How to test my light meter?

Discussion in 'Lighting' started by stradibarrius, Jan 13, 2011.

  1. stradibarrius

    stradibarrius Member

    Messages:
    1,382
    Joined:
    Jan 17, 2009
    Location:
    Monroe, GA
    Shooter:
    Medium Format
    I have an old Gossen Luna-Pro F meter and I am trying to decide if I need to replace it.
    I have tried metering a scene with different cameras and the Luna-Pro and comparing the readings but I can't seem to draw any conclusions.
    I need a meter when I shot my RB67 and I am under the impression that I will get more accurate readings with a hand held meter than a TTL camera meter????

    Is there a method to test the Luna-Pro that is not so complicated that I need a degree in physics to use?

    I have been looking at the Gossen Digi-Pro F or the Sekonic L-308S but why spend the money if my old Luna-Pro is just as accurate? Maybe I just do not know how to use it correctly???
     
  2. markbarendt

    markbarendt Subscriber

    Messages:
    7,198
    Joined:
    May 18, 2008
    Location:
    Beaverton, OR
    Shooter:
    Multi Format
    One testway is to shoot a roll or two in a variety of situations using the meter "normally" and see if the results match your expectations.

    If the shadows and neutrals fall "in the right spot" it's probably working fine.

    This assumes a "standard" is used when you develope and print too.
     
  3. Jim Jones

    Jim Jones Subscriber

    Messages:
    2,276
    Joined:
    Jan 16, 2006
    Location:
    Rural NW Mis
    Shooter:
    4x5 Format
    A very crude check of reflected light meters is to hold the cell against a flourescent light. The reading should approximate the "sunny sixteen" exposure. A better check is to compare the meter in question against meters known to indicate the correct exposure. TTL meters can certainly be as accurate as most hand-held meters under average conditions. My first camera with TTL metering, a new Nikon F Photomic T, exposed Kodachrome a little more accurately than the Weston Master III I'd relied on for some time.
     
  4. jeffreyg

    jeffreyg Subscriber

    Messages:
    1,337
    Joined:
    Jun 12, 2008
    Location:
    florida
    Shooter:
    Medium Format
    I definitely don't have a physics degree. You could try reading off a matte black card, 18% gray card and a bright white card and compare the readings. Use the 18% gray card reading and set your camera to that and photograph each of the cards. Pick a scene or subject that has a decent range of tones and take an incident reading and take shadow, mid-tone and highlight readings adjusting for the zone each falls in. Develop the film and contact print the negatives -- see what you get. You could also test by photographing the gray card having over and under exposing by half stops up to say 2 1/2 stops each way as well as the gray card reading. Cover the lens and click off a couple of frames which will give clear negatives plus the film fog. Make a test (step) print from the clear neg. see where it turns black and use that for your printing time. Cut pieces of print paper and print each negative at the same enlarger height, time and lens opening (having labeled which stop of the film it was). Then see which one of the prints is the very closest or hopefully right on matches the gray card. That will be the ISO for that meter, film, camera and chemistry.
    I'm sure there are more scientific ways to do this but this is simple and doesn't take long to do. It could be repeated for other films. All of course B&W. It should give you an idea of the equipment's accuracy because it takes in to consideration the meter and camera. Which will carry over to color (I think). For color you could take transparencies once you have the accuracy of the equipment established and if they look right no worry.

    http://www.jeffreyglasser.com/
     
  5. Christopher Walrath

    Christopher Walrath Member

    Messages:
    7,114
    Joined:
    Dec 30, 2005
    Location:
    In a darkroo
    Shooter:
    Multi Format
    All you need is a grey card and a cell phone. Hold the grey card up in front of a subject and shoot it in black and white. Find a comparable tone with the grey card and meter both that part of the subject and the grey card and see what you get. You can then meter up and down zones and see how they fall by comparing to some sort of zone strip. You could make one of those by step printing in the darkroom or just buy one on the cheap.
     
