Basic question about contrast and gamma

Discussion in 'Darkroom Developing and Printing' started by Monica Schulz, Feb 21, 2008.

  1. Hi ng,

    I'm just begining to dive into the photographic process, so my
    practical experience is still scarse. - I try to figure out the basics
    in a theoretical way first. In case of the negative/positive process
    (both b&w and color) there is the relationship of contrast and gamma
    and I simply can't get that into my head, no matter how much I read
    and think (maybe I read the wrong papers?!).
    One thing is that negative gamma and print gamma should combine to 1
    if the contrast reproduction should match the scene (or 1,1/1,2 if you
    take into account that most people prefer a slightly exaggerated
    contrast of the midrange values).
    If that´s true and the negative is developed to a kind of normal gamma
    of 0,6, print gamma must be about 1,7. In the case of b&w contrast
    must be balanced between the negative and the print if contrast is to
    be reproduced correct. So if the neg has, say, 1,3 log units it must
    be printed on a paper that is also capable of 1,3 log units. Not more,
    not less. So if you chose the paper grade according to the neg
    contrast, doesn´t that mean that you might have to chose a grade that
    leads to a gamma which doesn´t combine to 1 with the neg?
    Another thing that give me headaches in the same context are color
    prints. In the case of the color reproduction chain, the negative is
    normally developed to a gamma of about 0,6 and most papers have a
    gamma of more than 2,5, so the combined gamma is 1,5. - So how does
    that work? And why are color papers not available in different grades
    to balance with the neg contrast?
    You see I´m kind of lost with those concepts and I really hope that
    someone could shed some light on them to show me what´s going on.

    Monica Schulz
     
    Monica Schulz, Feb 21, 2008
    #1
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  2. Monica Schulz

    John Guest

    Color materials have a longer scale than black-&-white materials. Also
    there is in fact a significant difference in Kodak's RA4 Endura, Supra
    and Ultra papers. The Ultra is about 1 grade higher in contrast when
    compared with Supra which is about 1/2 grade higher than Endura
    Metallic.

    JD
     
    John, Feb 21, 2008
    #2
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  3. [snip theoretical worrying]

    If I might be so bold, it seems to me that if you're just starting out
    (at least in b&w photography, let's say), you're waaaay overthinking
    this one. Unless you absolutely need to feel comfortable with the
    theoretical basis of making negatives and prints, it would be far better
    to just go out and start shooting, assuming you have all the stuff you
    need. Shoot a bunch of rolls of film. Make contact sheets. Make some
    prints. Put them up and look at them.

    In other words, make mistakes. The more mistakes you make, the better
    (because you'll learn something from each one in a way that no
    theoretical discussion can match).


    What's that old saying? "My first 10,000 prints were garbage"?
     
    David Nebenzahl, Feb 22, 2008
    #3
  4. Hi ng,

    I'm just begining to dive into the photographic process, so
    my
    practical experience is still scarse. - I try to figure out
    the basics
    in a theoretical way first. In case of the negative/positive
    process
    (both b&w and color) there is the relationship of contrast
    and gamma
    and I simply can't get that into my head, no matter how much
    I read
    and think (maybe I read the wrong papers?!).
    One thing is that negative gamma and print gamma should
    combine to 1
    if the contrast reproduction should match the scene (or
    1,1/1,2 if you
    take into account that most people prefer a slightly
    exaggerated
    contrast of the midrange values).
    If that´s true and the negative is developed to a kind of
    normal gamma
    of 0,6, print gamma must be about 1,7. In the case of b&w
    contrast
    must be balanced between the negative and the print if
    contrast is to
    be reproduced correct. So if the neg has, say, 1,3 log units
    it must
    be printed on a paper that is also capable of 1,3 log units.
    Not more,
    not less. So if you chose the paper grade according to the
    neg
    contrast, doesn´t that mean that you might have to chose a
    grade that
    leads to a gamma which doesn´t combine to 1 with the neg?
    Another thing that give me headaches in the same context are
    color
    prints. In the case of the color reproduction chain, the
    negative is
    normally developed to a gamma of about 0,6 and most papers
    have a
    gamma of more than 2,5, so the combined gamma is 1,5. - So
    how does
    that work? And why are color papers not available in
    different grades
    to balance with the neg contrast?
    You see I´m kind of lost with those concepts and I really
    hope that
    someone could shed some light on them to show me what´s
    going on.

