Black and White Filters FAQ
Frequently Asked Questions

© Copyright 1997 and 2000 Lars Bergquist - Sweden

Black and white photography has become a minority concern. Most literature concerns color photography exclusively, and new practitioners find it difficult to obtain information on the use of color filters for B/W, which has thus become an arcane discipline. This is an attempt to clear the fog somewhat.

Contents

1. What filters are for
Spectral sensitivity of B/W film
Corrective filtering
Filtering for contrast
Technical filtering
Filter quality
2. UV filters
3. The Y--R series of filters
Yellow filters
Orange filters
Red filters
4. Black filters
5. Green filters
6. Blue filters
7. Polarizers in B/W photography
8. Filters and light meters
9. The how-to part

1. What Filters Are For

Spectral Sensitivity of B/W Film
The basic use of filters is to correct the way in which B/W film renders different subject colors in a grayscale. Since the early 'thirties, we have been using panchromatic films, which react to all parts of the visible spectrum (and then some). Earlier 'blue-sensitive' and 'orthochromatic' emulsions were sensitive only to blue, violet and ultra-violet, and to green, blue, violet and UV, respectively. Thus, an ortho emulsion rendered bright yellow as a dark gray and red as a very dark gray, as it reacted only to the small amounts of shorter-wavelength light in these subject colors. Real subject hues are of course never spectrally pure but mixed, sometimes in surprising ways.

Corrective Filtering
So, with pan film, all should be well? Well, no.

The spectral sensitivity of pan film is still different from that of the human eye. It reaches into the near ultra-violet, and it is much higher for violet and blue than is our own. Blue colors, then, give abnormally high density in the negative, and print abnormally light in the positive. The small difference in light intensity between a blue sky and white clouds, or a white building, may be completely blocked out in the negative, or at least unprintable (the sounds emanating from the darkroom are also as a rule unprintable). Conversely, green, yellow and red print too dark.

Corrective filtering is intended to rectify this by filtering away some of the light that the emulsion is over-sensitive to. So we 'put a brake' on the offending hues, giving the under-privileged green, yellow and red colors a fair chance to get some work done on the silver halides, producing more density in these parts of the negative and thus making them print lighter.

We 'brake' a color by filtering with the complementary color; so when we want to reduce e.g. blue, we apply yellow. This is the fundamental principle of all corrective and contrast filtering.

Contrast Filtering
Sometimes we do not want a 'correct' rendering. In the days when product photography was still done in B/W, it was sometimes found that two different but adjacent colors printed as the same gray tone. You could however create contrast by filtering down one of these hues. Nowadays, contrast filtering is mostly used for effect in landscape photography, where it may be desired to exaggerate contrast in the sky or in the subject --- as when we correct the sky, but more so. So we use not a light filter, but a dense one, but the basic principle --- apply the complementary color --- remains the same.

Technical Filtering
Much technical filtering too is actually contrast filtering. Here, it is a matter of blocking entirely certain wavelengths. In infra-red work e.g., we want to lock out most or even all visible light, which we do with a deep red or 'black' IR filter, which transmits IR. In fluorescence photography, we beam UV light at the subject, making it fluoresce (the same phenomenon as in day-glo paint). Here we want to keep out the strong UV light, which we do with a strong UV filter. If we do want to make an image in UV light however, we filter out visible light with a different 'black' filter which transmits UV light only. This is the kind of work quartz lenses are made for, as they let through more ultra-violet radiation than glass does.

Filter Quality
Anything you insert in the path of the light rays from the subject to the film can degrade your picture. The matter of filter quality is therefore not to be taken lightly. Filters by reputable manufacturers will work properly as advertised, but lots of bad stuff has been marketed in the past. I remember one case where the performance of a good 400 mm lens was degraded by a no-name UV filter with a sloppily fitted ring to the degree that it could not be focused. The glass surfaces were slightly skewed, and that was enough.

Filters are of five types:
1) massive glass filters in ring mounts
2) polarizers in ring mounts
3) square plastic filters -mostly 'trick filters' - in special sliding mounts
4) gelatin filters (brand name Kodak Wratten, as a rule)
5) acetate filters which should never ever be used as camera filters, only for filtering light sources.

