http://www.canemco.com/images/Ilford%20Contrast%20Control.pdf
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Contrast control with ILFORD MULTIGRADE papers can be achieved in several ways. These include the ILFORD MULTIGRADE hand filters and the ILFORD MULTIGRADE 600 equipment. Contrast can also be controlled with other variable contrast enlarger heads and with colour enlargers.
CONTRAST RANGE
This section describes how MULTIGRADE papers give different contrast levels. For practical information on selecting contrast levels turn to the next pages.
All chloro-bromide (black and white) emulsions are blue sensitive with a slight sensitivity to green light. To make an emulsion sensitive to colours in addition to blue, sensitising dyes need to be added. MULTIGRADE papers are coated with an emulsion which is a mixture of three separate emulsions. Each emulsion is a basic blue sensitive emulsion to which is added different amounts of green sensitising dye. Thus, part of the mixed emulsion is sensitive mainly to blue light, part to blue light with some sensitivity to green light and part to both blue and green light.
All parts of the emulsion have the same contrast. They also all have the same speed to blue light, but naturally, the part of the emulsion with only a small amount of green sensitising dye has a low speed (that is, is less sensitive) to green light. When the paper is exposed to blue light, all parts of the emulsion react and contribute equally to the final image. This image is of high contrast because of the additive effect produced by three emulsions with the same speed and contrast. The resultant curve has a narrow exposure range and is thus of high contrast. When the paper is exposed to green light, only the parts of the emulsion with the larger amounts of green sensitising dye react initially. This is because the three emulsions have very different sensitivities to green light. This image is of low contrast because of the additive effect produced by three emulsions with different speeds to green light, but with the same inherent contrast.
The resultant curve has a very much wider exposure range and is thus of low contrast. By varying the proportion of blue to green light, a contrast range between these two extremes can be obtained. The simplest way of controlling the colour of the light reaching the emulsion during exposure is by the use of filters: a magenta filter absorbs green light and transmits blue; a yellow filter absorbs blue light and transmits green. In this way, high and low contrast images can be made.
А вот рисуночек там неправильный, сделанный левой ногой. Ну да ничего, я его поправил. Ибо, рисовать надо не так как хочется, а так как правильно.
Сказано, что контраст одинаковый, так и рисуй с одинаковым контрастом S-образные кривые как полагается, а не хвостики не пойми от чего.
Я не успокоился и решил проверить, будут ли провалы в варианте "фильтр #2".
Блин!!!! Так откуда же эти провалы берутся? Если так всё красивенько должно было бы быть, если бы все три слоя имели одинаковую максимальную плотность.
Да потому что разная у слоёв максимальная плотность. Разная.
Типа: хотели как лучше. А если учесть, что над эмульсионными слоями бились пол века, и всё замечательно только в рекламных проспектах получалось, и всегда хуже, чем на номерной бумаге, то хотели одно, получили другое.
не знаю, не было такой надобности.