    Monica Schulz

    First of all "gamma" is a measure of contrast but is not
    often used now. Gamma is the slope of the straight line
    portion of the film or paper characteristic but, since most
    characteristics have only small straight line sections, or
    sometimes none another measure is used. This is average
    contrast or G-bar, the slope of a stright line drawn between
    a shadow and a highlight point on the curve. Kodak goes one
    step further and uses Contrast Index or CI which is the
    average contrast between two specified points on the curve.
    Since these are constant CI of different materials is easy
    to compare.
    You are right that the overall contrast of a print
    should be on the order of 1 but reflection prints are
    limited in the range of brightness they can reproduce so the
    unity contrast is usually true only of a range of mid tones,
    the shadows and highlights generally being compressed or
    simply going out of the range the paper can reproduce. Since
    transparencies don't rely on ambient light their range can
    be greater but the target contrast is still 1. However, the
    eye tends to judge "correct" reproduction of tones by the
    gray tones and, for the most part ignors deep shadows and
    bright highlights, so, if the mid-tone contrast is right the
    eye will see the print as being right even though the
    overall range of brightness that can be reproduced is
    substantially less than in the original scene, or, for that
    matter, less than what the negative will record.
    For many scenes of relatively high contrast a low
    contrast print that attempts to reproduce the full range of
    tones recorded on the negative will simply look flat.
    To some degree this can be compensated for by localized
    printing of details, either holding dark areas back to
    brighten them or printing in (burning in) highlights, but
    this must be done with skill or the manipulation will be
    obvious to the viewer.
    For black and white work, where there is little
    standardization, it is common to develop negatives to a CI
    of anywhere from about 0.4 to 0.7 depending somewhat on the
    kind of illumination used in printing. The printing light is
    important because of something called Callier effect. This
    is the effect on effective density caused by the relative
    diffusion or specularity of the light source. Because most
    B&W films have an image made up of descrete silver
    particals, which can scatter light, there is a fairly large
    Callier effect. Between a completely diffuse light source,
    like a color head or a cold light head, and the usual partly
    condensing head, the contrast difference is about one paper
    grade, the condenser head having the greater contrast. Note
    that Callier effect has no effect on the one reproduction
    scale other than overall contrast which can be completely
    compensated for by either a change in development time of
    the negative or choice of paper contrast. The method of
    compensation is a matter of convenience.
    Because color films, and here I include chromogenic B&W
    film, the image is made up of semi-transparent dye, there is
    very little Callier effect so the printing light makes
    little or no difference in effective contrast. While there
    is some choice in contrast of color printing materials it is
    much less than for B&W. This is because the eye is much more
    sensitive to variations in the contrast of color. If color
    is too low or too hight in contrast the effect is quite
    obvious. So, the contrast of color recording and printing
    materials is much more highly standardized than for B&W. In
    addition to this the three recording layers (some films have
    many more) for the three basic colors must have
    characteristics which match very closely if the color is to
    track for all brightnesses. Anothere way of putting this is
    that the neutrality of a gray scale must be constant. For
    this reason also there are strict limits on the contrast
    range of color materials.
    Color negative materials, which are intended to printed
    to make postives, have relatively low contrast and can be
    controlled to some extent in processing, as long at the gray
    scale tracking mentioned above remains acceptable. Color
    positive materials are generally meant for direct viewing
    and are much higher in contrast, generally slightly more
    than 1 to compensate for flare. There ARE some transparency
    materials intended for duplication or other printing which
    are lower in contrast bu they are rarely encountered by
    amateur photographers.
    Some color materials intended for direct reversal
    printing from positive transparencies offer a lower contrast
    grade for use when the usual grade is too contrasty. This
    was sometimes the case with Cibachrome/Ilfochrome. Another
    method of reducing contrast is masking, the use of a very
    low contrast negative image overlying the postive image to
    reduce the contrast.
    I think I am getting deeper into this than you want to
    go.
    You can find some elementary material on sensitometry,
    the name for the study of tone reproduction from film, on
    the Kodak site. Search for H-740. If you can't find it
    write me privately and I will send it to you.
     
