Panchromatic film responds to the full visible spectrum, but not evenly: most emulsions are slightly oversensitive to blue and somewhat depressed in the green, so a scene that the eye reads as balanced records with skies too pale, foliage too dark, and reddened skin too light. A yellow-green filter, designated X1 by several manufacturers and corresponding to the Kodak Wratten No. 11, sits between a yellow and a green filter and addresses several of these imbalances at once. It is one of the few filters useful for both outdoor portraiture and vegetation-heavy landscape, which is why it earned the older nickname of a “correction” filter.

Spectral Behaviour

The X1 passes green strongly, transmits some yellow and red, and absorbs much of the blue and violet that panchromatic emulsions over-record. Kodak originally specified the Wratten 11 as a correction filter that brings panchromatic film’s colour response closer to the brightness response of the human eye, and noted it reproduces greens slightly lighter in daylight while matching the eye more precisely under tungsten light. The practical result is a tonal scale that reads as natural rather than dramatic: the X1 makes adjustments where the film departs from vision, instead of exaggerating contrast the way an orange or red filter does.

Foliage and Sky

Because green is the dominant transmitted band, the filter lightens green foliage that would otherwise record as a heavy, undifferentiated grey. Ilford notes in its guidance on colour filters that a green filter is used almost exclusively for foliage, lightening dark green leaves that “can record very dark without a filter”; the yellow-green retains much of that benefit while adding the yellow filter’s tendency to separate shades of green from one another, so meadow grass, conifer, and olive-toned shrub render as distinct values. At the same time, absorption of blue darkens an open sky moderately, increasing the separation between sky and white cloud. The effect is gentler than a yellow filter alone and far gentler than orange, which keeps it from overstating the landscape.

Skin Tones in Portraits

The same blue absorption that darkens the sky also tempers the way panchromatic film over-lightens reddish complexions. Kodak’s specification for the Wratten 11 describes it as heightening the contrast of skin tones in black-and-white photography, and the filter is conventionally used in daylight portraiture to bring out facial modelling and separate the subject from a green outdoor background. Skin records with a more natural, fuller tone, and lips and ruddy areas hold value rather than washing out.

Exposure Compensation

The X1 carries a measurable filter factor. Kodak lists a factor of 4, or two stops, for the Wratten 11 in daylight, while manufacturer data for equivalent yellow-green glass such as the B+W 060 cites a factor of 2x, or one stop. Through-the-lens meters read the filtered light directly and compensate automatically, but a handheld reading must be opened up by the stated amount, and any difference between daylight and tungsten ratings should be respected, since the filter’s correction is calibrated differently for each source.