  6. Rick A

    Rick A Subscriber

    Messages:
    7,384
    Joined:
    Mar 31, 2009
    Location:
    northern Pa.
    Shooter:
    Multi Format
    Take it outside and set it for ISO 100 and check the reading. It should be 1/100 or 1/125 @f16 in bright sunlight.
     
  7. johnnywalker

    johnnywalker Subscriber

    Messages:
    2,257
    Joined:
    Sep 23, 2002
    Location:
    British Colu
    Shooter:
    Multi Format
    I tested my Luna-pro F against my F100, FM3A and F65 using a grey card, making sure the cameras and the light meter were the same distance from the card, from the same spot, and that the light isn't changing between readings. Make sure the lenses on the cameras match the angle on the light meter (I think the angle on the Luna-pro matched an 80mm lens). Make sure all the adjustments on the meter are set to zero. After a lot of fooling around I was satisfied that the meter was very close, but there was even a small difference in the readings between the cameras so no readings matched up exactly.
    It wouldn't have to be a grey card, a towel would do just as well.
     
  8. jerry lebens

    jerry lebens Member

    Messages:
    256
    Joined:
    Feb 21, 2004
    Location:
    Brighton UK
    Shooter:
    Med. Format RF
    When it comes to using a light meter to create an accurate exposure regime, out and out accuracy is not as important as consistency.

    A light meter may be "inaccurate" by one or more stops and yet be perfectly good enough to create an operable exposure regime for a specific camera. It would only start to go wrong if you attempted to apply the same exposure regime to the camera using a different (maybe accurate) light meter.

    Creating an accurate exposure regime for a specific camera/light meter combination is, in effect, setting up a closed loop ; change one element and the loop breaks...


    Regards
    Jerry
     
  9. 23mjm

    23mjm Member

    Messages:
    450
    Joined:
    Jun 25, 2005
    Location:
    Rocklin, Cal
    Shooter:
    Medium Format
    I have used a Luna Pro-F and have used it for years. The meter has always performed flawlessly. I have tried to check it against other meter with limited results. You have to use a uniformly lit surface with a uniform color. The Luna Pro-F seem to have a pretty wide angle of view and that makes it hard to compare the reading with other meters. I got a Vari-Angle attachment which turns the meter into a semi-spot meter with a 7 & 15 degree angle of view. Since getting the Vari-Angle it has made comparing much easier, you know exactly what the meter is seeing. I have compared the meter with my Sekonic L-558, F100, N80, D300, D50 & 645ProTL and they are all within 1/3 stop of each other. (all those cameras have spot meters)

    One thing you need to do is zero in correctly, if you don't it will be off. Download the manual from the link below it tells you how to zero it.

    http://www.butkus.org/chinon/flashes_meters/luna-pro_f/luna-pro_f.pdf
     
  10. Diapositivo

    Diapositivo Subscriber

    Messages:
    2,080
    Joined:
    Nov 1, 2009
    Location:
    Rome, Italy
    Shooter:
    35mm
    Pardon my ignorance, my sensation is that the Gossen Luna-pro being a lightmeter with silicon blue sensor and integrated circuits, if it works, it should work just as factory new. Electronics don't "decay" with years. Selenium meters might lose sensitivity, I don't think SBC metres will show any kind of decay. So basically you are just checking for damage (needle out of calibration, "lens" displaced, who knows) that would put reading sensibly off.

    Anyway, for testing any lightmeter, I would just do this:

    - Find a large surface of uniform neutral colour: a white or a grey wall, that is, in shade (you must be in the shade as well). ("neutral" colour because CdS metres respond differently to different colours. SBC light meters should not have this problem, or have it very much reduced. Neutral is neutral anyway).

    - Measure reflected light from a distance of at least a metre, paying attention to the angle between lightmeter and wall. Keep it orthogonal to the wall.