    Richard Knoppow, Feb 22, 2008
    #4
  5. On the risk of beeing pushing I must still insist on the relationship
    between neg contrast and paper contrast.
    As we agree that both must balance to about 1 and color negative film
    is normally developd to about 0,6, the paper should show gamma of 1,7.
    But almost all color printing papers have gamma values far in excess
    of 2. So how does the steeply raised contrast of the print fit into
    your above statement? Does the contrast have to be raised that much so
    offset the unavoidable flare? If that´s true paper contrast cannot be
    calculated by simply using the reciprocal of the neg. Maybe that´s all
    what blocked me from recognizing the true connection?!

    Best regards!
    Monica Schulz
     
    Monica Schulz, Feb 24, 2008
    #5
  6. Monica Schulz

    ____ Guest

    Neg contrast= 1.45 max
    Paper contrast .85 <

    Gamma is not a good indicator of true paper capability try .85 or if you
    must use film gamma subtract .20 from the bottom and top of the
    Characteristic curve your using, use a shoulder around 1.45 or less more
    like 1.25

    Flare should not be an issue when using a modern lens if using something
    older producing flare you should replace it with a modern Apo lens.
     
    ____, Feb 24, 2008
    #6
  7. On 22 Feb., 10:57, "Richard Knoppow"

    On the risk of beeing pushing I must still insist on the
    relationship
    between neg contrast and paper contrast.
    As we agree that both must balance to about 1 and color
    negative film
    is normally developd to about 0,6, the paper should show
    gamma of 1,7.
    But almost all color printing papers have gamma values far
    in excess
    of 2. So how does the steeply raised contrast of the print
    fit into
    your above statement? Does the contrast have to be raised
    that much so
    offset the unavoidable flare? If that´s true paper contrast
    cannot be
    calculated by simply using the reciprocal of the neg. Maybe
    that´s all
    what blocked me from recognizing the true connection?!

    Best regards!
    Monica Schulz

    I may be going over territory I covered in private
    e-mail but want to answer it here if possible.
    First of all I am not sure about your numbers for
    negative and paper contrast. 0.6 is probably about right for
    B&W negatives but I think color negs are somewhat lower,
    perhaps around 0.4. In any case gamma can be misleading
    because it applies only to the straight line portion of the
    film or paper characteristic. More modern methods of stating
    "effective" contrast are to use an average. This is done by
    drawing a straight line between the shadow density and
    highlight density and stating its slope. Average contrast,
    or G bar, does not have a fixed interval between highlights
    and shadows, the Kodak method called Conrtast Index does.
    G-bar and CI can be the same. In general gamma will not
    always agree with either.
    The idea is that multiplying negative contrast and
    positive contrast will give the contrast of the final print.
    Target contrast is usually somewhere around 1.0 but can be
    varied for a special effect or to compensate for something
    such as flare.
    Because the range of brightness that can be reproduced
    by a reflection print under normal illumination is, in
    general, less than the original scene some compression must
    take place. Compression means some part of the image is
    reproduced with lower contrast than the original. Since the
    eye tends to judge the "correctness" of an image based on
    its mid gray values or equivalent in color, the compression
    is typically in the shadows and highlights. Since the eye
    tends to compress these values in direct viewing the results
    are acceptable. If the mid tone contrast is too little, in
    an attempt to compress the entire tone scale, the results
    will look "flat" even though details are visible in shadows
    and highlights that are not in a more "normal" looking
    print. If the print is made to have higher contrast than the
    original scene it will still not look natural but may be
    acceptable if the original scene was very low in contrast.
    Since they eye is constantly compensating for brightness
    variations our impressions of original scene contast may not
    be too accurate. For instance, on a brightly lighted day
    with deep shadows the eye will adjust somewhat when looking
    into the shadows and see detail there that is not visible
    when looking at bright objects, but, because one's attention
    is not concentrated in the shadows then we are not generally
    aware of it. Film, OTOH, is fixed in its sensitivity. It can
    record a wide range of brightness but that range is centered
    according to the exposure its given. Then, when a reflection
    print is made, there is a further reduction of range since
    the reproducing medium is not capable of reproducing the
    full range of original scene brighnesses. Note that I am
    talking about the presentation to the eye. If one prints a
    long scale negative will onto a low contrast paper its
    possible to reproduce the full range of tones on the
    negative, but, the print will look very flat to the eye
    because the original brightness scale will be compressed.
    There are times when such prints are desirable, for instance
    for scientific or evidentiary purposes but, in general, they
    are not acceptable for pictorial purposes.
    Where there is not a clear reference for the eye
    distortion of contrast is acceptable. This is very true of
    B&W but somewhat less for color. If there is no
    psychological expectation the eye simply accepts what is
    presented.
    There is a great deal more to this and I have
    oversimplified to some extent but I think the main answer is
    that I think the gamma numbers you have are somewhat off.
     