Of these, type 1 give the best optical quality. The glass surfaces have been carefully ground and polished flat and then coated. polarizers come next; they are usually uncoated but may otherwise be of excellent quality. Plastic filters (3) are usually so-so, and clumsy to use, but inexpensive. Gelatin filters (4) are carefully controlled, but are by nature uncoated and horrendously sensitive to scratches, thumbprints and water drops which will all ruin them completely. They must use specialized holders. It is said that because of their thinness, they are the only filters which can be used behind the lens --- the exception being such lenses which are part of the lens design, as with some mirror and fisheye lenses --- but I would not use them.

Fisheye lenses cannot take front-mounted filters. They usually have a basic B/W filter set instead built into an inboard revolver. These are normally a clear UV, yellow and orange.

In the past, some filters were made of gelatin foils cemented between two thin glass plates, and mounted in rings. These are now mostly delaminated and useless (they were quite sensitive to humidity and mold).

It goes without saying that you should treat your filters with care, preserving them from scratches, keeping your greasy little fingers off them and cleaning them by the same gentle means which you would use for the outer lens surfaces. They are after all not cheap.

2. Ultra-Violet (UV) Filters

The human eye is sensitive to light from about 400 nm (nanometers) at the violet end of the spectrum to about 760 nm at the deep red end (one nanometer is one-millionth of a meter). Silver halides, however, react to ultra-violet too, and where there is a great amount of this invisible radiation, it builds density on the negative. Such situations are bright days at sea or by the seaside, medium to high altitudes in the mountains etc. The main problem in B/W photography is that UV light is strongly dispersed by the atmosphere; this light, then, is not image-forming but spreads a haze over the entire negative. UV filters, which correct this, are thus often called 'haze' filters.

The optical glass of the lens does not transmit UV at wavelengths shorter than about 340 nm. The remaining UV radiation down to 400 nm is capable of doing considerable mischief however.

Modern UV filters, in the strict sense, are clear, i.e. colorless filters. Additives to the glass absorb the unwanted UV light. On the filter ring, they are labeled UV, 0, Haze or L-39. They do not absorb any visible light, and they can therefore be kept on the lens in all situations where no other filter is needed, as they do protect the front lens to some extent. But never add two filters --- the result may well be vignetting (shading of the corners of the image), especially with wide angle lenses.

Color photographers sometimes use UV filters which have a faint pink tinge and are for 'warming' an image with too much blue, as during a overcast day. These are skylight filters (1A or 1B) or correction filters in the 81 series (81A to F). Skylight filters block UV nearly as well as real UV filters do, though they do not cut it off quite as abruptly, and can be used in B/W work too. Avoid the 81 filters. --- 'Sky' filters, then, are UV filters with an additional reddish color component. Remember though, that even in the absence of a visible blue tinge in the subject, UV radiation can register as blue on color films. Thus color films sometimes require UV protection of a different kind than that used by black and white emulsions.

3. The Y--R Series of Filters

The filters from light yellow to deep red form a series with similar properties and are the most useful of all B/W filters. They all cut off short-wave light rays quite abruptly, beginning in the violet and then progressively at ever longer wavelenghts down to green (for a red filter). The yellow filters thus address immediately the problem of over-sensitivity to blue. The range is as follows:

• Light yellow, (Y, K1), cutting at about 440 nm. This is really a 'strong UV filter' with little correction of color.
• Medium yellow (Y, K2, Y-48), cutting at c. 480 nm. The old standby in landscape photography.
• Dark yellow (Y-52), about 520 nm.
• Light orange (G), cuts at 550 nm or so.
• Deep orange (O, O-58), approximately 580 nm.
• Red (25-A), 600 nm.
• Deep red (various designations), cuts all wavelengths much above 700 nm.

Of these, the medium yellow and the orange filters are the most useful. The first is essentially a correction filter, holding back the blue and increasing contrast out of doors when the lighting is such that shadows are cast (these shadows are predominantly blue, as they are mainly lighted by diffuse light from the blue sky, and they are darkened somewhat by the filter. Increase exposure a bit to compensate). The orange filters are mostly for effect, darkening the blue sky and modelling clouds in a most dramatic manner. Remember however that for yellow or orange filters to have any effect, there must actually be some blue for them to filter! They are useless under overcast conditions, and indoors, of course.