    - Repeat measure with other cameras, and lightmeters, you have, from the same distance, paying attention to the angle.

    You need a uniform wall because this way, the larger or the smaller the angle of reading, you can compare readings;
    You need to be a bit distant, let's say at least 1 metre away, so that small differences in the angle of incidence does not play a role, and so that small lack in uniformity in the wall do not cause two different readings with two different lightmetre having a different angle of reading.
    You need a neutral wall so that you can use CdS metres for comparison.
    You need a wall in the shade so that it is more uniform, and there is less parasite light entering camera viewfinders (obviously you must be in the shade yourself).

    All that said, you should have the same reading from all lightmetres, and maybe from all cameras. You might see small differences between lightmetres, but no more than 1/3 EV. I would be more careful in comparing readings from cameras, as those can be influenced by lens vignetting or lens flare (you can have slightly different readings when you change your lens) and by parasite light entering the viewfinder if you have the sun at your back.

    I would not use grey cards, as a reading from a grey card is influenced by the angle formed by sun - card - camera. Grey cards are tricky, basically not very reliable, objects, unless you mount them on a tripod, stick them to a wall etc. so that you make the readings with exactly the same angle of reading and the same angle of incidence of the light. Also be careful not to project a shadow on the grey card if you use this method. With grey cards, a small movements in the card, changing the angle of reflection of the light, changes the light that is measured and this is possibly why comparing lightmetres is unreliable with grey cards, you can easily get small random differences.

    A wall in the shade is more reliable because it is in a fixed position, so you only have to check that the angle of the lightmeters-cameras that you compare is always the same.

    That's what I would do.

    Fabrizio
     
  11. dmr

    dmr Member

    Messages:
    494
    Joined:
    Sep 9, 2005
    Shooter:
    35mm
    Not to be a total snot here :smile: but I've been seeking the answer to this for several years now and I've about given up on it! I really fail to see, here in the 21st. century, how we have very simple, precise, and accurate means of measuring almost all physical quantities, but no convenient means of checking the accuracy of a light meter! :sad:

    For distances we have rulers and meter sticks. For mass/weight we have scales and balances. For time we have stopwatches accurate to the femtosecond! For temperature we have thermometers. Yadda yadda!

    It's really hard to believe that the only even-close-to-satisfactory answers to checking the accuracy of a light meter fall along these lines:

    1. Send it to a calibration lab for $$$ many times more than the meter is worth.
    2. Purchase a $$$$$$ laboratory standard light source.
    3. Compare it to a meter or camera which you believe is accurate.
    4. Go outside on a sunny day with a grey/white card or whatever.

    For all other quantities it seems like it's easy to trace to a known standard and verify measurement well within a fraction of a percent.

    Why is this so {f-bomb}-ing difficult with a light meter?

    (Yeah I know, b*tch b*tch b*tch!) :smile:
     
  12. SMBooth

    SMBooth Member

    Messages:
    1,132
    Joined:
    Jun 29, 2008
    Location:
    Melbourne, N
    Shooter:
    Multi Format
    OK I'll bite, whats the cell phone for....:whistling:
     
  13. OldBikerPete

    OldBikerPete Subscriber

    Messages:
    369
    Joined:
    Aug 29, 2005
    Location:
    Melbourne, A
    Shooter:
    4x5 Format
    The easiest way is to check it against a meter of known accuracy. If one is not available:

    As a first approximation, set the ASA at 125.
    On a sunny day point the meter at a field of green grass or an 18% grey card. The meter should read f/16 at 1/125sec. White skin should read f/22 at 1/25 sec on such a day.

    If it passes that test then expose a roll of 35mm color transparency film using the meter readings, bracketing exposures by 1/3 and 2/3 stop and recording your settings and meter readings.

    You should finish up with a meter calibrated to an accuracy of 1/3 stop WITHIN THE LIGHT RANGE YOU HAVE USED FOR CALIBRATION. If you want to use the meter for, say, night exposures, you will have to calibrate again for that.
     
