Kelvin & contrast

Discussion in 'B&W: Film, Paper, Chemistry' started by David Lyga, Mar 22, 2011.

  1. David Lyga

    David Lyga Subscriber

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    There is a difference in contrast when taking pictures under daylight (shade) and indoor tungsten roomlight. When I shoot a roll of BW film under both conditions this situation inevitably manifests.

    Why? I think that it has to do with the bluishness imposing a very slight fog veil over the outdoor image. Even looking at something under the shade, and then immediately placing a K2 (yellow-green) filter over your eye to judge the same scene, shows a slight increase in contrast when using the filter.

    I took photos of a gray scale with color patches under both conditions at the proper exposure. Obviously, the colors imparted somewhat greater contrast with the K2 but the gray scale ALSO did the same. The difference was not much but was at least half a paper grade.

    I prefer the tonal rendition under the lower Kelvin. Indoor tungsten lighting seems to just differentiate the tones better and are actually more believable when printed. It's frustrating to see a roll with negatives with differing contrasts. Are my assessments correct? - David Lyga
     
  2. David Lyga

    David Lyga Subscriber

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    I apoligize for the double posting: I was trying to eliminate the % from the first's title and replace it with an ampersand. I was not able to this in edit mode. I thought I had lost the post and reposted with the title corrected. - David Lyga.
     
    Last edited by a moderator: Mar 23, 2011
  3. Michael R 1974

    Michael R 1974 Subscriber

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    Wouldn't this have more to do with a given film's spectral sensitivity than haze? Of course, distance is an important factor when outdoors since there is more haze between you and the subject as distance increases, but if you are talking about a subject right in front of you and comparing clear daylight to tungsten I would assume haze to be a negligible factor and instead consider spectral sensitivity.
     
  4. Doremus Scudder

    Doremus Scudder Member

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    If a film's sensitivity and contrast rendering are different in different areas of the spectrum, this could be true.

    David Kachel mentions this in his articles and I have observed an increase in contrast of Tri-X when using a #25 filter, meaning that the strongly red light source seems to render more contrast than "neutral," unfiltered light.

    This could be caused by sensitizing dyes, differences in emulsion layers etc. Modern films are very complex and there are many possible causes to look into. Since unmanipulated silver halide crystals are not usually sensitive to green and red light, the things done to make them so would likely be the place to look, however, not in the nature of the silver halides themselves.

    Best,

    Doremus Scudder
    www.DoremusScudder.com
     
  5. David Lyga

    David Lyga Subscriber

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    To clarify, I am not talking about haze, Michael R. The gray scale was only about 3 feet away. There really IS an increase in contrast and it is, I believe, beneficial when it comes to printing. And Doremus is correct when he says that it is NOT in the nature of the silver itself: Just place a K2 filter in front of your eye and then quickly remove it (in shaded conditions). There really IS something about bluish light that lightly veils contrast. I have always noticed this even without taking the photographic process into account. I simply can see (and perceive) better with lower Kelvin temps. The tones just 'fit' better. I am neither scientist nor philosopher but the warm light is more accurate both in tonal values and in subjective perception. - David Lyga.
     
  6. Michael R 1974

    Michael R 1974 Subscriber

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    Sorry about that - I read your post but then mysteriously forgot about the part where you described the test you performed.

    A film's rendering of values is obviously influenced by its spectral sensitivity. But leaving film aside, I'm still puzzled as to why the human eye would see/perceive more contrast in a grey scale under lower temperature light at close distances.
     
  7. Bruce Watson

    Bruce Watson Member

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    If there's an answer, it's probably in Livingstone's book, Vision and Art: The Biology of Seeing. Highly recommended.
     
  8. Tim Gray

    Tim Gray Member

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    I'd think it has to do with spectral sensitivity of the film, which also ends up affecting the effective speed of the film. Shooting indoors under tungsten light can be thought of as equivalent to shooting through a yellowish filter (which your TTL meter might not compensate for properly). It increases contrast a little bit and reduces the amount of light reaching your film. For motion picture, Kodak gives daylight and tungsten ratings for its B&W film. For the most part, I think they tell you to rate it 1/3 of a stop slower indoors, e.g. 250 outdoors, 200 indoors.
     
