Light meter's strange behaviour

Discussion in 'Medium Format' started by paul_ericcson, Dec 26, 2013.

  1. Hi guys!
    So I recently bought a used Gossen Luna Pro light meter and it seems to not work properly. And I wasn't able to find out what the problem could be and if there's a problem there at all.
    So here's the thing. I zero adjusted the meter and I use proper 1.35v batteries for it. When I hold the meter against a bright light and use a low light scale it maximum shows me a reading of 10 (the max on the scale is 12 though). Then when I switch it to the high light mode it shows me a reading of around 17. And this behaviour confuses me.
    Here's how I think it should work. The meter is held against a bright light. Let's say a proper reading should be 17. So if the meter's in the low light mode the indicator should go all the way to the right (around 12). Then if switched to the bright light mode it should read 17.
    Has anybody experienced a problem like this? What do you think could cause the problem? Should it be recalibrated?

    P.S. When checking the battery the indicator doesn't go all the way to the red marked area on the scale although the batteries have just been bought (these are Wein cells).
    Thanks in advance!
    Observe the meter needle while turning the "zero adjustment screw" and record exactly how much and what direction you turn the screw. Turn the zero adjustment in one direction or the other, your choice, until the needle goes to a limit and returns back to zero. Test the meter as you describe in your first post. If it improves but is not correct repeat the adjustment in the same direction as before, if it is worse return it to its starting position and try in the opposite position. Turn the zero adjustment screw until the meter indicates zero and moves the full range of the scale.
    The zero adjustment can be turned without encountering a limit and while indicating zero prevent the meter movement at the limit(s). The mater should move to the line left of the 1 to 12 or just beyond. Dropping or subjecting the meter to sharp blows can damage the movement also.
  3. If you don't have the manual, you can download it at That may have information to supplement what Charles has offered.
  4. If you point the meter at something that has a Gossen LV around 11~12, it should read close to the same value on both high and low scale. If not...
    Make sure the batteries are good, wein cells are zinc-air batteries, once exposed to air, thier life time is limited. Cells fresh out of the pack may not really be fresh.
    Check that the lens and cds cell is clean, some of these meters have dust, haze, salt crystals inside blocking the light. The are relatively easy to clean.
    Lastly there are 4 pots inside for adjusting the meter movement, and instructions on line describing the circuit and calibration procedure (if that is within your means). If you need to go that route, I'd recomend re-adjusting the meter for a Silver Oxide cell.
  5. to Charles Monday:

    I now tried to zero adjust the meter again, but the needle only goes to the area of around 3 and doesn't move any more to the left. So while being zero adjusted now the needle moves between 3 and 6 only.
    Regardless of what the zero position was set up at, while metering the needle range is between 3 and 10 on the low scale and between 12 and 20 on the high one.
    to tom chow:
    The lens and cds are clean. Will try out other batteries, thanks, and will post here the results. Though I think it's not the batteries, because I'm having problems with the range that the needle covers. It doesn't go all the way to the zero and it doesn't go any further than 10 and 20. Zero adjust didn't help either.
  6. The zero adjustment may have to be turned 3 or more revolutions to get the needle to move its mechanical limit. My Luna Pro F needed 2 1/2 turns to correct. I sent it to Bogen and had it calibrated several months later then sold it later on.
  7. to Charles Monday:
    Thanks, you were right. Turned it a couple of times more and set the needle at the zero. Now the max reading the needle reaches is 9 and 18 accordingly. It seems like the range that the needle covers is here somehow shrinked.
    Do you think the battery can cause this kind of problem?
  8. Its time to turn it in the other direction and see if the range extends. If it continues to decrease suspect the battery.
    Battery adapter at B&H is getting pricey:
  9. to Charles Monday:

    Tried turning it in both directions - still the range doesn't extend.
    Do you think something like that could work well with the meter (1.4V)?
    Just want to try them instead of Wein cells, cause those are little pricey.
  10. Try an alkaline battery, maybe the meter has already been recalibrated for current batteries.
    Does the needle move freely throughout the whole range, or is it hanging up on the clamp mechanism?
  11. to tom chow:
    I'm not sure about alkalines. As far as I know they give a pretty unstable voltage.
    Yes, the needle moves freely throughout the whole range (well, "the whole range" in my case means from 0 to 9 on the low scale and 18 on the high scale).
  12. I think the current capacity of the battery is playing an important part but not being addressed. I do not know if other types of hearing aid batteries will work or not. I doubt that a hearing aid pulls as much current as the meter so if they do work they will have a short life.
    Another possible adapter .
    I believe your battery is weak. Do you have a Volt meter to test it on?

