Exposure indexes and exposure

Discussion in 'Darkroom Developing and Printing' started by Uranium Committee, Oct 11, 2004.

  1. From Kodak's book 'Kodak Films for Black-and-White Photography', 1960.
    The text was written before the change in ASA speeds.


    EXPOSURE INDEXES

    Exposure indexes are numbers assigned to photographic films and plates
    for use in conjunction with exposure meters and other exposure
    computing devices to aid the photographer in obtaining correct camera
    exposure. They thus relate the relative sensitivity of the film and
    the brightness level of the subject to the lens aperture and shutter
    speed settings of the camera. They are determined by a sensitometric
    procedure which is specified in The American Standard Method for
    Determining Photographic Speed and Exposure Index, PH2.5-1954.

    "Correct camera exposure" can be defined only in terms of a careful
    evaluation of the desired final result which, in the case of
    continuous-tone negative materials, is the final print or positive
    image. An extensive program of research which was carried on by the
    Kodak Research Laboratories and which led to the sensitometric
    procedures specified in the standard, demonstrated that, for any
    particular negative material, a certain minimum camera exposure was
    required to yield excellent prints. If the camera exposure was reduced
    below this point, there was a marked drop in the quality of the best
    prints that could be obtained from the resulting negatives. If the
    camera exposure was increased beyond this minimum, the resulting
    negatives increased in density, but there was no immediate drop in the
    quality of tone reproduction obtained in the resulting prints.

    Since there may frequently be some error or uncertainty in the
    estimation or measurement of the brightness of the subject, it would
    be unwise to attempt to give the minimum exposure as a general
    practice, because any errors leading to less exposure would cause a
    definite loss in quality. The investigations mentioned above indicated
    that the black-and-white negative materials then in use could stand
    considerable increases above the minimum exposure without any drop in
    print tone-reproduction quality. Therefore, in order to avoid
    underexposed negatives, it was considered desirable to include a
    safety factor above the minimum exposure. The safety factor adopted
    for the black-and-white continuous-tone negative materials was 2.5. In
    other words, the exposure indexes were selected so that, on the
    average, when used with exposure meters calibrated in accordance with
    the American Standard they would lead to two and a half times the
    minimum exposures needed to produce top-quality negatives.

    Subsequent practical experience and more recent research
    investigations have indicated that the use of such a large safety
    factor is sometimes undesirable. For one thing, while considerations
    of tone reproduction alone do permit great exposure latitude, other
    factors which affect the final image quality, such as minimum
    graininess and optimum sharpness or definition, generally do not. This
    is particularly true for the currently popular small negative sizes
    which are enlarged in printing. With these, it is advisable to keep
    the exposures as close to the minimum as practical, in order to obtain
    the lowest graininess and best definition. Furthermore, many common
    causes of exposure errors lead to more rather than less exposure than
    expected.

    For instance, shutter speeds are marked for the effective exposure
    time for shutter operation at maximum aperture, where shutter
    efficiency is lowest. At the high shutter speeds and small apertures
    generally used with the modern fast films, the shutter efficiency is
    considerably higher, thus giving more exposure. Also, any changes with
    age will tend to slow down the shutter at its higher speeds and lead
    to still more exposure.

    Therefore, it now appears that, in order to obtain the highest quality
    in the largest percentage of negatives, it is desirable to set the
    exposure aim point closer to the minimum exposure required. Thus, for
    subjects of normal brightness scale, it usually is better to use
    indexes approximately two times the published values, in order to
    avoid unnecessarily heavy exposure and its attendant disadvantages.

    These considerations become particularly important for both the
    extremely fast films and also, at the other extreme, the slow, very
    fine-grain films. The latter are generally used only when extremely
    low graininess and the best possible definition are needed, and these
    may be impaired by unnecessarily heavy exposures. The very fast films
    are normally used only in situations where it is difficult to obtain
    adequate exposure. Thus, the use of any unnecessary safety factor
    would offset their most important characteristic, the high speed. This
    is the reason why the instructions for Kodak Royal-X Pan Film suggest
    that an index of 1600 should be used for most applications of the
    film. With subjects of average brightness scale, this leads to the
    minimum exposures which yield top-quality negatives with the
    recommended normal development. Thus, it contains no safety factor.
    The American Standard Exposure Index for this film, containing the
    normal safety factor, is 650, which would lead to undesirably dense
    negatives.

    It should be emphasized that the sensitometric procedure prescribed in
    the standard gives a reliable measurement of the relative speeds or
    sensitivities of negative materials. The only question is with regard
    to the magnitude of the safety factor that should be used in exposing
    black-and-white, continuous-tone negative materials. It has long been
    known that some other types of materials, such as reversal films, have
    very little camera exposure latitude, so that their exposure I indexes
    can contain only a small safety factor. It is now being recognized
    that, for the maximum attainable picture quality, this is frequently
    true also for black-and-white negative materials.
     
