Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,283   Posts: 1,534,991   Online: 1012
      
Page 2 of 4 FirstFirst 1234 LastLast
Results 11 to 20 of 31
  1. #11
    dmr
    dmr is offline
    dmr's Avatar
    Join Date
    Sep 2005
    Shooter
    35mm
    Posts
    493
    Not to be a total snot here 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!

    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!)

  2. #12
    SMBooth's Avatar
    Join Date
    Jun 2008
    Location
    Melbourne, North/West
    Shooter
    Multi Format
    Posts
    978
    Images
    7
    Quote Originally Posted by Christopher Walrath View Post
    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.
    OK I'll bite, whats the cell phone for....

  3. #13
    OldBikerPete's Avatar
    Join Date
    Aug 2005
    Location
    Melbourne, Australia
    Shooter
    4x5 Format
    Posts
    348
    Images
    3
    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.

  4. #14

    Join Date
    Dec 2010
    Location
    Los Angeles
    Shooter
    Medium Format
    Posts
    100
    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.

  5. #15

    Join Date
    Dec 2010
    Location
    Outside Boston, MA
    Shooter
    35mm
    Posts
    179
    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.

  6. #16
    michaelbsc's Avatar
    Join Date
    Dec 2007
    Location
    South Carolina
    Shooter
    Multi Format
    Posts
    2,106
    Images
    5
    Quote Originally Posted by dmr View Post
    Why is this so {f-bomb}-ing difficult with a light meter?
    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 michaelbsc; 01-13-2011 at 08:43 PM. Click to view previous post history.
    Michael Batchelor
    Industrial Informatics, Inc.
    www.industrialinformatics.com

    The camera catches light. The photographer catches life.

  7. #17
    ic-racer's Avatar
    Join Date
    Feb 2007
    Location
    Midwest USA
    Shooter
    Multi Format
    Posts
    6,179
    Quote Originally Posted by stradibarrius View Post
    Maybe I just do not know how to use it correctly???
    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 ).
    Last edited by ic-racer; 01-13-2011 at 08:46 PM. Click to view previous post history.

  8. #18
    2F/2F's Avatar
    Join Date
    Apr 2008
    Location
    Los Angeles, CA
    Shooter
    Multi Format
    Posts
    8,008
    Images
    4
    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 2F/2F; 01-13-2011 at 09:01 PM. Click to view previous post history. Reason: added info and fixed typos
    2F/2F

    "Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."

    - Rob Tyner (1944 - 1991)

  9. #19
    ic-racer's Avatar
    Join Date
    Feb 2007
    Location
    Midwest USA
    Shooter
    Multi Format
    Posts
    6,179
    Quote Originally Posted by dmr View Post
    Not to be a total snot here 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! )
    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.

  10. #20
    CPorter's Avatar
    Join Date
    Feb 2004
    Location
    West KY
    Shooter
    4x5 Format
    Posts
    1,662
    Images
    24
    Quote Originally Posted by stradibarrius View Post
    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???
    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.

Page 2 of 4 FirstFirst 1234 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin