Comparing Spectral Sensitivities

Discussion in 'Exposure Discussion' started by MattKing, Aug 19, 2012.

  1. MattKing

    MattKing Subscriber

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    As a result of a fairly spirited discussion yesterday I've been thinking about spectral sensitivity.

    In particular, I've been thinking about how the spectral sensitivities compare between human vision, black and white film and various types (Selenium, CDs, Silicon, Gallium etc.) of exposure meters.

    Can someone here provide a reference that would include some comparisons (ideally in graphical form) that would show the similarities and differences?

    I'd like to be able to take better account of the differences when I am using exposure meters - particularly considering the fact that I have and use a variety of different exposure meters.
     
  2. MattKing

    MattKing Subscriber

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    Anyone?
     
  3. benjiboy

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  4. Photo Engineer

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  5. Bill Burk

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    Hi Matt,

    I consider all this out-of-date. They don't include the latest cells. They don't include the lastest films.

    But Todd-Zakia show all that you ask...

    [​IMG]

    [​IMG]

    [​IMG]

    I am intrigued by this too, but worry that it will drive me insane to try to correlate all this in the field.

    For example, I was surprised that shots from a few weeks ago on TMY-2 gave me realistic skies without a yellow filter. I didn't even see the clouds the film picked up. Thought for sure the skies would be blown out. So my eyes don't match the film.

    I know from a test of an hour-long exposure to near-infrared that TMY-2 is fairly blind to deep red. I also know that my light meter responds to the same near-infrared so I already know the meter doesn't match my film's response.
     
  6. MattKing

    MattKing Subscriber

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    Thank you all.

    This helps me deal with the situations at the relevant ends of the spectrum - at least as far as choosing which meter to use in which (relatively extreme) situation.

    Any further thoughts about where I could enquire about spectral sensitivity of a silicon blue cell ("SBC") photo diode?
     
    Last edited by a moderator: Aug 26, 2012
  7. benjiboy

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    Selenium cells have the widest spectral sensitivity that is closest to that of film, but I suggest you contact Gossen about S.B.C cells.
     
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  8. Leigh B

    Leigh B Member

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    The only comparisons that are meaningful are between the film sensitivity curves and those of various exposure meters.

    While the meter sensor technology has a major impact on its spectral sensitivity, it does not define same.
    Meter manufacturers incorporate filters to modify the sensor response, in an attempt to duplicate that of film.

    For a meaningful analysis you need to obtain the sensitivity curves for the actual meters (make and model) of interest,
    and compare those with the curves for the actual films.

    - Leigh
     
  9. Chan Tran

    Chan Tran Member

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    You're right Leigh! The problem is that while I can obtain spectral response charts of most film online but not so with meters. Almost no meter manufacturer published spectral response of their meters.
     
  10. Leigh B

    Leigh B Member

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    I cannot dispute your statement. I know I have seen spectral response curves for meters in the past,
    but I don't know how commonly they are available or from which manufacturer(s).

    Good luck in your quest.

    - Leigh
     
  11. Photo Engineer

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    When comparing sensitivities of films, meters and the eye, one must compare not the peaks but rather the area under the entire curve.

    Also, the eye is linear and has the capability to extend its sensitivity in darkness and bright light by changing sensitivity and effective "speed". In addition, the meter and films are not linear and have a pronounced toe and shoulder which the eye does not have by virtue of its variable sensitivity.

    This has no simple answer and requires a lot of understanding of the entire topic. Some of it is loaded with concepts that are best explained by math.

    PE
     
  12. MattKing

    MattKing Subscriber

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  13. Bill Burk

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    You're right Leigh, comparing the spectral response of the fully assembled meter versus the specific film in question is important ... especially for example the Zone VI Modified meters which tried to emulate the spectral response of Tri-X.

    The human eye response is relevant when using the SEI photometer, which uses the eye as a sensor. But I think Matt may be more interested to know why results don't match what he "felt" at the time of the shoot.
     
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  15. Bill Burk

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    I dislike the conclusions in that butzi.net article that rely on a Macbeth Color Checker test.

    I believe the relevant way to test colors that don't meter "linearly", such as Periwinkles, Greens and Shadows, is by using live subjects - for example green pigments don't reflect infrared to the same extent that living foliage does...
     
  16. Leigh B

    Leigh B Member

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    The Macbeth chart is widely used for tests because it renders colors accurately and consistently over a wide range of light sources.

