A contrast filter is often treated as a fixed tool: a red filter darkens skies, a yellow filter holds them. In practice its effect depends as much on the light striking the scene as on the glass itself. A filter removes part of the spectrum, but the source decides how much of that part was present to begin with. The same filter on the same film can produce a markedly different tonal separation, and demand a different exposure increase, under tungsten light, open daylight, or the blue light of shade.

Why the source changes the result

A contrast filter works subtractively. It transmits its own colour and absorbs the complementary band, so a deep red filter passes long wavelengths and blocks blue and green. The tonal shift it produces depends on how much of the absorbed band the source actually emitted. Daylight at roughly 5500 K is rich in blue and ultraviolet; tungsten at roughly 3200 K emits a continuous spectrum weighted heavily toward red, with comparatively little blue output. A red filter under tungsten therefore discards very little energy, because little blue was present, while the same filter under daylight removes a large share of the incident light. Open shade, lit mainly by blue skylight rather than direct sun, pushes the balance further toward the short wavelengths still. The filter is constant; the spectrum it filters is not.

Tonal rendering shifts with colour temperature

Because panchromatic film responds across the visible spectrum, the proportion of blue to red in the source maps directly onto how light or dark a coloured subject records. Under daylight, a blue sky carries enough blue energy for a yellow or red filter to darken it strongly. Under tungsten there is little blue to remove, so the same subject is already rendered dark and the filter adds little. The effect is most visible with orthochromatic emulsions, which are blue and green sensitive and blind to red: Ilford’s datasheet for ORTHO Plus rates the film at ISO 80/20° in daylight but only ISO 40/17° under tungsten, a full stop slower, precisely because tungsten’s output sits largely in the red region the emulsion cannot record.

Filter factors are not constant

The exposure compensation a filter requires, its filter factor, follows the same logic. A factor assumes a particular mix of wavelengths reaching the film; change the mix and the factor changes. Ilford’s ORTHO Plus data sheet lists separate daylight and tungsten factors for this reason. Its 104 Alpha yellow filter carries a factor of 2.5 in daylight but only 1 under tungsten, because the yellow glass removes blue that daylight supplies in quantity and tungsten barely emits. The relationship inverts for blue filters: the 304 tricolour blue filter requires a factor of 3 in daylight but 5 under tungsten, since blue light is scarce in a tungsten source and more exposure is needed to pass enough of it. Ilford’s FP4 Plus data sheet adds that even within daylight the factor drifts, noting that in late afternoon or winter, when the light contains more red, green and blue filters may need slightly more exposure than usual. Treating a single published factor as universal therefore introduces error whenever the working light departs from the conditions it was measured under.