Convert camera meter to incident meter?

Discussion in 'Photography' started by Martin, Oct 2, 2005.

  1. Martin

    dadiOH Guest

    Both may have given *acceptable* exposure but one was either under or
    over exposed.
    If you are getting more then yours is. Or something in the way you are
    using them is.

    Think about it...what possible good are two metering systems if one is
    telling you to increase/decrease exposure by a factor of four times
    relative to the other meter? How would you know which to use? Answer
    is, you wouldn''d have to let the films latitude cover the error
    as you did with your daughter. And you don't get 4x latitude to play
    with when using transparency film.

    I've been using meters for close to 60 years. All kinds...extinction
    meters, reflected meters, incident meters, spot meters, in camera meters...expensive meters...Weston, GE (loved the PR-1),
    Norwood, Balcar, Gossen, Seconic, and a whole bunch more. I simply
    can't imagine how you can possibly use two different but properly
    calibrated meters and feel that each is giving you a proper reading when
    they are two stops apart. Not under the conditions that are being


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    dadiOH, Oct 4, 2005
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  2. Martin

    Colin D Guest

    This is most definitely a wrong statement, and indicates a wrong idea of
    exposure measurement. An 18% gray card is designed to reflect 18% of
    the incident light falling on the card, and a reflected-light meter is
    calibrated accordingly. An incident-light meter has an integrating
    diffuser which is designed to allow for incident light at different
    angles of incidence, e.g. partial side lighting, and the meter is
    calibrated to provide the correct exposure for the average subject,
    which has an 18% average reflectance.

    Therefore a reflected-light reading from a gray card AND the reading
    from an incident meter will be the same. No difference. A moment's
    thought will show that there cannot be two *correct* readings that
    differ by two stops, as you stated.

    The original idea behind the concept of the incident meter was to
    eliminate the possibility of incorrect reflected readings when the
    subject did not represent an average gray, as with incident vs reflected
    readings of snow or coal. A reflected-light meter will underexpose snow
    and overexpose coal, rendering both the same shade of gray, whereas the
    incident meter will place the tones correctly.

    BUT, when the subject DOES reperesent an 18% gray, both meters will
    yield the *same* reading.

    Colin D.
    Colin D, Oct 4, 2005
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  3. Martin

    Colyn Guest

    First off.. was the lighting even?? Second..You did right with
    incident measurement however 7-8 inches is too close to get an
    accurate reflected measurement based on your 30 degree FOV. You only
    measured the light on a small part of the card therefore it cannot be
    correct. You have to measure the same FOV in both tests...otherwise
    you are doing nothing more than a spot meter reading.

    In addition low light is not an accurate way to test. You need enough
    even lighting to get medium speed measurements.

    I conducted the same test as you with both my Minolta Autometer and
    Weston Ranger 9 taking into consideration FOV of each meter (10 degree
    for the Minolta and 18 degree for the Weston) and used a 500w tungston
    light set 4ft from the card to give even lighting.
    1/125th f/8 incident
    1/125th f/16 reflected
    Both reading with the Minolta were just slightly on the high side of
    each full reading
    1/125th f/8 incident
    1/125th f/16 reflected

    The Ranger 9 was calibrated to factory specs back in June of this year
    by QLM. The Minolta was sent back unadjusted with a note that it was
    in spec..
    Colyn, Oct 4, 2005
  4. Unfortunately, your results say the opposite. I can't tell what
    you are doing wrong, but it is a *fact* that if you are indeed
    measuring 2 or 3 stops difference between an 18% grey card
    reflectance reading and an incident light reading, *something*
    is wrong. The precise specification (if the meters are
    calibrated to ANSI standards) would be *exactly* 1/2 f/stop.
    Not all incident light meters are calibrated exactly to ANSI
    standards though, and furthermore the usual method for measuring
    reflected light from a grey card will reduce the brightness by
    almost 1/2 stop due the angle of illumination. Therefore the
    actual measured difference might be anything from 1/2 f/stop to
    0 f/stop.

    A two or three stop difference is *clearly* an error of some kind.
    Technically, if your meters are calibrated to ANSI standards the
    incident meter would read exactly the same as a reflected meter
    on a 12% grey scene. An 18% grey card reflects 1/2 stop more
    light than a 12% grey scene.

    Note that real incident meters may be in fact calibrated to
    anything from 12% to 14%, resulting in a slightly adjusted
    reading. And of course real reflected measurements using a grey
    card will be at an angle, and the reflectance will be more like
    12.5% than the 18% a head on reading would give, but at the same
    time that reduces the reflected reading by nearly a 1/2 stop,
    making it almost identical to the incident reading.
    Neither have I, so I don't see why you would do whatever it is
    you are doing.
    You get "proper exposure" when you use 2 stops difference?????

    I'll grant that 1/3 of a stop is often hard to notice, and in
    many cases 1/2 stop makes little difference. But 1 stop is a
    very distinct difference.