    Read this link:
  13. Charles Monday:

    Too bad those adapters cost more than the light meter itself cost me :) And too bad I don't have a Volt meter.
    That's true, the hearing aid batteries are said to have a short lifespan of about 2-3 months, but that's fine with me for now. Will try them out tomorrow, will keep you guys posted. Just want to find out if it's the meter that's broken or the batteries that are dead.
  14. With respect, why not let this relic go and get a newer meter? Used Sekonic 308/318/328 meters are less problematic, deadly accurate and take available-everywhere AAs. Film and processing aren't getting any cheaper, so why use a meter with iffy accuracy?
  15. to Charles Monday:
    So I tested the batteries. Their both's voltage output is 1.3V. So they're 0.05V weaker than they should be.
    I don't think this little difference could make such an impact on the meter's readings. Probably the meter itself is broke..
  16. I'm not sure about alkalines. As far as I know they give a pretty unstable voltage.​
    The meter movement is primarily a voltmeter, it is calibrated such that 1.35 volts causes the meter to swing full range. If someone has recalibrated the meter to use Silver Oxide batteries then you need 1.55 volts to get full range. If you then insert a wein cell (1.35V) then the meter will only swing to somewhere around 9 on the Luna-pro "low" scale. Alkaline cells run 1.55 volts new, are cheaper then Silver Oxide, and will maintain that voltage for longer than a wein cell will last, so it's a good candidate to test if your meter.
    If you get full range with an Alkaline cell at 1.55 volts (or a silver oxide battery) then your meter is probably been re-calibrated.
  17. to tom chow:
    Now I see what you mean. Thanks for taking your time to explain it :)
    Will try it with Alkalines or Silver Oxides tomorrow then.
  18. At 1.3V the battery is suspect to borderline.
    A volt meter does not put a battery under load.
    Its been over 20 years since I used a Wein cell and I do not remember what their low limit is but probably around 1.28V-1.29V. The Wein cell is suppose to have a stable voltage under load so a battery that reads less than 1.35V on a volt meter should be suspect especially if the piece of equipment it powers is not functioning correctly. If memory serves correctly the Wein cell does not last very long under regular equipment use.
    AA carbon zinc and alkaline batteries rated at 1.5V read 1.55V to 1.6V when new and 1.47 when "dead".
  19. I carefully recalibrated an old Gossen Lunasix to take Silver oxides. One year later the stupid thing died on me completely - open circuit in the meter coil and pretty much unrepairable. So +1 to getting something more modern and reliable.
    Almost anything digital is going to be a better bet. A Minolta Autometer or Quantum CalcuLight X or XP would be my first choices. I like the CalcuLights because they're small, can be read in the dark, are incredibly sensitive and with a good narrow angle for reflective metering. OTOH an Autometer xF can take flash readings as well, but eats batteries quite quickly. If only the Sekonic Studio L398 was about 6 stops more sensitive and took flash readings .......
  20. I'm not sure that I have followed the discussion properly, but would mention that the instructions with the Wein MRB state that the full voltage is not available immediately after the seal is removed. I was caught out by this, thinking the first time I used one that it was partly exhausted or defective out of the package, but finding it reached full voltage some time later.
  21. to Charles Wass:
    That's true. According to the instructions the paper tabs should be removed 30 minutes prior to use. Which I did, but it seems like it didn't help :)

    How long did it take you to reach the proper voltage of the batteries?
    to Rodeo Joe:
    Yes, the old light meters might sometimes be a pain in the rear :) But the digital meters are not as cheap as the analog ones. In the end I think I'll end up buying a digital one, but for now I'm looking to go for an old one.
  22. The Luna Pro F uses 9V batteries, reads reflective, incident, and flash. Has a 30° angle of view, its only drawback IMO.
  23. to Charles Monday:
    So I tried the 1.55V silver oxides today - the needle's behaviour stayed just the same. Must be the meter that's broken.
  24. Due servicing/explains why it did not cost much.

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