A single sheet of variable-contrast paper can print a flat negative at high contrast and a harsh one softly, with nothing changed but the colour of the enlarger light. That flexibility replaces the boxes of fixed-grade paper a darkroom once needed, but it relies on a specific piece of emulsion engineering: the paper carries more than one light-sensitive component, each tuned to a different part of the spectrum. Understanding how those components respond explains both how a grade is selected and why exposure does not stay constant across the range.

Two spectral responses in one coating

All silver chloro-bromide printing emulsions are inherently sensitive to blue light, with only slight sensitivity to green. Variable-contrast paper exploits this. According to Ilford’s technical sheet Contrast Control for ILFORD MULTIGRADE Variable Contrast Papers, multigrade emulsion is not a single coating but a mixture of separate emulsions, each a basic blue-sensitive emulsion carrying a different amount of green-sensitising dye. One part responds mainly to blue, another to blue with some green, another to both. Crucially, all of the components share the same inherent contrast and the same speed to blue light; they differ only in how sensitive they are to green.

That shared behaviour under blue light, and divergent behaviour under green, is the entire mechanism. The components effectively span two spectral characters — a high-contrast blue response and a low-contrast green response — blended so that the printing light’s colour balance selects between them.

Why blue is hard and green is soft

When the paper is exposed to blue light, every component reacts and contributes equally. Because they share the same speed and contrast, their characteristic curves stack, producing a combined curve with a narrow exposure range — that is, high contrast.

Under green light the picture changes. Only the more heavily green-dyed components respond readily, and they do so at markedly different speeds. The curves now spread out rather than stack, and the additive effect of several differently sensitised layers yields a wide exposure range and therefore low contrast. Every grade between these extremes is simply a mixture of blue and green reaching the emulsion.

Setting the grade with magenta and yellow

The colour balance is controlled by filtration. A magenta filter absorbs green light and transmits blue, pushing the response toward the high-contrast blue channel; a yellow filter absorbs blue and transmits green, biasing toward the soft green channel. Ilford’s multigrade filter set numbers twelve filters from 00 to 5 in half-grade steps, the lowest number being the softest. On a dichroic colour head the same effect is dialled in: published Ilford tables place grade 2 at zero filtration, with increasing yellow toward grade 00 and increasing magenta toward grade 5.

Exposure does not hold constant across this range. Ilford specifies that the exposure time is the same for filters 00 through 3½, then doubles for grades 4 and 5. At the hard end the magenta filtration removes so much green that the paper draws almost entirely on its blue-sensitive component, sharply reducing the light it can use and demanding the longer exposure to compensate.