    How you can say that "both gave proper exposure" when they are 2
    stops different is beyond me! That is 4 times as much light on
    one of them as the other. Even if you balance it, one is too
    dark by 1 stop and one is too light by 1 stop. Or, more likely,
    one of them is right and the other is either extremely
    under exposed or extremely over exposed.
    I think you didn't know what they were doing. Given what you've
    stated above it is doubtful that you would be able to make valid
    They are *absolutely* *not* what you say they are. They are
    "correct" in the sense that the meters are no doubt giving you
    valid figures, but you are not understanding them. (Don't be
    horribly insulted or upset about that... it *is* a very
    technical aspect, and not one that comes easy to everyone or
    even quickly to those who do understand it thoroughly. With
    time and discussion, we can probably find ways to greatly
    improve your basic understanding.)
    You mean the overall exposure ratios, which is how much light is
    reflected into the lense. If you want to measure lighting, use
    incident metering.
    It is rather easy to demonstrate that the *precise* difference
    should be 1/2 stop. That is the difference between a 12% grey
    scene and an 18% grey card. ANSI PH3.49-1971 says a reflected
    light meter should be calibrated for 12% reflectance, +/- 2%.
    In fact, many meters are intentionally calibrated with a value
    slightly higher than 12% reflectance. 12.5%, 13% and 14% are
    often quoted figures, and hence the difference between that and
    an 18% grey card reading might commonly be less than 1/2 stop.

    Here are some URL's that will help you get a better grasp on
    this topic:

    "For an ANSI calibrated meter ... the luminance value used
    translates into a reflectance of 12%. ... one half stop
    lighter than 18%, by the way."

    "I anticipated a roughly 1/2 stop difference in exposure and
    got it when comparing a used Minolta Autometer IIIf and my
    Nikon F4s's and Nikon FA."

    Now, after you read all of that, and get the idea that an 18%
    grey card *will* in fact be 1/2 stop brighter than a 12% "middle
    grey" scene... let me point out that it won't happen that way
    in practice! Instead, the lighting will be at some angle and
    the reflected metering will be at some angle. If the two are at
    about 45 degrees... the brightness of an 18% grey card will be,
    wha-laa, about 12.5%! The gray card and a reflected meter would
    exactly agree with the incident meter.

    So, the *typical* reflected light reading from an 18% grey card
    will be less than 1/4 stop different than almost all incident
    light meters!
    Floyd Davidson, Oct 4, 2005
  5. Can you cite something to indicate where you are getting these
    Floyd Davidson, Oct 4, 2005
  6. [Interesting description of a valid test deleted for bevity.]
    Whether a spot meter is used or not, makes *no* difference. As
    long as *only* the gray card is measured, the reading should be
    the same. The entire gray card is 18% gray, and as long as the
    only object within the FOV of the meter is the gray card it
    makes no difference whether the whole card is measured, or just
    1/10th of it. The intensity of the light reflected will be the
    same if the illumination is the same.

    However the angle of the light and the meter does make a
    difference, and typically might be about 45 degrees from the
    light to the meter, which would reduce the light reflectance by
    perhaps just under 1/2 stop.
    All he needs to do is get the reading away from either end of the
    I have no idea what you are doing wrong, but your readings are
    *clearly* about 1.5 stops off.

    The reflected reading off an 18% grey card should, if *very* carefully
    done, be about .5 stop brighter than an incident light reading.

    If it is true that your meters are in calibration, then *you*
    are doing something wrong. That is *not* a guess, it is a
    technical fact that is easily demonstrated. The ISO/ANSI
    documents describing light meters work out to 1/2 stop brighter
    reading from an 18% gray card than an incident reading of the
    light shining on it. Your 2 stop brighter is a gross error.
    Floyd Davidson, Oct 4, 2005
  7. Martin

    McLeod Guest

    In point of fact, both the Gossen and the Minolta meters are not
    calibrated for 12% grey, they are calibrated for 18% grey. AFAIK, the
    Sekonic meters are the only ones calibrated for 12% grey. So there
    should be no difference between the incident and reflected grey card
    McLeod, Oct 4, 2005
  8. Martin

    McLeod Guest

    Please explain to me your theory behind a full 2 stop difference,
    because quite frankly, you don't seem to have a clue.
    McLeod, Oct 4, 2005
  9. Martin

    dadiOH Guest

    Sell them both and use the bright sun/hazy sun/cloudy bright/cloudy
    dark/open shade routines. You'll get far more accurate exposures.


    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at
    dadiOH, Oct 4, 2005
  10. Martin

    Martin Guest

    Thanks - yes, I look forwards to studying the thread fully and properly - so
    far it seems fairly friendly and very interesting.

    Martin, Oct 4, 2005
  11. Martin

    Colyn Guest

    A 2 stop under/over exposure using slide film will ruin the photo. All
    test done with both meters using both modes give perfectly exposed
    90% of my work is done on slide film..
    All of my testing is also done on slide film since it has a narrow
    exposure latitude. I never test with print film especially since it
    can give acceptable results with as much as 4 stops difference
    Because all of my film tests are conducted shooting real subjects not
    gray cards.
    Colyn, Oct 5, 2005
  12. Martin

    Colyn Guest

    With incident metering you don't measure the 18% reflectance of a gray
    card. You read the light falling on the gray card.