  14. Sponsored Ad
  15. btaylor

    btaylor Subscriber

    Messages:
    148
    Joined:
    Dec 28, 2010
    Location:
    Los Angeles
    Shooter:
    Large Format
    Why not just send it in for calibration and KNOW that it's accurate? I like George at Quality Light Metrics, I send meters to him regularly, I think he charges about $60, two or 3 day turnaround.
     
  16. Peter Simpson

    Peter Simpson Member

    Messages:
    229
    Joined:
    Dec 10, 2010
    Location:
    Outside Bost
    Shooter:
    35mm
    There's very little to a Luna-Pro: a CdS cell, a meter and some resistors. So while it *may* be slightly out of calibration, it's still worth having calibrated. You can even adjust it to use silver oxide cells in place of the unavailable mercury cells: http://www.graphic-fusion.com/lunapro.htm

    Camera and Luna Pro both aimed at an evenly lit blank wall. Use a pair of incandescent lamps on a dimmer to light the wall evenly.
    Compare readings. I bet it's still pretty accurate, the used one I bought was. But, there are places that will recalibrate it for you.
     
  17. michaelbsc

    michaelbsc Member

    Messages:
    2,106
    Joined:
    Dec 18, 2007
    Location:
    South Caroli
    Shooter:
    Multi Format
    All your other examples are a macro measurement. But a light meter is measuring a phenomenon down in the quantum range.

    If the light source was intense enough to heat a pail of water, you could measure the temperature easily. But that's a pretty intense light beam. Probably EV 5673 or something preposterous.
     
    Last edited by a moderator: Jan 13, 2011
  18. ic-racer

    ic-racer Member

    Messages:
    7,401
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    If you are not using it correctly, any test you do is going to fail. Do you have the manual? There are manuals at http://www.butkus.org/
    (I will admit that a friend got a Lunapro F, and even with the manual, I had some difficulty showing him how to use it; its not the easiest meter to figure out. Its a good meter but its almost like trying to show a beginner how to use an SEI :smile: ).
     
    Last edited by a moderator: Jan 13, 2011
  19. 2F/2F

    2F/2F Member

    Messages:
    8,005
    Joined:
    Apr 29, 2008
    Location:
    Los Angeles,
    Shooter:
    Multi Format
    To test any reflected meter for accuracy, you can go outside on a sunny and entirely clear day, near the middle of the day. Meter a grey card in direct sunlight, with the card at a 45 degree angle to the Sun; make sure the card is free of glare and shadows. Fill the metering pattern entirely with the card. The reading should be close to being half a stop brighter than a sunny 16 exposure (i.e. the meter will tell you to use 1/2 f stop higher than what sunny 16 sez).

    In other words, to get the correct exposure from a grey card when actually shooting, you take the reading and open up half a stop. In this case, the meter should read f/16-1/2 when shutter speed is set 1/3 step above EI (e.g. shutter of '500 with 400 film).

    Better yet, so you don't have to monkey with a gray card and the 1/2 stop adjustment off of it, just use your Luna Pro as an incident meter. Point the dome at the sun in the same conditions I described above. You should get a reading that says pretty much exactly '500 at f/16 with a 400 film, '125 at f/16 with a 100 film, '60 at f/16 with a 50 film, etc.
     
    Last edited by a moderator: Jan 13, 2011
  20. ic-racer

    ic-racer Member

    Messages:
    7,401
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    Actually we have a fantastic means. Better than a 'standard candle' and better than the sun.

    FILM! Yes, the ISO of every batch of Kodak film is exactly tailored to a specific ISO (the one on the box) with trimming dyes (per info from our own PE).

    I calibrate my sensitometer with Kodak film.

    Since you need a camera to expose film, you are right in that you can't easily calibrate the meter and camera separate. But since you usually use them together, that should not be a problem to calibrate them as a system.
     
