Filters for film photography

What we think of as "white" light is, in fact, composed of an entire spectrum of colors, both visible and invisible. Each color of the spectrum has its own wavelength, measured in nanometers. The visible spectrum ranges from red on the one end (700 nanometers) to violet on the other (400 nanometers). Beyond this visible spectrum on the red side are invisible light waves known as infrared, and beyond the spectrum on the violet side are wavelengths called ultraviolet. Most of the light we see is composed of unbalanced amounts of a the different colors in both the visible and invisible portions of the spectrum.

Because light which appears white is not pure white, an impartial recording medium like film may record the scene as "reddish" or "bluish." You have probably noticed this phenomenon in your photographs, but the effect is even noticeable to the naked eye. By turning your eyes quickly from daylight to an incandescent bulb, you will notice that for a few seconds the bulb seems unusually "warm" looking, strongly orange in color. This is the way that "daylight" film records scenes lit with incandescent light. This is one reason why filtering is necessary in many photographic situations.

Filters function by holding back portions of the spectrum and allowing others to pass freely through. In this way they can correct what our eye perceives to be imbalances in color tonality. This imbalance is as true for black and white (B&W) as it is for color film, though its effect is less pronounced in B&W.

Film vs. the Eye

In addition to its ability to adjust to different light sources the eye has other qualities which differ from the qualities of films. For example, the eye is more sensitive to yellow than are most panchromatic B&W films, meaning that yellow items are rendered in a darker shade of gray relative to other colors than we would expect. The sensitivity of the eye diminishes to those colors progressively farther from yellow on the spectrum. Ultraviolet, which is practically invisible to the human eye, is still recorded by the film, and causes tonal relationships unnatural to our eyes. As a result, skies and foliage, which often reflect high amounts of ultraviolet may appear unnaturally light or even white in a B&W print. So to restore a natural tonal relationship which corresponds to our perception we need to filter.

Enter Filtration

Whether used to shift the color balance with color film or to improve contrast and tonal rendition with color or black-and-white films, filters function by absorbing selected areas of the visible and invisible portions of the spectrum.

So a good way to think of filters is not so much by the color they appear to the eye, but rather in terms of the colors they hold back from the film.

Essentially a filter is a subtractive device, absorbing certain wavelengths of light and thereby seeming to boost the value of other wavelengths.

Filters for Black-and-White Photography

In black-and-white photography, the principal aim of filtration is to change the way certain colors are rendered on a "gray scale" of black to white. As mentioned earlier, film does not respond to the brightness of colors in the same way as does the human eye, meaning that some colors are represented lighter than we see them in nature, and some are rendered a darker shade of gray than we find "natural." To highlight details, to improve contrast, or to create special effects we can choose filters that suppress certain portions of the spectrum while allowing others to pass. For example, skies reflect ultraviolet and blue to which film is quite sensitive. The result is a "washed-out sky-look" that can take away from the effectiveness of a black-and-white print. By using a filter which absorbs blue and ultraviolet, less light from the sky passes on to the film and the result is a more dramatic photograph.

Contrast can be improved with the use of filters by choosing one color to be darkened with respect to the others and then choosing a filter which absorbs that color. A flower, for example, can be made to stand out against darkened foliage by choosing a filter which passes the color of the flower while absorbing the green of the foliage.

To create strong pictorial effects, deep filters can be used for unusual renderings like black skies, very high contrast, and "nighttime" effects taken in daylight.

Filters for Color Photography

Color photography poses its own problems, and filtration often becomes an essential tool to obtain the desired result. No color film can ever see light or color the same way the human eye sees it. The best that can be hoped for is a reasonable balance. Our eye and brain working together can compensate for such affecting variables as the color and brightness of objects surrounding the main subject, its gloss, and the color temperature of the light which illuminates the scene. To get a truly correct rendering of the subject with the kind of film we are using, we have to take all these factors into account and choose a filter accordingly.

In practice, the process is not as complicated as it sounds for most types of photography. Once again, the key is to approach filtering from the idea of subtraction; what colors need to be suppressed? For example, the A-series filters cut blue while allowing other colors to pass. So rather than thinking of the A-series as amber filters, which they are, we ought to look at them functionally as "blue-suppressing" filters. This helps us to see that this series is useful for eliminating the blue tints encountered when photographing outside on heavily overcast days or with film balanced for the warmer light of electric bulbs.

This brings us to the subject of the different types of color film. Daylight film is ideally balanced for soft noonday sun. However, the morning sun tends to have a slightly lower color temperature (more yellow and red component) than noon sun, and late afternoon sun is redder still. On these occasions a blue filter will suppress the reddish quality of the light and deliver a more neutral rendition. Incandescent lights produce so much red that it is difficult to filter it all satisfactorily. However, photofloods can be used with daylight film when filtered, and conversely, tungsten film can be used in daylight when the blue-suppressing amber filter is used.

For special effects, seemingly inappropriate or strongly colored filters can be used to create a mood, add drama, or give a painting-like quality to color photographs.

Filters for Color and Black-and-White Films

There are some filters that have no strong effect on color rendition, and whose properties are effective with color and B&W. Contrast, darkened skies, and reduced transmission can be effected with these filters regardless of the film used.