  9. Bob-D659

    Bob-D659 Member

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    David, the eye is more sensitive to yellow light than blue, that's why you can see much better at night with 2700-3200k headlamps on your car that 6500 or higher ones. The european countries had a reason for mandating yellow tinted bulbs decades ago. :smile:

    Not that this is relevant to film. :smile: :smile:
     
  10. Doremus Scudder

    Doremus Scudder Member

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    There are two different issues here that we should be careful not to conflate.

    First is the question of whether different colored light (be it filtered or "warmer" due to lower color temperature) has an effect on the contrast rendering of a particular film. If this effect exists (and I think it does), then it is a characteristic of the film, i.e., how it reacts to different colored light. I would speculate that there is different contrast rendering in the differently sensitized emulsion components that explains this.

    Second is the way the human eye works; what its spectral sensitivity is and how we perceive visually. Human eyes are most sensitive in the green/yellow area of the spectrum, as mentioned above. Another characteristic of the eye is that it cannot focus red and blue wavelengths at the same spot, which causes the "vibration" one "sees" when pure red and blue are juxtaposed, and why we may see more "sharply" when we filter out one or the other of the extremes (e.g., looking through a yellow or orange filter). This also explains the "sharpening" effect of yellow-tinted sunglasses.

    The film sensitivity/contrast issue is easy to test and then use in the field. David has made a good start by shooting his gray scale with a yellow filter. I test all my filters this way and find that different films react differently to different colors, and that the most extreme filters, i.e., the deep reds, blues and greens, are the ones with the most effect.

    If you are interested, I've detailed my views on metering through filters and my testing methods here a time or two. A quick search should turn them up.

    As I mentioned earlier, David Kachel talks about this in great detail. Check his article for more at http://www.davidkachel.com/historical/zsfilter.htm


    Best,

    Doremus Scudder

    www.DoremusScudder.com
     
  11. jford

    jford Member

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    The eye focuses light better at the red end of the spectrum than the blue (it's a simple lens remember). I would be surprised if this caused any kind of haze in the viewing test you did, but it might. An example of this is little blue LED marker lights which seem to be the fashion in some areas of outdoor public area lighting these days. In the dark they appear to have haze around them when you look at them from a distance of at least about ten metres. Another example is seeing an ambulance from a distance on a dark night: here in Australia they are now using red and blue LEDs for their party lights. The red ones seen at least about 500 metres away appear sharp, the blue ones hazy.

    John.
     
  12. jerry lebens

    jerry lebens Member

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    I think you're barking up the wrong tree in blaming degrees K alone. Shaded sunlight is inherently less contrasty than tungsten light. In shade, (fairly even) diffused light is probably approaching the subject from almost 360 degrees. Whereas, under tungsten, there will be a discrete number of small, hard, light sources. So, even accounting for reflections from walls etc, the majority of the light hitting the subject under tungsten will tend to be harder and more directional than shaded sunlight, creating an effectively more contrasty scene.
    Studio photographers spend a lot of effort diffusing hard tungsten and flash sources in order to make them less contrasty. Many would like to use shaded north light instead, but it isn't practical.

    Also, although light temperature may play some part, I'd suggest that flare from scattered light probably plays a more significant role in this contrast difference. There's a lot of incoherent light flying about outdoors in shady conditions. Being sunlight, there's a larger blue component than in tungsten light and, I seem to recall, this wavelength has an even greater tendency to scatter as it strikes dust and water droplets etc in the atmosphere.

    UV filters are useful in combatting scatter but a really good lens hood is probably better. The typical 35mm/MF lens hood is highly compromised. Few photographers would want to carry around a truly efficient lens hood because, in many situations, it would need to be as big, if not bigger, than their camera.

    Regards
    Jerry
     
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