Red filters are predominantly used for IR photography. With normal film, they give effects reminiscent of horror movies of the 'thirties, with violent contrast and black skies. In the days of B/W cinematography, cameramen used them, together with a slight underexposure, for night effects in daylight. They can be useful with long lenses, as they can bring out detail masked by bluish water-haze. They have no effect on man-made smog however.

All these filters absorb progressively more light, from c. 30 percent in the case of the light yellow to 50 with the medium, 75 with the deep orange and even more for the red. See the section on light meters below.

Red being the complementary color of green, you might think that orange and red filters would tone down green foliage to near-black. This is not the case, as chlorophyll reflects strongly in the deep red and infra-red, a fact which gives rise to the 'snow effect' in IR photography where leaves and grass can be rendered a frosty white. Green paint, through, may well photograph very dark.

4. Black Filters

These are used in IR and UV photography and filter out all or nearly all visible light, cutting at c. 700 and 400 nm, respectively. The IR black filters are in fact very extreme red filters. --It should be kept in mind that IR film, while optically sensitised for IR radiation, also retains the normal UV, violet and blue sensitivity which silver halides possess by nature. These wavelengths must therefore be kept out if you want an IR image pure and simple (it may be possible to compromise in 'pictorial' IR work). Photography in the ultra-violet can be done on standard film, it being sensitive to near-UV.

5. Green Filters

Many early pan films were over-sensitive not only to blue but also to red, leaving a dip for green in the sensitivity curve. Kodak Panatomic was originally of this type. So would it not be logical to keep down both the blue and the red simultaneously with a green filter? Many thought so, the filters were duly produced --- and discussed. No issue of a photo periodical in the 'thirties was complete without an article about 'the filter question', and especially the green filter question. The matter was scientifically investigated. Nearly all tests showed that even with extra red-sensitive film, green filters did only what the yellow filters did, and with more light loss. They did not even enhance foliage better than the yellow filters. Still, the argument about complementary colors seemed so self-evidently true that the articles sometimes ended with conclusions which flatly contradicted the published, quantitative test data! So the filters (green X-1 and yellow-green X-0) are still made. They are even sold with the same hoary argument.

A note on 'skin tones': Blue-sensitive (once called 'ordinary') and ortho emulsions tended to over-emphasise the mostly reddish patches of un-even skin tone in white-skinned people, keeping retouchers in work. Pan film removed the problem, but high red sensitivity made people look unnaturally pale, especially in incandescent lighting, the 'panchro disease'. The green filter was supposed to cure that, and you can still read in filter catalogs that it does, even though it did nothing of the sort and the problem went away with the introduction of balanced pan emulsions in the late 'thirties. Conclusion: you do not want any green filters.

6. Blue Filters

Filters in the '80' series are pure light blue, changing a pan emulsion to something reminiscent of an ortho one, brightening blue and darkening everything else. The only earthly use for these things is as effect filters for exaggerating haze in the background, which they do admirably.

7. Polarizers in Black and White Photography

These do exactly what they do in color photography: they darken the blue sky in certain directions, and they can remove or reduce such reflections that are themselves polarized (this happens when some of the light can penetrate below the reflecting surface, as on water, glass, car body paint or leaves, but not on metal, such as chrome, which is totally reflecting). They were in fact introduced for B/W use in the 'thirties, before modern color films. They are less useful as 'sky darkeners' than the Y--R filters, as they are dependent on the direction to the sun --- they work best at ninety degrees from it --- and therefore tricky to use with short wide angle lenses. They do however reduce contrast in many back-lit pictures out of doors, as they can tone down the reflections from the glossy upper surfaces of the leaves.

Note that when the TTL light meter in a camera meters light which has passed through a semi-silvered part of the main mirror, then this light is to some extent polarized. A linear polarizer may exaggerate this effect, throwing the meter off. So for use on such a camera, use circular polarizers if you want to be on the safe side. Frankly, I have never seen any real difference with negative films, color or B/W, which differ from reversal films in having exposure latitude, at least on the 'over' side.