  21. CPorter

    CPorter Member

    Messages:
    1,662
    Joined:
    Feb 2, 2004
    Location:
    West KY
    Shooter:
    4x5 Format
    I have the Luna Pro F, it's a good meter.

    First, check that the meter is properly "zeroed". Note, next to the 3 stops "UNDER" designation, notice that there is a small green mark. Remove the battery and check that the needle rests on that green mark. If it does not, you need to turn the zero adjusting screw on the back of the meter until it does; the screw has a curved line above it with arrow marks on each end. Use a small screwdriver.
     
  22. Galah

    Galah Member

    Messages:
    481
    Joined:
    Feb 4, 2009
    Location:
    Oz
    Shooter:
    Multi Format
    I have a boxfull of "old" meters (as well as many cameras with built-in meters).

    The only two which actually agree with each other are my Gossen Lunasix-3 and my Gossen Lunasix Pro-F :tongue:.

    Despite this, most of my exposures are quite acceptable.:lol:

    Modern (negative) films are amazingly tolerant. :smile:
     
  23. jerry lebens

    jerry lebens Member

    Messages:
    256
    Joined:
    Feb 21, 2004
    Location:
    Brighton UK
    Shooter:
    Med. Format RF
    It's been said before in other threads but, whilst the 'sunny 16' method works well in some climates, it doesn't hold here in the uk (and, I'd guess, the rest of northern europe).

    Of course, not intending to disparage a technique that clearly works well in places like Oz and the US. Simply a heads up for anyone from northern europe thinking of trying it.

    ********

    It's not a technique I'd use but, if you must calibrate a meter, I like i-c racers suggestion of using the film to 'backwards calibrate'.

    It reminds me of something said to me by one of the boffins at Ilford, during a get together of the 'Ilford Master Printers', when I asked him a question about making film speed tests towards a zone system... "Why on earth do you want to do that? If you develop our films according to the instructions, they're already accurately calibrated."
    Not sure I fully agree with him, because each photographer works under slightly different conditions, but it certainly gave me pause for thought...

    Regards
    Jerry
     
  24. markbarendt

    markbarendt Subscriber

    Messages:
    7,198
    Joined:
    May 18, 2008
    Location:
    Beaverton, OR
    Shooter:
    Multi Format
    Doesn't even work all over the US, Sunny 16 for me is Sunny 11.
     
  25. picker77

    picker77 Subscriber

    Messages:
    122
    Joined:
    Feb 24, 2009
    Location:
    Oklahoma, US
    Shooter:
    Multi Format
    Don't give up too soon on that Luna Pro. I have used a Luna Pro F for years, and love it. I actually prefer using it to my L-608. I got enamored a while back with digital readouts, spot metering, and AA Zones and paid a bunch of money for the Sekonic, and I'm sorry I did. The Luna Pro F's instant analog F-stop under/over exposure indication and ability to see the entire range of available optional apertures and speeds at one glance is so simple and fast to use it puts the fancy L-608 to shame, at least in reflective mode. Great meter. Mine reads less than 1/3 stop different than the L-608 under quick and dirty reflective use, which is how I usually meter with old cameras anyway. And in the reflective mode it's FAR easier and faster to use than turning the L-608's cumbersome optics sideways and pushing buttons right and left, and then turning the thing back and studying a readout to see what happened. Angle of view is pretty wide, but so what? Unless you really need spot metering for zone work or do a lot of studio work and need automatically accumulated and computed flash readings, the Luna Pro F is plenty of meter.
     
  26. flatulent1

    flatulent1 Subscriber

    Messages:
    1,326
    Joined:
    Oct 7, 2008
    Location:
    Seattle USA
    Shooter:
    Multi Format
    If you have a local camera repairman, he should have a gizmo that puts out a known light value and can quickly test the accuracy of your meter.