    Uranium Committee, Oct 11, 2004
    #1
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  2. message
    Kodak article snipped...
    The big change came in 1958 when the ASA changed from
    measuring using the Kodak speed method to the DIN method and
    dropped the very large safety factor it had attached to the
    Kodak method. The Kodak method and the DIN method come up
    with very similar speeds but reducing the safety factor from
    2.5 to 1.25 effectively doubled the speeds of all films on
    the market which were measured by this method (not color or
    reversal films or specialty films like X-Ray film). There
    have been a number of changes in the ASA standard, now the
    ISO method, over the years. Originally it specified a
    developer to be used in the measurements. Then a standard
    and a fine grain developer, neither of which was much like
    actual developers used in practice. That was dropped in the
    last revision of the standard which now allows the use of
    any developer for measurement provided the developer is
    stated in the results.
    Kodak now uses the term Exposure Index or EI to mean
    effective film speed when processed in some way other than
    that used to get the ISO speed. Since negatives are often
    developed to contrast indices other than the one produced by
    the ISO method the effective speed will be different. For
    instance, the ISO method results in a contrast index about
    right for diffusion enlarging or contact printing. If
    development is reduced to reduce the contrast to the right
    amount for condenser enlarging the speed will be reduced
    about 3/4 to 1 stop.
    Kodak's explanation of practical exposure is a good one.
    The extensive research they refer to is mostly that done by
    Loyd A. Jones of Kodak Research Labs, over a thirty year
    period. Jones is the one who came up with the Kodak Speed
    method, which takes into account the minimum contast
    necessary in the toe region of the film to obtain good
    shadow detail. This proved too difficult to use routinely so
    the fixed minimum density DIN method was adopted eventually.
    Jones idea was to find the minimum exposure which would
    result in good tonal rendition. The reason is that, in
    general, grain and sharpness are optimized when the overall
    density of the negative is minimal. Tonal rendition will
    remain good for a very considerable range of exposure above
    the minimum but grain will be increased and sharpness
    reduced. Unfortunately, the large saftey factor adopted by
    the ASA eliminated this advantage. Jones and his associates
    at Kodak also photographed a large number of scenes of
    different tonal characteristics. They measured the contrast
    of the scenes and photographed them over a wide range of
    exposures. They derived the minimum exposure criteria from
    making the best prints possible from each negative and
    presenting them to a very large group of judges who were
    asked to pick the "first excellent print" from the series of
    increasing exposures.
    Jones published perhaps a dozen or more papers between
    about 1920 and the early fifties, all worth reading.
    Unfortunately, most of them are in somewhat obscure
    scientific journals, like the Journal of the Franklin
    Institute.
     
    Richard Knoppow, Oct 15, 2004
    #2
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  3. message
    Kodak article snipped...
    The big change came in 1958 when the ASA changed from
    measuring using the Kodak speed method to the DIN method and
    dropped the very large safety factor it had attached to the
    Kodak method. The Kodak method and the DIN method come up
    with very similar speeds but reducing the safety factor from
    2.5 to 1.25 effectively doubled the speeds of all films on
    the market which were measured by this method (not color or
    reversal films or specialty films like X-Ray film). There
    have been a number of changes in the ASA standard, now the
    ISO method, over the years. Originally it specified a
    developer to be used in the measurements. Then a standard
    and a fine grain developer, neither of which was much like
    actual developers used in practice. That was dropped in the
    last revision of the standard which now allows the use of
    any developer for measurement provided the developer is
    stated in the results.
    Kodak now uses the term Exposure Index or EI to mean
    effective film speed when processed in some way other than
    that used to get the ISO speed. Since negatives are often
    developed to contrast indices other than the one produced by
    the ISO method the effective speed will be different. For
    instance, the ISO method results in a contrast index about
    right for diffusion enlarging or contact printing. If
    development is reduced to reduce the contrast to the right
    amount for condenser enlarging the speed will be reduced
    about 3/4 to 1 stop.
    Kodak's explanation of practical exposure is a good one.
    The extensive research they refer to is mostly that done by
    Loyd A. Jones of Kodak Research Labs, over a thirty year
    period. Jones is the one who came up with the Kodak Speed
    method, which takes into account the minimum contast
    necessary in the toe region of the film to obtain good
    shadow detail. This proved too difficult to use routinely so
    the fixed minimum density DIN method was adopted eventually.
    Jones idea was to find the minimum exposure which would
    result in good tonal rendition. The reason is that, in
    general, grain and sharpness are optimized when the overall
    density of the negative is minimal. Tonal rendition will
    remain good for a very considerable range of exposure above
    the minimum but grain will be increased and sharpness
    reduced. Unfortunately, the large saftey factor adopted by
    the ASA eliminated this advantage. Jones and his associates
    at Kodak also photographed a large number of scenes of
    different tonal characteristics. They measured the contrast
    of the scenes and photographed them over a wide range of
    exposures. They derived the minimum exposure criteria from
    making the best prints possible from each negative and
    presenting them to a very large group of judges who were
    asked to pick the "first excellent print" from the series of
    increasing exposures.
    Jones published perhaps a dozen or more papers between
    about 1920 and the early fifties, all worth reading.
    Unfortunately, most of them are in somewhat obscure
    scientific journals, like the Journal of the Franklin
    Institute.
     
    Richard Knoppow, Oct 15, 2004
    #3
  4. The booklet has a supplementary insert page which is dated May, 1960,
    describing this change. The data sheets in the rest of the book refer
    to the old ASA speeds, and is dated 'Seventh Edition, 1956. First 1958
    printing'. So the change must have occurred between 1958 and 1960.
    Kodak specifically mentions in the data sheets that the exposures you
    get using the published ASA speeds are double the minimum needed.
     
    Uranium Committee, Oct 15, 2004
    #4
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