Tri-color Filters Making Tutorial from Early 20th Century - 1922

Tri-color Filters Making Tutorial from Early 20th Century - 1922

  1. Mustafa Umut Sarac

    Mustafa Umut Sarac Member

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  2. Mustafa Umut Sarac

    Mustafa Umut Sarac Member

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    An alternative method is to leave the hinge on, place
    the filter on a thickness or two of blotting paper on a
    level surface and place a card on top with a good-sized
    weight, four pounds not being too heavy for a 12x12
    cm filter. Direct heat cannot be used for drying, as this
    causes the edges to dry first and gives rise to distortion.
    At the end of the three weeks the exuded balsam
    should be scraped off, and the glass cleaned with alcohol
    and newspaper, then with bits of cloth and finally
    polished. Do not try to be sparing with the cleaning
    cloth, or use one large cloth; little bits and each piece
    thrown away as soon as it gets sticky is the easiest way.
    The final polishing should be done with tissue paper and
    alcohol, following the same plan, that is, fresh pieces
    continually. Benzol, xylol or chloroform should not be
    used, as they are energetic solvents of balsam and will
    almost inevitably creep in between the edges, in which
    case the job will have to be done all over again.
    Those who would like to make their own preparation
    of balsam may purchase some dried Canada balsam from
    a lens worker or optician. This should be roughly
    powdered, which is most easily done, though it is rather
    wasteful, by tying it up in a cloth and hammering it
    with a heavy hammer; a fine powder is not wanted but
    the big pieces should merely be broken up. Then place
    this in a wide mouthed bottle, place in the water bath
    and bring the latter slowly to a boil, stirring the balsam
    all the time; add about one fifth of its weight of xylol,
    stirring well and then letting it get cold in the water
    bath. This preparation requires a much higher temperature
    to melt and must be used hot. It then sets very
    quickly and at a pinch a filter thus cemented may be
    used the next day. The only difficulty likely to be met
    with is the setting of the balsam before an even film is
    obtained, but warming the glasses, or keeping them on
    a hot plate for some time under pressure will soon make
    the balsam spread out.
    There are four possible positions for the filter; in
    front of the lens; between the combinations close to the
    diaphragm; behind the lens; and immediately in contact
    with the sensitive surface. Between the lenses is
    the very worst place to choose, although this requires
    the smallest filter, as is obvious. In the first place, it
    is very likely to upset the corrections of the lens, particularly
    with the later forms of anastigmatic lenses, and
    with these there is often not enough room to insert any
    other than a film filter. Secondly, it is not easy to
    change the filters in this position without some special
    fitting, so that we can dismiss this at once. Either in
    front of or behind the lens may be chosen, which one
    being a matter of indifference, provided focusing is
    always effected through the filter, a matter that we
    shall have to deal with later on when talking of screen
    plates (See Chapter XII). In either case some sort of
    sliding fitting is advisable, although this is not conve
    lent in some cases inside the camera, as not only may
    the rear lens protrude beyond the lens board, but one
    has to have some means of shifting the filter between
    exposures, which necessitates a light-tight fitting.
    It is possible in many cases to arrange a frame to
    slide over the camera front, and to fit the ordinary lens
    panel on this, so that the filters will be behind the lens.
    The sliding frame can be made on the same lines as the
    usual lantern slide carrier, and if velvet is used to line
    the outer frames there will be no trouble in making it
    light-tight. Or it may be possible to fit such a frame on
    the lens barrel itself, but here it must be so securely
    fastened that there is no chance of its slipping off.
    Really the simplest plan is to obtain one of the square
    slip-on cells, which, fitting on the lens hood or barrel,
    may be always retained in position and the filters merely
    lifted out and inserted as required. It is advisable, if
    possible, to remove the lens hood and fit the holder on
    the barrel, as this means not only slight reduction in
    size, but as a rule a firmer hold. In order to obtain the
    correct size of fitting, the diameter of the lens tube
    should be taken with a pair of sliding calipers. Failing
    these, the next best plan is to take a narrow strip of hard
    writing paper and wrap round the lens barrel so that the
    ends overlap by about half an inch, then with a sharp
    penknife cut right through both pieces of paper midway
    of the overlap, not at the end.
    Placing the filter close to the plate means that the
    filter must be of the same size as the plate. Defects in
