Panchromatic film is sensitive across the visible spectrum, but its response is uneven. Green foliage, which the eye reads as bright and varied, tends to record as a heavy, undifferentiated grey. A canopy of overlapping leaves collapses into a single dark mass, and the structure a woodland scene depends on disappears. A green filter recovers it by reweighting exposure toward the wavelengths the foliage reflects.

How a Green Filter Reweights the Scene

A colour filter passes light of its own colour and absorbs its complement. A green filter transmits green wavelengths and absorbs red and, to a lesser degree, blue. Subjects that reflect green therefore reach the film with more exposure and print lighter, while those reflecting red are held back and print darker. This is why a green filter lightens grass and leaves while darkening a red barn, red brick, or sunlit skin, all of which carry a strong red component. The Kodak Wratten designations reflect this division of labour: the Wratten 58 is a green tricolour filter intended for colour separation, while the lighter Wratten 11 (the older X1) is a yellow-green that, according to the Wratten reference, heightens the contrast of skin tones in monochrome work.

Where Green Outperforms Yellow

A yellow filter is the common first choice for landscape work because it darkens blue sky and cuts haze. Ilford notes that yellow gives differentiation between the colours of foliage while keeping flesh tones natural. Its weakness is the canopy itself: yellow lightens green only modestly, so closely related leaf tones stay compressed. A green filter acts more selectively on the green band, opening the gradation between light spring growth, mid-toned deciduous leaves, and dark conifers. For woodland subjects, where the picture is built almost entirely from variations within green, this finer separation is the deciding advantage.

Exposure Cost and Restraint

Filtration absorbs light, so exposure must increase by the filter factor. Ilford gives a typical green filter a factor of 2, or roughly one stop, and the stronger yellow-green Wratten 11 carries a factor of about 4, or two stops. Through-the-lens metering corrects for this automatically; a hand meter requires the compensation added manually. The effect is also easily overdone, since a deep green renders a red barn or unfiltered skin unnaturally dark. The choice of green over yellow is therefore governed by whether foliage separation, rather than sky tone, is the priority.