B&W Zone System - why is the colour of the spot metered area unimportant ?

Discussion in 'Exposure Discussion' started by PeterB, Jan 6, 2012.

  1. PeterB

    PeterB Subscriber

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    I have searched for hours trying to find the answer to this question.

    In the Zone System (for B&W) - why is the colour of the spot metered area unimportant?

    A hypothetical example - the reflectivity of indigo is much less than yellow. If I had two scenes , one with indigo in a shadow, the other with yellow in shadow (same incident light level to both colours) and I wanted to place them both in zone III, they will both turn out to be the same shade of grey. That isn't good since I'd like the indigo to be darker than the yellow.

    What am I missing ??!!
     
  2. Eric Rose

    Eric Rose Subscriber

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    It is important as different meters react to different colors differently. Same goes for metering through colored filters. I remember seeing somewhere in a book the color sensitivity of the Pentax Spotmeter. It certainly was not linear with respect to color and that was taking the relative reflectance of the color materials into consideration.

    However taking your question into consideration if you metered a yellow object in the shadow you would still underexpose by 2 stops from what the meter said. Same goes for the indigo object. The meter will give you two very different readings depending on which object you metered. In either case underexpose by 2 stops. This will place whichever object you metered off on Zone 3.

    Now if you were able to find out that your meter gave a +3/4 stop error when reading yellow you would have to take that into consideration when determining your calculation.

    In the real world I have never really worried about specific color sensitivity or lack thereof.
     
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  3. Bill Burk

    Bill Burk Subscriber

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    Incident light meters are used for BTZS which doesn't have you place readings on Zones.

    Zone System uses spotmeters (or get up close) to find the reflectivity of each part of your subject. So when you aim at your indigo, the needle would drop. When you aim at the yellow the needle will go up. So then you might place the indigo in Zone III but the yellow would fall in Zone IV.

    How color affects film versus how it affects your eye versus how it affects your light meter... Now that is an interesting issue that you generally dig into when you start working with filters. A yellow filter on your scene would make the indigo darker still without affecting the yellow much. So you might end up with indigo in Zone II and yellow in Zone IV.

    ...edit... Oh you're talking two different pictures. Yea, you might be wise to consider the color of your significant shadow. You might want to expose the two scenes exactly the same. Thus you place the shaded indigo in Zone III and you turn the camera and shoot the next shot without changing the aperture/shutter speed. (Or keeping in mind color, don't pick a lighter-color subject in your shadow as the significant shadow.)
     
  4. PeterB

    PeterB Subscriber

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    OK, thanks Bill and Eric. I'm happy to see that you both think it is important to compensate for the different colours - as that too was my thinking but I suspected I was wrong because I can find absolutely NO, NADA, ZIP reference to taking this into account on any forum posts or in any decent Zone System books ?? OK, it might be in one in an obscure place, but surely this should be first principles stuff that is commonly taught. (Assuming one is using a spot meter for metering and not an incident meter as per BTZS)

    Why isn't this concept mentioned anywhere?
     
  5. MattKing

    MattKing Subscriber

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    If you are talking about two different scenes and two different photos, if you place the yellow spot in Zone III, and want the indigo spot to be darker, you won't be placing it in Zone III - you will have to place it in Zone 1 or 2.

    If both spots are in the same scene and photo, if you place the yellow spot in Zone III, the indigo spot (if it is darker), won't fall in Zone III - it will fall in Zone 1 or 2.
     
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  6. PeterB

    PeterB Subscriber

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    Now I get it thanks Matt. I knew there was a flaw in my logic.
     
  7. Collas

    Collas Member

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    If you then use colour filters to increase or decrease contrast, then the intensities of the colours (or where they fall on the Zone scale) will alter again. And the filter effect will also change with time of day, altitude and weather conditions. It's down to testing to find out what effect the filters will have.

    Nick
     
  8. Harrison Braughman

    Harrison Braughman Member

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    "In the Zone System (for B&W) - why is the colour of the spot metered area unimportant? ... Why isn't this concept mentioned anywhere?"

    The majority of zone system books address your question during the discussion of exposure meters. The answer is not always straight away, but generally state light meters do not know the subject matter nor its colour. The light meter wants to make the subject, regardless of colour, a middle grey.
    Therefore, from the lightmeter's point of view, the colour of the subject is unimportant. However, our individual interpretation of the subject's reflective surfaces is what is important.
     
  9. Leigh B

    Leigh B Member

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    They're both the same shade of gray because you chose to put them there.

    Anything placed on Zone III will have the density corresponding to that level, regardless of the original color.

    If you want the yellow brighter you need to put it on a higher zone.

    - Leigh
     
  10. polyglot

    polyglot Member

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    As long as the spectral sensitivity of the meter and the film are a decent match then the colour doesn't matter. You're just choosing an intensity. If on the other hand you metered a red area while shooting ortho film (that can't see red!) then that region is going to come out a lot darker than you'd hoped for.

    If you place two colours (in separate exposures) on the same zone, they will end up the same shade of grey in the prints. If they're in the same scene/exposure and one is darker than the other, then obviously you can't place them on the same zone unless you start filtering the light.

    If you have yellow and purple areas that meter identically and you want the yellow to look brighter, you could use a yellow or orange filter on the lens to reduce the brightness of the purple area. You would need to meter each area through the filter to observe the exact effect that the filter has on each colour.
     
  11. PeterB

    PeterB Subscriber

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    Thanks for the additional replies. There were two important lessons I learnt from this thread.

    1. It is up to the photographer to place "darker" colours on lower zones, lest they be rendered the same shade as "lighter" colours.
    2. Light meters, panchromatic film and our eyes all have different spectral sensitivity curves. Refer to the following excerpt from p.190 of WBM Ed.2, Copyright 2011 Ralph W. Lambrecht, Chris Woodhouse (I'm satisfied I followed the "fair use" rules).

    [​IMG]
     
  12. RalphLambrecht

    RalphLambrecht Subscriber

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    the snswer is in the different spectralse sitivitiest of film ,meter and the human eye
     

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  13. Stephen Benskin

    Stephen Benskin Member

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    Peter, I know what you are talking about, but you've stated it incorrectly. If the reflectance is different for two objects under the same illuminance, they will have different luminances; and barring any spectral sensitivity problems, the resulting exposure will produce different densities. What you're talking about is producing different tones from two objects of different colors having the same luminance.

    As Bill said, color filters are the answer. I've attached the Maxwell Triangle which "can be used to predict the effect of color filters on black and white film." As the color of the filter moves further from the subject's color, the resulting tone progressively darkens. Complementary colors darken. Supplementary colors lighten.
     

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  14. Leigh B

    Leigh B Member

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    All(?) modern exposure meters incorporate filtration to match the response to that of "average" photographic film.

    The native sensitivity curves of silicon or selenium are not relevant.

    - Leigh