    Richard Knoppow, Feb 24, 2008
    #7
  8. Gamma is a measure of the slope of exposure vs: density
    of the straight line portion of a film or paper curve. Since
    nearly all film or paper characteristics are curved and have
    either small or no straight line portions gamma can be
    misleading about overall contrast. More modern measures are
    average contrast or G-bar and Contrast Index or CI. These
    two are very similar but CI has a specified interval of log
    exposure.
    In any case, whether one is using gamma or G-bar or CI
    the numbers combine by multiplying, not addition or
    subtraction. That is, a gamma of 0.5 will combine with a
    gamma of 2.0 to result in a gamma of 1.0. A gamma of 1.0
    means that the contrast is the same as the original subject.
    Flare comes from other sources than just lenses, for
    instance light scattered in the optical path of a projector.
    Also, ambient light in a room where slides are being shown
    has an effect similar to flare. Projection transparencies
    and motion picture prints are often slighly higher than a
    gamma of 1.0 to compensate a bit.

    Another note: The prefix APO on most modern lenses in
    meaningless. It is permitted in German made lenses because
    of an error in the German DIN standard which mis-represents
    the term "apochromatic" and allows its use for lenses which
    are not so corrected. I think this has been changed but the
    terminology continues.
    A true apochromat is corrected to bring three colors to
    a common focus. The curve of longitudinal chrmomatic
    aberration will be S shaped crossing the zero line three
    times. Most lenses are achromats, that is they are corrected
    for two colors. The curve for an achromat will cross the
    zero line twice. More important than the number of zero
    crossings is the deviation from focus between the corrected
    points: the less the deviation the better the lens is
    corrected. Its possible to have an achromat that has better
    overall correction than an apochromat. In any case, nearly
    all of the current crop of lenses with APO as a prefix are
    NOT apochromats but are simply well corrected achromats. I
    think the term APO used for such lenses is misleading and
    should be eliminated.
    In any case, an APO lens, regardless of its true color
    correction, has no less flare than any other. Flare is
    mainly a function of the number of glass-air surfaces but
    other things can contribute such as reflections from the
    inside of the lens mount. Surface reflections from the glass
    can be controlled by anti-reflection coating. Such coatings
    have been known for over a century but were made
    economically practical only after WW-2. Modern coatings are
    complex and are effective over the entire range of colors of
    interest in photography. Modern complex lenses, especially
    zoom lenses, would be impossible without advanced coating
    techniques.
    Even single coated lenses have substantial reduction in
    flare and modern multiple coated lenses are nearly flare
    free. Probably the use of a modern lens would pretty much
    eliminate having to increase print gamma to compensate for
    flare but one might still want to compensate for other
    factors such as the viewing conditions.
     
    Richard Knoppow, Feb 24, 2008
    #8
  9. Uuh, now I see what you mean. You talked about the difference between
    gamma and contrast index before but I didn´t realize it´s importance -
    sorry for the delay! So using the contrast index method leads to other
    figures for the negative and the paper and they combine to about 1. So
    I finally got rid of this problem I had with the curves of these
    materials. - Thank you!
    Now there is only one point left from my starting question: Why is
    there not such a wide variaty of paper grades in color reproduction as
    there is in b&w? As far as I understand, in b&w we need this wide
    variaty to make the dark tones of a low contrast negative as black as
    the dark ones of a high contrast neg. What´s thze difference in color?
    Is it because a high density there really means a more saturated color
    and not so much a darker color?