Filter Factor

Since filters are subtractive devices, they do cut down the amount of light which reaches the film. Therefore most filtered photographs require additional compensation to ensure a proper overall rendering of tones. Cameras equipped with TTL metering measure light through the lens and therefore through any mounted filter. Because the light is measured after the filtration process, no manual exposure compensation is required*. For cameras without TTL metering, the lens aperture or shutter speed must be adjusted from the normal reading to compensate for the reduced light transmission of filtered exposures. The amount of adjustment is a geometric progression represented numerically - the filter factor. A filter with a factor of two requires one extra "stop," a factor of four requires two stops, a factor of eight requires three "stops" and so on.

* Except when using the R60 under tungsten lights.

Filters for Black-and-White Film

Yellow (Y44, Y48, Y52)

Because black-and-white films are more sensitive to blue and ultraviolet than the human eye, blue objects are often rendered lighter than we want them to be. Skies, for example, often appear white and lifeless in prints where we see them as a darker blue. Yellow filters absorb blue, producing darker skies with more pronounced clouds. They are particularly useful for landscapes which often reflect a large quantity of ultraviolet, which is invisible to the eye but registers strongly on the film. Y44 is a mild filter, while Y48 and Y52 have progressively stronger effects.

Orange (056)

The orange filter absorbs more of the spectrum than the yellow, suppressing ultraviolet, blue, and green, while passing orange and red. The effect of the orange filter is more pronounced than the yellow in darkening skies and heightening contrast. Because orange absorbs green, the orange filter is very effective for pictures of flowers and trees, where the darkened foliage emphasizes the texture of rocks, tree trunks, and similar objects. Another situation which calls for an orange filter is telephoto landscape photography. Long lenses can reduce contrast with distant scenery, but filtering restores this contrast to normal.

Red (R60)

A very high contrast filter which absorbs all colors of the spectrum except red. Skies are darkened, and red objects appear to jump out of a dark background. Dramatic cloud effects and simulated night effects make use of the severe cutting action of the red filter. It should also be considered when photographing misty landscapes where the additional contrast may be needed to provide sufficient detail. The red filter is also used with infrared film.

Green (X0, X1)

Green filters absorb both red and blue, and so are effective for highlighting both green and yellow. X0 has a less pronounced effect in subduing red than does X1, though both absorb blue light about equally. X0 is a good all-around filter for both panchromatic and orthochromatic films, while X1 is particularly recommended for portraiture where its red-absorbing quality can be used to balance the reddish cast of floodlights to render skin tones in a more natural tonal relationship.

Selection of Filters for Black-and-White Film

Filters for Color Film

Amber (A2, A12)

Amber filters absorb blue, and are therefore useful in counteracting the bluish tint that sometimes affects daylight-balanced color film. A2 is mild in effect and is normally used to warm up the bluish cast of subjects in the shade or under cloudy skies. It is also used to neutralize the effect of light from a north window when shooting daylight film indoors during the day. A12 is a stronger amber and is used with tungsten film when shooting outdoors in fair weather.

Blue (B2, B8, B12)

B2 is a mild blue filter which can cool off the normally reddish cast of daylight within three hours of sunrise or sunset. It is used with color film balanced for daylight. B8 is a medium-blue filter which gives correct color rendition on daylight film used with clear flashbulbs. B12 is a strong blue fiIter which converts daylight film for use with 3400° K photofloods when shooting indoors.

Selection of Filters for Color Film

Filters for Both Color and Black-and-White Film

Skylight (L1A, L1B, L1BC)

Daylight type color film, used in fair weather at midday, often displays a slightly bluish cast, owing to the film's sensitivity to invisible ultraviolet rays. Skylight filter cuts excessive blue-green and ultra-violet, as well as haze, to make the effect more natural and to uncover more details. The blues of a summer seashore, or a winter scene under blue skies, for instance, will appear toned down. This filter also serves as a lens protector.

Ultraviolet (L37, L37C, L39)

Absorbs the ultraviolet light invisible to the naked eye without affecting visible light to effect clear-cut, non-hazy pictures with B&W film-and pictures without bluish or violet tints when color film is used. Ultraviolet filters are colorless and transparent, requiring no exposure compensation, and are thus useful for general photography; they also serve to protect the lens. The L37 is for general use while the L39 is particularly recommended for B&W photography where ultraviolet rays are apt to be strong, as in mountain photography in midsummer.

Neutral Density (ND2, ND4, ND8)

Used with B&W and color films, Neutral Density filters absorb all colors equally, and are therefore employed when color balance is to be retained while restricting the amount of light entering the camera. They are useful in controlling depth of field by allowing a larger aperture to be used while the shutter speed need not change. They can also be used when no combination of aperture and shutter speed is sufficient to prevent overexposure, as with high-speed films in daylight, or where a slow shutter speed effect may be wanted.

Polarizing Filters

Polarizing filters can be used with both color and B&W films, and reduce the reflected light from non-metallic surfaces such as glass and water. The polarizer is the only filter which can darken the sky in color photography while not affecting the overall color balance. It also reduces glare from grass and trees, giving them a deeper, more saturated color. Polarizing filters may be used to photograph through window glass, or spectacles. The effect of a polarizer can be observed on a focusing screen it is being rotated (up to 180°) on its mount. For maximum control of reflections, the camera position may need to be altered because best results are obtained when the camera's optical axis is at an angle of 32 ~ 37 degrees from the target surface. For maximum sky darkening effect, the camera should be at right angles to the sun.

Source: Nikon Sales Manual: General Camera Accessories.