Panchromatic film responds across the whole visible spectrum, but not in the proportions the eye uses. The trouble starts in the silver halide itself. Bare emulsion is intrinsically sensitive only to blue and ultraviolet; H. W. Vogel’s discovery of dye sensitisation in 1873 dragged that response first into the green and later into the orange and red, producing the panchromatic films we still use. What it never fully cured was the residual surplus of blue sensitivity. Look at the published spectral-sensitivity curve for Ilford HP5 Plus or FP4 Plus and you see the signature: a strong response in the blue, a relative dip through the blue-green and green around 490 to 540 nm, and a recovery into the red. The eye does the opposite. The photopic luminosity function V(lambda) peaks at roughly 555 nm in the green. So a scene the eye reads as balanced records with skies too pale, clouds dissolved, foliage too dark, and reddened skin pushed too light.

A yellow-green filter sits between a yellow and a green filter and addresses several of these imbalances at once. It cuts the blue surplus while supporting the green, exactly where film is weakest and the eye is strongest. That is why it earned the old nickname of a correction filter, and why it is one of the few filters genuinely useful for both outdoor portraiture and vegetation-heavy landscape.

Naming the Filter

The numbered gelatin filters trace back to Frederick Charles Luther Wratten; Eastman Kodak acquired Wratten & Wainwright in 1912, after which the Kodak Wratten series became the reference standard still cited today. Two related filters matter here, and they are easy to confuse. The Wratten No. 11 (yellowish-green) is the version Kodak specified to match panchromatic response to the eye under tungsten light. The Wratten No. 13 is its daylight counterpart. Kodak’s old letter designations map them as X1 (= Wratten 11) and X2 (= Wratten 13), and B+W codes the yellow-green as 060 (and 061 for the deeper version). Note that a manufacturer’s own letter code need not follow this scheme — Hoya, for instance, sells its yellow-green as the X0 and reserves X1 for a green filter — so buy by the Wratten number, not the colour name or the house letter, and you will not mistake the tungsten-corrected 11 for the daylight-corrected 13.

Spectral Behaviour

The filter passes green most strongly, transmits some yellow and red, and absorbs much of the blue and violet that the emulsion over-records. The mechanism is the realignment described above: it both removes the blue the film has too much of and lifts the green where the film is comparatively deaf, pulling the recorded tonal scale toward the 555 nm peak of human vision. Kodak’s specification for the Wratten 11 says as much, noting that it reproduces greens slightly lighter in daylight and brings the response close to the eye’s under tungsten. The practical result reads as natural rather than dramatic: the X1 corrects where the film departs from vision instead of exaggerating contrast the way an orange or red filter does.

Foliage and Why Greens Separate

Green is the dominant transmitted band, so the filter lightens foliage that would otherwise record as a heavy, undifferentiated grey. Ilford’s own guidance on colour filters states that a green filter is used almost exclusively for foliage, lightening dark green leaves that “can record very dark without a filter,” and that the yellow #8 gives “more differentiation between the different colours of foliage.” The yellow-green keeps both behaviours.

Separation works because no two greens are the same green. Spring grass and new growth carry more yellow; conifer needles and waxy, mature leaves carry more blue and sit darker. A filter that passes yellow-green strongly while suppressing blue spreads these across the grey scale by their yellow-versus-blue balance rather than collapsing them into one tone. There is a second reason to prefer it over a red filter on vegetation: the Wood effect, named after R. W. Wood. Chlorophyll absorbs most visible light but turns nearly transparent above about 700 nm, and leaf cell structure then reflects near-infrared (roughly 700 to 900 nm) strongly. A red filter, which passes that band, is partly defeated by this hidden reflectance and renders leaves lighter and less predictably than expected; conifers, reflecting less infrared, stay darker. The X1 works wholly in the visible, so what you meter is what you get.

The Sky, in Stops

Blue absorption darkens an open sky and separates it from white cloud, but only moderately, which is the whole point of this filter. Setting it among the family makes the magnitude concrete. The Wratten 8/K2 yellow has a factor of 2 (one stop); deep yellow Wratten 15 is 2.5 (about one and a third stops); the green Wratten 58 is 4 (two stops); orange Wratten 21 runs about 4 in Ilford’s table (two stops); the red Wratten 25 runs 4 to 5 (about two to two and a third stops). Ilford’s published factors run yellow 2, green 2, orange 4, red 4 to 5, blue 2. The yellow-green’s sky-darkening lands in the middle of this spread: more than a plain yellow, well short of the heavy, near-black skies an orange or red will give you.

Skin Tones in Portraits

Place an average Caucasian face in open daylight on Zone V’s 18% grey and meter it, and convention puts the face one stop up, on Zone VI, with its brightest highlights near Zone VIII. Unfiltered panchromatic film fights you here: it over-records the cool, blue component of skin and renders red light lighter than the eye expects, so lips and ruddy cheeks drift up toward Zone VII and lose their value. The X1’s blue suppression plus green pass holds the face nearer the Zone VI you intended, while a green leaf background lifts toward Zone VI to VII, giving subject separation without a printed halo.

A worked frame, on glass-filter ratings: meter the face at EV 13 and the foliage behind it at EV 12. Fit the X1, open one stop for its factor, and expose as if the face were at EV 12. The face holds Zone VI; the foliage, already a stop down, is lifted by the green pass back toward VI to VII rather than sinking to a muddy IV. The filter literature’s shorthand that the 11 “heightens the contrast of skin tones” describes this realignment, though it is worth treating as the shorthand it is: the gain is tonal placement and modelling, not added grade. (Kodak’s own Wratten datasheet is more sober, saying only that the 11 alters panchromatic response to match the eye under tungsten and reproduces greens slightly lighter in daylight.)

Exposure Compensation, and the Factor That Disagrees with Itself

The X1 carries a real filter factor, and the published numbers contradict each other unless you know why. Kodak’s gel Wratten 11 is rated at a factor of 4, two stops, in daylight. Modern glass yellow-greens such as the B+W 060 and Hoya X0 are rated 2x, one stop. The difference is formulation, not error: the Kodak gelatin filter is a denser, more saturated yellow-green, while the glass filters are mixed lighter. Trust the factor printed for the filter in your hand. With the gel you open two stops; with the glass you open one. Apply the wrong one and you are a full stop out, enough to drop a Zone VI face to Zone V or float it to VII.

Through-the-lens meters read the filtered light and compensate automatically, but treat this cautiously, especially with stronger filters: a meter cell’s spectral response is not the film’s, so metering through a strongly coloured filter can mis-read. The safer habit is to meter unfiltered, then apply the published factor to that reading by hand, or to bracket when the light is doing anything unusual.