    the filter, such as want of absolute parallelism of the
    surfaces, are here of the least consequence; but local
    defects, such as coating striae or bubbles, are more
    apparent on the negative image, though only locally.
    Special sliding backs can be? obtained commercially, fitted
    with the three niters and made to take three plateholders,
    or with some plate-holders, particularly of the
    English book-form pattern, the filter may be placed in
    actual contact with the sensitive surface; then naturally
    its thickness must be allowed for in focusing.
    One important point in the choice of filter fittings,
    particularly metal ones, is that there should be no
    abnormal pressure on the glasses, as this may cause
    strain and consequent degradation of definition. It
    should be possible to turn the filter round, or shift it,
    with the lightest pressure of the fingers. Neither is it
    advisable to use cells screwing into the lens hood, as
    this is almost certain to shake the camera and there
    is much loss of time in changing.
    For photomechanical work, in which long-focus lenses
    are nearly always used with half-tone screens, the glass
    must be optically worked, as carefully, in fact, as the
    lenses themselves. Such glasses are known commercially
    as
    "
    optical flats," and are very costly if of any size.
    They must all be absolutely the same thickness and be
    so arranged that they are always perpendicular to the
    axis of the lens.
    The ordinary filters may be used for making the separation
    negatives for photomechanical work, as if there is
    not absolute coincidence of size, this can be corrected by
    the operator when making the screen negatives from the
    transparencies, though he will not be pleased at having
    to do this.
    It may possibly be as well to interpolate here a note
    as to the making of the constituent negatives for photomechanical
    purposes. The use of the panchromatic
    gelatine plate for this work is largely on the increase,
    and in some cases the slow panchromatic plate is used
    for making the color separation and the screen negatives
    in one; but the usual practice is to make the separation
    negatives first, from these a set of transparencies, and
    then the screen negatives. It may be noted that the
    transparencies for such work should be as little like a
    lantern slide as possible. They should be fully exposed,
    quite "soft" in character, and with practically no bare
    glass except in the very deepest shadows. Full exposure
    should be given to the plates, and it is better to use slow
    negative rather than transparency plates for this work,
    as giving a longer range of gradation and less tendency
    to brilliancy. The exposure should be full and development
    not pushed too far, so that the highest densities
    are quite transparent.
    In many commercial process establishments collodion
    emulsion, and even the wet plate process, still hold their
    own for the making of the separation negatives, and also
    the combined separation-screen negatives. Usually the
    emulsion is obtained commercially with its special sensitizers,
    and the makers issue instructions for the making
    of the filters, which are usually of the liquid cell type,
    for use with the same. On the other hand the method of
    sensitizing already advised may be adopted, or the dye
    may be added to the enrulsion, and in this case 80 ccm
    of sensitol violet stock solution should be added to 1000
    ccm of the plain emulsion, and the plates washed in running
    water or under a rose tap for fifteen minutes. The
    washing increases the sensitiveness of the plates about
    five times.
    By some writers it has been proposed to use different
    plates for the different color separations, that is to say,
    an ordinary, non-color-sensitive plate for the minus
    yellow negative; an orthochromatic plate for the minus
    red negative; a panchromatic or red-sensitive one for
    the minus blue negative. This plan may at first sight
    appear to have certain advantages, but this method is
    not one that should be adopted. It is a well established
    fact that the degree of contrast, or gamma, differs
    with different kinds of plates, and in fact with different
    batches of the same kind of plate, to say nothing of the
    development velocity of the plates, and one of the most
    important essentials in making separation negatives is to
    have them of the same degree of contrast as far as possible.
    That is to say, in the three negatives the range
    of densities of a black and white scale should be the
    same; and with three totally different kinds of plates
    this is almost an impossibility. One kind of plate
    should be used for all three separation negatives, and
    they should be, as already pointed out, as far as convenient,
    developed together. The adoption of this plan
    will save no end of after manipulation and dodging in
    getting concordant results; and it may be taken as an
    axiom that hand work, except for the removal of purely
    mechanical defects, such as pinholes, etc., cannot be successfully
    executed with color negatives.