Color Filters for Black and White Film: What Each Does

The one rule behind every filter

A coloured filter lets its own colour through and holds back the opposite colour. On black and white film, anything that matches the filter prints lighter. Anything opposite prints darker. That is the entire trick, and once it sinks in you can work out most of what follows without a chart.

Why bother? Because modern panchromatic film (Tri-X, HP5 Plus, T-MAX, FP4 Plus) sees every colour, but it collapses them all onto one grey scale. Clear blue sky and white cumulus clouds can land a stop apart, which looks like nothing on the negative. Red geraniums against green leaves can come out as two patches of the same mid grey. You saw a contrast that the film simply cannot record by brightness alone.

Ansel Adams worked this out on a ledge in Yosemite in April 1927. He had exposed Half Dome with a yellow filter, realised the sky would print far too pale for what he felt, and spent his last plate with a deep red Wratten #29, giving it the full 16x factor. That plate became Monolith, the Face of Half Dome, with its sky gone nearly black.

Yellow #8: the one people leave on

The old Kodak name was K2, and plenty of photographers in the 1950s and 60s just kept one screwed on for everything outdoors. It darkens blue sky a little, enough for clouds to read as clouds instead of a white mush, and it cuts a touch of distant haze. Skin barely changes. Kodak's factor for panchromatic film in daylight is 2x, so one stop.

Its effect is mild, and a lot of people expect more. Panchromatic film is a bit oversensitive to blue compared with your eye, and a yellow filter mostly pulls the film back toward what you saw. If you shoot one frame with and one without, the difference is real but modest. I keep one on for landscapes and take it off for anything indoors.

Orange #21: the honest middle

If I could only carry one filter it would be this. The sky goes a properly dark grey while still looking like weather and not a night shot, and the far ridge you could barely see through the haze comes back. Brick lightens a fair bit. Where it really shines is a beech wood in late October: the copper leaves glow against the dark trunks and whatever green is left, and on bare film the whole canopy would have been one muddy tone.

You won't find the #21 on the Tri-X or T-MAX data sheets. Most people work with about 4x in daylight, so two stops, and that is a sensible place to start on HP5 Plus as well. Watch faces, though. Orange smooths skin and fades freckles, which flatters a teenager and does nothing good for an old farmer whose face is the whole story.

Red #25: the sky turns black

The #25 (Kodak's old A) is the one behind "the red filter look". Point it at deep blue sky, the kind you get in the mountains or at ninety degrees to the sun, and the sky heads for black with the clouds sitting on it like torn paper. It also darkens any shadow that was lit only by blue skylight. Contrast goes up across the whole frame, and on a hard sunny day that can be too much.

Red ruins more frames than any other filter. Trees go heavy and almost black. Put someone in a red jumper and you get back a ghostly white sweater and lips that have nearly disappeared. Kodak gives it 8x in daylight, three stops, and if you stack a polarizer on top (2.5x on the Tri-X sheet) you are over four. With 100 speed film and anything less than bright sun, bring the tripod.

One trap. Ortho film such as Ilford Ortho Plus has almost no red sensitivity, so a red filter on it blocks nearly everything the film can see. Use red only on panchromatic stock.

Green #11 and blue #47: the specialists

The #11 (the old X1) is a yellowish green. It lightens foliage and separates the different greens in a summer landscape, which on unfiltered film tend to merge into one dull tone. It darkens reds, so red brick, flushed cheeks and lips go deeper. For outdoor portraits of men it gives a tanned, ruddier face that a lot of people like. Kodak's daylight factor is 4x, two stops. That figure surprises people, since green glass does not look dense, but the filter eats a lot of the red and blue the film is sensitive to.

Blue #47 runs the other way from red. Sky goes pale or white, haze and fog get thicker, distance fades out fast. Red and orange subjects go dark, and skin can look blotchy with every freckle marked. It roughly imitates old blue-sensitive plates, and it is lovely for misty mornings when you want the mist to win. Kodak lists 6x for Tri-X in daylight and 8x for T-MAX 100, so call it 2.5 to 3 stops.

Filter factors, and why they move

A filter factor is how much longer you expose to make up for the light the glass removes. Every doubling is one stop: 2x is one, 4x is two, 8x is three. A 6x factor works out to about 2.6 stops. Stack two filters and the factors multiply, or the stops add, which is easier to do in your head.

So on Tri-X 400 in clear sun, Sunny 16 gives 1/500 at f/16. With a #25 mounted, open three stops: 1/500 at f/5.6, or 1/250 at f/8, or 1/60 at f/16 if you want depth.

Now the catch. Those factors were measured in daylight on specific films. Change either and they shift. Tungsten light is heavy in red and thin in blue, so warm filters lose less of it: Kodak's Tri-X sheet drops the #25 from 8x to 5x and the #8 from 2x to 1.5x under tungsten, while the #47 climbs from 6x to somewhere between 10x and 12x depending on which edition of the sheet you read. Film matters as well. T-MAX 100 lists 1.5x for the #8 and 3x for the #11, a little gentler than Tri-X. Treat any factor as a starting point and bracket half a stop either side the first time you use a filter on a new film.

Meter without the filter

With a handheld meter there is no choice: it reads the scene, not the light behind your glass. Take the reading bare and apply the factor yourself, or lower the meter's film speed to fold the factor in (Tri-X behind a red filter set as ISO 50).

Through-the-lens meters can read through the filter, and with yellow they usually get close. With red it goes wrong more often. The cell in the camera does not share your film's colour response, and silicon cells in particular see plenty of red, so the camera thinks more light is getting through than the film will record. Negatives come back thin. Meter without it, add the stops, and you are on known ground.

A small refinement I use: when the deep sky is the whole point of a red filter shot, I give it two and a half stops instead of three. Extra exposure builds density in the sky and softens the very effect you paid for. Check it against your own developing before you trust it.

Previewing it in Zone Light Meter

The hardest part of filters is guessing what the print will look like. In the app, turn on the viewfinder's B&W preview, then pick the filter on the filter ring the way you would for the exposure. With a yellow 8, orange 21, red 25, green 11 or blue 47 mounted, the live view, the frozen still after you meter, and the photo saved to the shot log all render the way panchromatic film sees through that filter. The meter already carries the filter's factor, because it is the same filter. Set it back to none when the glass comes off.

The weights come from Kodak's published Wratten transmission tables and the spectral sensitivity curves of T-MAX 100 and Tri-X 400, balanced so a neutral grey stays the same grey. It is a simulation. A phone sensor is not film, and colours the camera clips cannot be brought back. The B&W Filter Advisor now has a Preview in viewfinder button too, and this release fixed the Green #11 factor, which the app had at one stop instead of two. The B&W filter preview page has the details.