    Best!
    Monica Schulz
     
    Monica Schulz, Feb 25, 2008
    #9
  10. Maybe I did not make myself clear enough in my last post. What I mean
    is that there is no need for such a wide variaty of paper grades in
    color printing as there is in b&w because contrast can´t be
    manipulated that much in color as in b&w. I came thinking about that
    after reading another old thread so I will use the important words of
    the relevant post.
    To adjust contrast means to make the image brighter or darker. The
    only way to do that is to make the dyes thinner. This lets more light
    reflect off the paper. But as a dye gets, say, thinner it removes less
    of its anti-color until it finally disappears and no anti-color is
    removed. And the other way around if the dye gets thicker. So the
    scale doesn´t go from, say, a dark magenta to a light magenta but from
    dark magenta to white. Or from a saturated magenta to an unsaturated
    magenta.
    The digital process can compensate for this unwanted increase or
    decrease in saturation if contrast is adjusted in the luminosity
    channel of lab-mode. In this case neither hue nor saturation is beeing
    changed. There is a nice little example of that on
    http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm. But
    we can´t do the same in analog color printing.
    It could well be that this is complete garbage as far as the
    subtractive mixing of the analog process is concerned and if so I´d be
    happy if someone tells me so that I can get this idea out of my head
    and can think in another direction.

    Best regards!
    Monica Schulz
     
    Monica Schulz, Feb 25, 2008
    #10
  11. Maybe I did not make myself clear enough in my last post.
    What I mean
    is that there is no need for such a wide variaty of paper
    grades in
    color printing as there is in b&w because contrast can´t be
    manipulated that much in color as in b&w. I came thinking
    about that
    after reading another old thread so I will use the important
    words of
    the relevant post.
    To adjust contrast means to make the image brighter or
    darker. The
    only way to do that is to make the dyes thinner. This lets
    more light
    reflect off the paper. But as a dye gets, say, thinner it
    removes less
    of its anti-color until it finally disappears and no
    anti-color is
    removed. And the other way around if the dye gets thicker.
    So the
    scale doesn´t go from, say, a dark magenta to a light
    magenta but from
    dark magenta to white. Or from a saturated magenta to an
    unsaturated
    magenta.
    The digital process can compensate for this unwanted
    increase or
    decrease in saturation if contrast is adjusted in the
    luminosity
    channel of lab-mode. In this case neither hue nor saturation
    is beeing
    changed. There is a nice little example of that on
    http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm.
    But
    we can´t do the same in analog color printing.
    It could well be that this is complete garbage as far as the
    subtractive mixing of the analog process is concerned and if
    so I´d be
    happy if someone tells me so that I can get this idea out of
    my head
    and can think in another direction.

    Best regards!
    Monica Schulz


    Something very like the "curves" effect can be done in
    chemical photography be means of elaborate masking and by
    using such techniques as coltrolled flashing. Both
    techniques were widely used in making the plates for
    four-color printing. However, its very much easier to do it
    in Photoshop and one can see immediately the results. Most
    of the tailoring done in Photoshop can be done in analogue
    photography but can be very complex to do there.
    Contrast and brightness are different. This is
    especially true of most reproduction media where the maximum
    brightness is determined by illumination. What adjusting
    contrast does is to fix the points that are the minimum and
    maximum brightness of the image. As contrast is increased
    something must be lost. That is where the "brightness"
    adjustment comes in. That controlls what is to be the
    midpoint of the gray scale. Its possible to extend the range
    of tones that will reproduce without falling off one end or
    the other by shaping the transfer curve and that is what
    Photoshop is doing. Many films and papers have curves which
    affect the reproduction of gray tones because they are not
    straight lines. Similar curves are available in color films
    and papers. This one of the things that determines the
    difference between "portrait" material and commercial
    material. The overall conrast may be the same but the
    placement of reproduced tones in relation to the originals
    is different.
     
    Richard Knoppow, Feb 27, 2008
    #11
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