8. Filters and Light Metering

All filters except the clear UV types do absorb some light, necessitating the setting of a larger f-stop or a slower speed than otherwise, or dialing in some compensation if on auto. Nearly all filters have a 'filter factor' printed on the mount ring, mostly as a multiplier (2 x, 4 x or the like). If it says '+ something', then it reads directly in f-stops. The multiplier means that the total exposure has to be multiplied by this amount. Remember that each change of f-stop by one full stop doubles or halves the exposure (the largest apertures may differ), and the shutter speed scale and compensation dials are arranged in a similar fashion. So if the filter factor is 2 x, open up one f-stop or change the speed from, say, 1/250 to 1/125, if it is 4 x, use two stops or 1/60, and so forth. This of course is for hand-held meters, or cameras which do not meter the light through the lens. Yes Victoria, such cameras are made, such as the lovely square-format and 6x7 Mamiya rangefinders, and the classic rangefinder Leicas.

You have probably been told that as your camera meters through the lens, and thus through the filter in front of it, you can forget about light loss. Well, not quite. First, the spectral sensitivity of the meter cell is not necessarily identical with that of the film, which may matter unless the subject happens to be neutral gray. Silicon cells e.g are over-sensitive to red and IR. This is only partially compensated for by a blue filter above it (which is what a 'Silicon Blue Cell' or SBC means). So a strongly colored subject can throw the meter off. Second, remember that Y--R series filters increase contrast, so that overall exposure may have to be increased over and above what the filter factor says in order to maintain detail in the shadows.

All filter factors, and meter readings, should therefore be interpreted as minimum exposures, or even below minimum. Remember that all negative films, B/W and color, are very tolerant of over-exposure. In fact, they can be 'over-exposed' by several hundred percent before the highlights begin to slide up on the shoulder of the characteristic curve, losing contrast and printable detail. One or two stops just serve to move the shadows up above the toe of the curve and on to the reasonably straight part of it, improving tonal separation in them.

Modern films are of the thin-emulsion type, so that neither detail nor grain suffer from moderate 'over-exposure' (i.e. above the bare minimum). I once loaded T-Max 100 and proceeded to make a series of different exposures of a very contrasty townscape. I developed carefully (no brutal cooking!) and printed the resulting images. I found that I could 'over-expose' by 1,600 percent (four f-stops, no increase is 100 percent!) without either losing highlight detail or increasing grain. The deep shadows improved with increases of one and two f-stops above the metered value (measured as incident light in sunlight) but with further increase, the improvement ceased. This was probably because stray light diffused into the shadows, decreasing contrast, in fact 'flashing' them.

Out of doors, it is actually possible for an experienced practitioner to make easily printable negatives under quite a large range of conditions without any light meter whatsoever. I know --- I used cameras long before I could afford my first selenium meter, in 1958 or so!

9. How to Use Filters

The first rule for using colored filters is: don't, unless there is a clearly statable reason to do so.

Yellow filters very seldom do any harm, except to lengthen exposure. This may of course be harm enough in low light. Remember that for general photography, Y--R series filters have no beneficial effect except for toning down excess blue; indoors, in deep shadow and in overcast or foggy conditions there is nothing for them to do, and other colored filters are of little use under any circumstances. Modern pan film --- and few of you have ever touched any other B/W material --- gives quite acceptable gray-scale rendering of normal subjects. But when walking through a lovely landscape in fine weather, by all means put the medium yellow on the lens first thing you do, keeping the orange at hand for a dramatic effect with billowing cloud or white-washed buildings in the Greek archipelago. There is no harm in bringing the polarizer along either. From your perch on the cliff twenty meters above the water, you may want to get detail below the surface in the shallows. You can safely forget about the rest of the supply, except the UV, which, if it does nothing else, at least protects the front element. Also, a proper lens shade can do more for your pictures than any filter, multi-coating or no multi-coating.

One good idea is to make two exposures of certain subjects, one with and one without a specific filter, and to carefully compare the resulting negatives and prints.

Note that at great altitudes, where there is progressively more blue in the light and skies tend to darken, the effect of the Y--R filters also increases, sometimes giving rise to too great contrast and jet black skies.

Load up with black-and-white film, learn to develop and print - and enjoy!

- Lars Bergquist

About the Author

Return to the contents page of ACE Camera photography magazine. Go