    That is not true of other color swatch products.

    - Leigh
     
  17. Photo Engineer

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    I'll have to agree with Bill.

    The Color Checker does not "handle" IR - or for that matter, UV - the same as the real outdoors.

    That said, it is a very useful tool and can be used for determining spectral sensitivities and speeds in comparisons. One must be careful to use the right conditions though.

    PE
     
  18. Leigh B

    Leigh B Member

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    Why should it? It's a visual standard. By definition 'visual' does not include IR nor UV.

    We may need to revise the definition after the Denisovans take over the planet, since they have extended visible range.

    - Leigh
     
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  19. Photo Engineer

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    Leigh;

    The problem is that you must then meter and photograph the real world with an IR and UV filter to make things even out. See my point?

    PE
     
  20. Bill Burk

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    Hi Leigh,

    The color checker is fine for standards - for color reproduction for copywork or studio illumination, calibration etc. But it is made of ink pigments.

    When photographing nature, living things have anomalous ultraviolet and infrared reflections that the color checker doesn't represent. If you tell me the Macbeth company deliberately selected metameric inks representing "foliage green" and "periwinkle blue", then I'd concede the point, because it is possible to select substances that reflect light "the same way". But I think they chose certain Pantone Matching System inks and left it at that.

    These are the things that come out looking illogical in photographs because the "system" (un-modified meter, color checker, film) didn't properly evaluate them.
     
  21. Leigh B

    Leigh B Member

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    Bill,

    All colored objects that are not self-luminescent derive their color(s) from pigments.
    The number thereof available in the printing industry is staggering.

    Many of these are inorganic, and can exhibit very stable colors over centuries or millenia.
    One recent example is a particular blue that was used by the Mayan culture in Central America 2000 years ago.
    It is still as bright and vibrant as when it was originally mixed. The same is true of their red pigment.

    The choice of specific pigment(s) to use for a particular application will take into account many factors.
    When one is designing a standard, stability and longevity are top priorities. Macbeth did its homework.

    - Leigh
     
  22. MattKing

    MattKing Subscriber

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    I agree with everyone :smile:.

    That being said, when wrestling with the question "what is this meter reading, and how does it compare with what I am seeing?" I'm reassured by tests that use a visual reference.

    I'm also reassured by sources that seem to indicate that the spectral sensitivity of meters is closer to the sensitivity of both films and human vision than what I had been led to believe.

    I think I can now confidently move from Cds to SBC to selenium cell meters, with just minor modifications of technique at times where either UV or near infrared light may be more prevalent.

    And I think I'll pass on limiting myself to a search for a Zone VI modified Pentax Spot Meter.
     
  23. Bill Burk

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    How does a photograph of the color checker look when shot on Infrared film? Does the green patch "light up" like crazy?

    I'm saying film (red-extended) is sensitive to infrared reflected by green leaves. And film (any) is sensitive to ultraviolet reflected by periwinkles.

    This is what the Zone VI modifications were meant to address (except UV... you are supposed to use a UV filter), and this advantage of Zone VI modification would not be fairly tested under studio lighting of a Macbeth color checker.
     
  24. Leigh B

    Leigh B Member

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    Bill,

    I'm sorry but your question is OT. The product was not designed nor intended for use with infrared film.

    The results you get are the results you get.

    This obsession with IR and UV sensitivity of film is pretty specious in the first place.
    The errors introduced, if any, would be significantly below the magnitude of other errors in the photographic process.

    - Leigh
     
  25. Photo Engineer

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    The comment is not OT when considering the various methods of exposure to the various types of light.

    PE
     
  26. Bill Burk

    Bill Burk Subscriber

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    I don't know if you have seen these charts that Fred Picker gave in Zone VI Newsletter Number 37, November, 1983...

    They get right to the Matt's original question. They involve IR and UV.

    [​IMG]

    This also gives us the missing SBC photocell spectral response Matt was looking for!

    Fred asserted that with Tri-X tests of 21 scenes, including natural objects such as a Cortland apple and an unripe orange, an unmodified Soligor II read 2 stops or more off in 23% of the scenes. After modification, all exposures were within 1 stop.

    Leigh, you know I'm just making the specific argument that it wasn't fair to debunk Fred Picker's Zone VI modification of spotmeters using a Macbeth color checker, because it's not as good a test of natural scenes as the test Fred did.