Meter accuracy

Discussion in 'Darkroom Developing and Printing' started by Terry Davis, Sep 20, 2007.

  1. Terry Davis

    Terry Davis Guest

    This really isn't a darkroom question but since most who read these
    posts are competent photographers I will ask.....how do I calibrate or
    check the accuracy of a handheld light meter? I use it in the ambient
    light mode.
     
    Terry Davis, Sep 20, 2007
    #1
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  2. Well, the Really Obvious answer is the one you no doubt thought of:
    check it against a known good meter. If you can find one, that's really
    all you need to do. (And if it turns out to be off, you can find out how
    far off it is and adjust your settings to compensate.)
     
    David Nebenzahl, Sep 20, 2007
    #2
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  3. Terry Davis

    Peter Guest

    On a really clear day, when the sun is high enough
    that your shadow isn't much longer than you
    are tall, a grey card shouldn't be more than
    a third stop off f/16. I have checked this many
    times and it is remarkably repeatable.
    If it is an old selenium meter it may have gone
    non-linear. Check it under low light and sunny
    f/16 conditions. If it has two ranges then find
    some light which is at the top of the lower
    range and the bottom of the upper range and
    see if it agrees with itself.

    I have a 50 year old GE meter which is actually
    pretty accurate, but many old selenium meters
    have not fared as well.

    Old CDS meters which were meant to take mercury
    cells may be non-linear if the meter has been
    inexpertly adjusted to cope with a different voltage.

    Peter.
     
    Peter, Sep 20, 2007
    #3
  4. Take it outside on a sunny day, set for ASA 125, should indicate
    f11-f16 at 1/125th

    The other method is to compare it to _two_ other light meters.
    If you only compare it to one, and they don't agree, you
    don't know which is wrong.
     
    Nicholas O. Lindan, Sep 20, 2007
    #4
  5. If it does seem to be off, check it in reflected light mode. If it's
    correct in reflected and off in ambient, look around to make sure there
    is not a problem with the switch.

    Most meters need some adjustment in ambient light mode. Usually this is
    done by a small switch which is pressed by putting on or moving in place
    the ambient light sensor. It may be dirty or broken.

    Keep in mind that in ambient light mode it has to be pointed at the light
    source. If you are in the shade, pointing it up may not work.

    It may also be your camera. Still cameras with built in meters are calibrated
    so that they expose properly, not meter accurately. Lenses are calibrated
    in "F" stops which are not always 100% consistent between lenses.

    Movie cameras are calibrated in "T" (transmission) stops which are.

    Geoff.
     
    Geoffrey S. Mendelson, Sep 20, 2007
    #5
  6. Terry Davis

    Henry (k) Guest

    Dnia Wed, 19 Sep 2007 20:53:24 -0400, Terry Davis napisal(a):
    Correct measure is the one which gives you correct photos - not the
    one which is the same as for other light meters.
    Buy roll of Velvia, put it in camera and make a few photos using
    your light meter and bracketing. Develop and look at results.
    This is a base - later make test for your favorite process and film (BW?)
    and tune it up - details you find in google ("Zone System" +calibrating).

    Greeting from EU,
    Henry
     
    Henry (k), Sep 20, 2007
    #6
  7. Terry Davis

    Rob Morley Guest

    But it's handy if the setting on the meter is the one that works on the
    camera, without having to apply some form of compensation. If a meter
    is linearly off relative to a camera you just have to tweak the film
    speed, but if the discrepancy isn't linear it's a pain to translate the
    readings.
     
    Rob Morley, Sep 20, 2007
    #7
  8. Terry Davis

    Henry (k) Guest

    Dnia Thu, 20 Sep 2007 11:48:22 +0100, Rob Morley napisal(a):
    Of course - I assumed that light meter has some calibration screw
    and keeps linearity. If not then I suggest give it to children as
    a toy ;-)
    Some old light meters have only EV range - then it's easier
    to print table/draw graph with translation.

    Greetings,
    Henry
     
    Henry (k), Sep 20, 2007
    #8
  9. Terry Davis

    Ray Guest

    Bracket some exposures in a particular scene, then when you determined the
    correct exposure turn the adjusting screw to indicate that exposure. Forget
    about "zeroing it in" covering the cell and turning it to "zero".

    Ray
     
    Ray, Sep 20, 2007
    #9
  10. Dr. Richard J. Henry in the second edition of his book, "Controls in Black
    and White Photography" gives a procedure for calibrating a light meter. It
    is on pages 176-179 and are intended for a spot meter.

    Calibrating a light meter by comparing it with others is fraught with
    difficulties. At one time, I had four different light meters and they all
    worked well. On a clear day, I put a grey card on my front lawn facing up. I
    metered the card with each light meter and got readings from two stops over
    to two stops under the middle value. Now I was surely not getting 4 stop
    errors in my exposures. It turns out that each light meter had a different
    color sensitivity. One was a cadmium sulfide meter, one was "silicon blue",
    one was whatever is in Fred Picker's modified and color corrected spot meter
    (the digital model), and one is a Luna-Pro F. I tried the experiment indoors
    with 3400 degree incandescent light bulbs, and the meters were off in
    different amounts.

    In practical work, I notice that colors in nature are not usually very
    saturated, so the color differences make less difference to the film than to
    the light meters.
     
    Jean-David Beyer, Sep 20, 2007
    #10
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