Exposure Meter Linearity

Discussion in 'Exposure Discussion' started by dancqu, Mar 8, 2005.

  1. dancqu

    dancqu Member

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    Is meter linearity an issue? I don't recall the matter having
    been mentioned. I thought I'd calibrate some gray cards
    for zone testing of film. I worry that my Sekonic L-228
    may not be all that linear. How would one test? Dan
     
  2. chuck94022

    chuck94022 Subscriber

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    Actually I thought they were logarithmic...

    Maybe you can compare its readings to another meter on the same subject. Alternatively you could measure it against a calibrated luminance I guess. Probably something a camera shop can do for you.

    -chuck
     
  3. eumenius

    eumenius Member

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    Hello Dan,

    the meter linearity can be an issue, and a bad one. It's very easy to see if the measuring cell response is linear with an usual 4X neutral density filter, placing it before the meter window and take readings with it and without it. These filters are usually well calibrated and absorb the light evenly across the whole spectrum, so the colour won't be a problem. Just try it in under a different lighting conditions, and you'll easily see if your meter says something wrong. Don't mix artificial and sky light, the silicon measuring cell is usually much more optimistic about reds, in spite of blue filter placed before it - its high sensitivity to the red end can't be fully compensated without great loss in sensitivity.

    Cheers,
    Zhenya
     
  4. Ed Sukach

    Ed Sukach Member

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    I'm not quite sure what you mean by "Linear". If you mean reading properly at all levels of energy ... that is a valid concern.

    I haven't calibrated photographic "Exposure Meters", but I have calibrated something one or two magnitudes more accurate and sensitive - Cascade Photometers. That calibration required an Optical Bench (or similar) and a Standard Lamp ... we had Intralaboratory Standard Lamps, rotating to N.B.S. They would be powered by a constant AMPERAGE power source (not voltage ... the light output is three times more sensitive to amperage than voltage). Those photomultipliers read in Watt-seconds (or milli-watt seconds) and reacted to energy, in the form of light. An "exposure meter" does the same thing, and mathematically "massages" the readings to be useful in photography.
    Someone tried to use one of the original Honeywell 1/21 exposure meters in our optical work ... I wrung that puppy out, and, after an involved conversation with Honeywell, determined that it was within manufacturer's specifications; +/- one half "stop" ... altogether too coarse for what we were doing, but fine for photography.

    That exercise resulted in valuable information ... I cringe whenever I hear a photographer claim that he ALWAYS exposes his film, transparency or negative, "within a tenth of a stop". Yeah -- right. He would have absolutely no way of knowing if that was true. So far, the best accuracy claim I've heard of was +/- 1/3 "stop" and I'm wondering about the truth of that, using a handheld meter.
     
  5. Loose Gravel

    Loose Gravel Member

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    Linear on log graph paper.

    It is hard to design analog circuitry that will take a log over 6 orders of magnitude (20 stops is 2^20 is about 1,000,000). The Pentax spot tends to have a long curve in them. Some worse than others. Calibration can reduce, but not eliminate. No doubt this is true of many meters. Some newer meters are very linear because the log is done with a microprocessor and the photonic reading is done with a very linear sensor. Ed is right though, there is no way to get your exposure to 1/10 of a stop. Too many other variables (film response, film speed, meter/film spectral response, shutter speeds, f-numbers)

    So, yes, linearity can be an issue and the way to test is with ND filters. Over a short range, linearity will be pretty good, but over 10 stops you might see some variation. If you are worried about it, give it another stop or bracket.
     
  6. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    ND filters are a good way to test. Usually the problem is that the meter becomes unreliable at very low light levels.

    You can see this with the meter in a 35mm camera--try seeing if you get the same reading with the lens wide open as you get in stopped down mode with the lens at f:22. In most cases, the readings will be different. It's not as much of a problem with handheld meters, but if you think you're getting questionable readings, then it's worth testing.
     
  7. Walter Glover

    Walter Glover Member

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    Light meter linearity is a crucial factor in using a light meter in varying levels of illumiunation or luminance and should be checked prior to purchasing a meter if at all possible.

    I took one of those black card sheets for displaying a selection of 6x6 trannies and in each aperture I placed a different Wratten ND gel - all the way to an ND 3.0.

    Over that 10 stop range both my Zone VI modified Pentax Digital Spotmeter and my Minolta V perfromed within 0.10 of a stop over the entire range. Concversely there are major brands that I have tested in shops which have not even been acceptably linear in their response even over the 4 or 5 stop range of most colour photography for reproduction.

    It is a simple test to carry out once the sheet is made up and I regularly check to ensure that everything is happening just as anticipated. I work primarily for interior designers and architects and the values in an interior can be very critical in maintaining the design integrity of tonal differences.

    Another worthwhile test to carry out is to direct the cell at an electric stove element in a darkened room. Take a reading and then turn on the element and read again BEFORE any visible redness shows in the hotplate. If the meter indicates a difference in exposure then the IR screening is not satisfactory.
     
  8. dancqu

    dancqu Member

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    The EV scale on my Sekonic goes 3, 4, 5, ... ,18. Logs would run
    0.0, 0.3, 0.6 and on. Each of those logs is double the light as is
    each suceeding EV. The log of 2, twice as much light, is 0.3.
    For some reason the scale does not read in logs;
    convention I suppose.

    Measureing against a light source I think a good idea. The
    source need not be calibrated as we are not interested in the
    absolute value of the light. A meter which measures the light
    intensity in lamberts, or foot candles, or ... , likely should
    use a standard as light source. We make our correction
    when setting the ISO to this or that on our meters
    for our personall EI.

    The inverse square law as applied to a point light source in
    the vacuum of deep space would be extremly accurate. Something
    terrestrial of that sort might be plenty good enough. Dan
     
  9. dancqu

    dancqu Member

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    I suppose you back-lighted those 10 gels? That method may work
    well with my 8 degree L-228 Sekonic and many of the camera based
    meters in spot mode. I'd think those 10 ND gels cost something?

    Perhaps a point light source and measured distances will work
    well. I think a clear filament bulb against large dead black
    background worth a try. Any opinions? Dan