    Reflected metering gives an inaccurate measurement if you rely on its
    18%. It yeilds middle gray (muddy gray) in B&W and desaturated
    mid-tones in color. Incident metering does not read 18% reflectance it
    reads the whole light falling on the subject. However if you allow the
    meter to give a reading based on 18% it to will be inaccurate too. You
    should use 36% with incident light..
    Colyn, Oct 5, 2005
  13. Martin

    Colyn Guest

    Agray card is an inaccurate method to use so we both are going about
    it wrong..
    Most people use a small light source to light the gray card so in most
    cases it will not be evenly lit..

    The only time I use 18% reflectance is when measuring lighting ratio
    since 18% is inaccurate. I instead use 36% incident.
    Since I use slide film 90 % of the time and they come out properly
    exposed, I would have to say my readings are correct. A 2 stop
    over/under will render a slide useless..
    Colyn, Oct 5, 2005
  14. Martin

    Colyn Guest

    Please explain how you can point a meter at a gray card or any other
    object that reflects 18% of the light falling on it then point this
    same meter at the light source that will usually be 3+ stops brighter
    and come up with only 1/2 stop difference...
    Colyn, Oct 5, 2005
  15. Martin

    dadiOH Guest

    Colyn wrote:

    You originally said, "Actually I conducted several different meter
    tests.. I even used my
    daughter as a subject. I still got approx 2 stops difference between
    the two methods and both gave proper exposure"

    Now you say...
    Those last two sentences of yours are diametrically opposed. The first
    one is more or less correct; therefore, you couldn't - by your own
    statement - have gotten "perfectly exposed slides" when the exposure
    difference was two stops.
    The grey card isn't meant as a subject, it is a way of determining what
    an exposure should be for a subject of average reflectivity when
    illuminated by the same light that was measured from the grey card.


    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at
    dadiOH, Oct 5, 2005
  16. Martin

    dadiOH Guest

    Yes, we all know that.
    Oh, have your head screwed on oddly. When you point a
    reflected meter at something - *ANYTHING* - it busily averages the
    reflective characteristics of the objects to - guess what? - 18%. If
    the scene has a general mix of light/middle/dark areas the result will
    *not* be muddy. If you happen to be photographing a polar bear in a
    snow field then yes, the result will be has to have the
    experience to know what a meter is doing and the wit to know when and
    why it can be incorrect.
    More or less right.
    I haven't a clue as to what you are talking about here. What do you
    mean, "allow the meter to give a reading based on 18%", and, "you should
    use 36% with incident light"? There are no percentages...incident
    readings are reading the light falling on the subject, remember?


    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at
    dadiOH, Oct 5, 2005
  17. Martin

    dadiOH Guest

    JeeSUS! You don't point the *same* meter at the light source, you point
    an *incident* meter at the light source. If you only have one meter you
    put an incident accessory on of those little white things that
    both diffuse and transmit way less light - referred to above as "an
    integrating diffuser". If you only have one meter and don't have a
    little white thing you are SOL.

    We don't usually get trolls here...


    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at
    dadiOH, Oct 5, 2005
  18. You post more bullshit than Ferdinand.

    The gray card has 18% reflectance. Making a reflected light meter
    measurement from it produces *verifiable and repeatable* results.

    When done correctly, it is essentially the *same* as making an
    incident light reading.

    The method that is *not* accurate is to take a reflected light
    meter reading on a scene of unknown reflectance and then
    *guessing* at the average reflectance of the scene... or worse
    yet, not even bothering to think about it and instead simply
    assuming that it is very close to what the meter is calibrated
    for. Obviously that is the way the vast majority of photographs
    are taken, and that is why the average set of photographs have
    an exposure range of +/- 1 full fstop from correct.
    Oh? "A small light source" like the sun? A cloudy sky? Light
    reflected off the walls and ceiling of a room?

    In fact, hardly anyone illuminates a gray card with a "small
    light source".
    In what way is 18% inaccurate? And what on earth do you mean by
    "36% incident". Are you telling us that your incident meters
    are calibrated for 36% reflectance?

    Now we finally see why you get two stops difference between your
    incident and reflected meter readings! You've miscalibrated
    your meters!
    You've already said that you use both meters, get a 2 stop
    difference, and that you still get proper exposures...

    You obviously are not accurately describing what you do and what
    the results are.
    Floyd Davidson, Oct 5, 2005
  19. Then you are saying that you *clearly did not have a 2 stop difference"
    in the exposure actually used.

    Again, it is clear that you are not correctly using your
    exposure meters. Or at least what you are describing is not a
    correct usage...
    Then how do you explain your claim that your meters give you correct
    exposures when you say they are 2 stops different on the same scenes?
    And because you are not accurately describing either the tests nor the
    Floyd Davidson, Oct 5, 2005
  20. You clearly do not understand what the significance of the two
    different readings is, or how they relate to each other. I
    provided a couple of URLs previously, why not read them?
    No incident light meter that I've ever heard of is calibrated to
    match a 36% scene reflectance.

    Can you provide some kind of a cite or reference for that?
    Floyd Davidson, Oct 5, 2005
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