to register. Then one of them aimed it at the sky for several seconds to see if they could get it to even register. At that point the needle moved in the High range scales. Then we took readings again and got f/16 at 1/500 second in the bright sun, f/11 at 1/500 second in the less bright areas and f/8 at 1/500 second in the full shade! They said that CdS meters needed to be woken up especially if they have not been used in a long time.

    Has anyone else experience this?

    Steve
     
  2. Denis K

    Denis K Member

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    That is a god question and I don't know the answer; however, I do have one thing that you think about. Some of these high/low sensitivity meters really only have one electrical sensitivity (which is the high setting). The low setting is often a done by interposing an aperture in the light path to cut down on the amount of light getting to the sensor. If you have one of these types, then if it works well in the low sensitivity setting it should work in the high setting unless something is wrong with the aperture changing mechanism. If yours uses a different high/low mechanism, then just forget everything I've said.

    Denis K
     
  3. Sirius Glass

    Sirius Glass Subscriber

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    I think you meant:

    The high setting is often a done by interposing an aperture in the light path to cut down on the amount of light getting to the sensor.

    Yes, a lever moves the range by rotating the dial. It may change the aperture too.

    Steve
     
  4. Denis K

    Denis K Member

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    Steve, this high/low thing is hurting my head. I was thinking of the low setting as in "low sensitivity", which is probably backwards from how smart people would think about it.

    You can usually see the aperture if you look carefully in the front with whatever integrating sphere you might have moved out of the way. Swap between high and low and make sure you see the aperture mechanism operating smoothly and correctly. It sounds like you might have already done that. The aperture hole can be surprisingly small.

    Denis K
     
  5. Sirius Glass

    Sirius Glass Subscriber

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    Denis,

    I just now got up, walked into another room, turned on a bright light and looked. The High scale has a pin hole, the Low scale opens up the complete aperture. You are right.

    On two outings the Low reading were several stops too low and the High range did not work at all. I had to guess the exposure. This we reproduced inside the store. After the light meter was aimed at the sky both ranges worked outside and back in the store when compared to other reflected meters. And therefore they made the comment that CdS meters need to be woken up if they have not been used for a long time. [Note: CdS meters are slower to respond than the Silicon meters.] Hence the question.

    Steve
     
  6. Denis K

    Denis K Member

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    Steve, I was doing some reading in the book, "The manual of photography". by R. E. Jacobson and he had an interesting section on Cadmium Sulfide photocells. He claims that CdS has a "memory" effect; however, it would be the opposite of what you found. He claims that a CdS cell placed in a very bright light has a very noticeable delay before it responds properly to a dimly lit subject. According to your first post, your light meter experienced the opposite of this effect. Frankly Steve, I think your light meter has slipped into the Twilight-Zone.

    Denis K
     
  7. DWThomas

    DWThomas Subscriber

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    I not aware of any CdS wake-up phenomenon, but I might wonder if there was a dirty contact that got cleaned off by going through all the motions a few times.

    I've personally pretty much written off CdS meters. Too many years ago to remember reliably I had a Gossen LunaSix. It was an early CdS meter and I don't remember it being terribly slow. But later (after the LunaSix died a 2nd time) I acquired (new, back when) a Gossen Super Pilot which was much more sluggish to work with. When I got back to needing light meters again, it had the mercury battery issue, but even with a Schottky diode correction, it was still a slug to work with and I finally bought a Digisix, much less frustrating -- and smaller and lighter.

    I don't know if any CdS meters might use pressure contacts on the sensing cell; I think not for CdS, but it was not uncommon on selenium cells. I had, actually still have, a mid-1960s Waltz Coronet that after a few weeks of disuse would fail to read. If I pushed in gently on the plastic diffuser over the cell area. the needle would suddenly pop to life and it would work. When it read at all, it seemed to be right, but it was a tad frustrating and not exactly confidence inspiring.

    Assuming an analog meter, I might also wonder if this is/was a mechanical issue in the meter movement -- accumlated dirt, oxide, whatever -- that needed a really high current jolt via bright light exposure to break loose. My current preference for digital displays is an outgrowth of tedious mechanical difficulties with D'Arsonval meter movements over the years (not just in light meters.)

    DaveT
     
  8. Q.G.

    Q.G. Inactive

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    These are not just claims: CdS cells do have a memory effect.
    They need to recover after having been exposed to high(er) light levels.

    With CdS meters that offer two ranges, it is 'good practice' to try the 'high range' first, before using the 'low range' mode.

    I never heard about, of experienced, a 'wake up' issue with CdS cell meters.
    Could indeed be a mechanical thing. Or a battery issue.
     
  9. Prest_400

    Prest_400 Member

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    I've noticed that the meter of My OM-1 sometimes has a delay in response. At first, it didn't.
    When pointing the camera at low light levels, it does have this little delay. But if I suddenly point to the sky, it takes 3 seconds to the meter (needle) to response.
    I think it's the battery. It's still got 1.55v; I suppose the batteries tend to lose power.
     
  10. DWThomas

    DWThomas Subscriber

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    There is memory effect, but what I associate with CdS cells is measured in seconds, though, not hours or days. I might even argue it's not really "memory," just a very slow response to changes in light intensity. That is, if you sequentially take meter readings of two areas of very different brightness, on the second reading you will see the needle move some, then continue to creep along for as much as a few seconds before it settles at the new reading. I don't remember it being as noticeable with my old original Lunasix, long dead so I can't try it, but it's very obvious with the Super Pilot I still have. An attempt to take a quick reading and shoot could wind up being off a stop or more. Unless my latest silicon cell based units die suddenly, the Super Pilot will probably remain a shelf ornament.

    DaveT
     
  11. Q.G.

    Q.G. Inactive

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    The memory effect does not manifest itself in a lag in respons, nor in a slow moving needle (that's another of CdS cell's 'habits').
    It shows itself in values that are too high, after the cell has been exposed to high light levels.

    CdS cell's slowness also is not a lag in response, but rather that it takes the meter to stop moving and show the final measured value.
    It builds up, but it starts doing so immediately.

    I would think that a meter that stays motionless for a while, then jumps into action, has another problem than using a CdS cell.