Filters in Black and White Photography

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    Iain McCulloch

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    The Application of Filters in Black and White Photography

    Filters have a wide range of applications in photography. Essentially, a light filter is a

    transparent material which modifies the characteristics of the light passing through it. Some

    filters absorb or filter-out particular wavelengths of light. These colour correction filters

    will affect the image recorded on the film. Another group of filters create special effects likestar or diffuser filters. These create particular artistic effects on the photographic image. A

    third category of colourless filters are used to compensate for environmental conditions

    which the photographer wishes to eliminate from the final image. These include ultra-violet

    or skylight filters, neutral density filters, and polarising filters. The three categories of

    filters will be considered in turn.

    The response of photographic film to light is not exactly the same as the human eye. The

    difference for panchromatic film is illustrated in figure 1 below:

    Figure 1: Colour reproduction of panchromatic film relative to the eye's response

    (after Langford, 1986)

    Light of shorter wavelengths (violets and blues), and of longer wavelengths (orange-red) thus

    appear lighter than we would judge by eye. By contrast, green shades, nearer the middle of the

    visible spectrum, appear darker. The vast majority of black and white films have a

    panchromatic sensitivity to colour [Langford, 1986, p163]. Spectral sensitivity curves are

    published by manufacturers with the data sheets for their films. The spectral sensitivity curves

    for Ilford HP5+ and Ilford Delta 100 black and white film (taken from the relevant Ilford

    data-sheets) are shown in figures 2 and 3 below for comparison.

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    Figure 2: Ilford HP5+ Spectral Sensitivity Curve Figure 3: Ilford Delta 100 Spectral Sensitivity Curve

    A frequently encountered effect of the spectral sensitivity of panchromatic film can be seen

    when photographing white clouds in a blue sky. The sky records much paler than it seemed to

    the eye at the time, with the result that clouds are barely visible in the final print. This may

    change the dramatic effect that the photographer was trying to achieve. Colour filters may be

    used to modify the characteristics of light reaching the film. For example, an orange-red filter

    used in combination with panchromatic film will absorb much of the blue light making thesky darker and causing white clouds to stand out.

    Coloured filters normally affect the whole image (but see the comments on graduated filters

    below). This must be considered when selecting the correct filter. Consider an image

    consisting of a landscape with green foliage, blue skies, and white clouds. A described above,

    an orange-red filter would darken the sky making clouds stand out. However, it would also

    darken the green shades of the foliage. A green filter would darken the sky almost as much

    but would make green tones lighter. The effect some of the most commonly used coloured

    filters is illustrated in figure 4 below.

    Figure 4a: Unfiltered Image Figure 4b: With Yellow Filter

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    Figure 4c: With Orange Filter Figure 4d: With Green Filter

    (After Cokin, n.d.)

    Effects and applications for selected coloured filters are presented in table 1.

    Filter filter-factor

    (daylight)

    f-stop

    Yellow

    Lends contrast to clouds. Gently subdues ambient light. Lightens skin and blond hair and softens freckles in portraits. Provides light black and white contrast.

    x 2 + 1

    Orange

    Darkens skies; accentuates clouds. Strongly subdues ambient light. Brings out texture in stonework. Darkens greens. Provides medium black and white contrast.

    x 4 + 2

    Red

    Darkens blue skies, almost to black. Turns red stains white. Makes greens darker. Reveals grain in wood. Provides strong black and white contrast. Used with infra-red film.

    x 8 + 3

    Green

    Darkens blue skies, accentuates clouds. Turns green stains white. Makes green vegetation lighter. Slightly darkens colour of skin & lips in portraits, but will exaggerate

    red skin blemishes.

    x 4 + 2

    Yellow-Green

    Tones down blue skies. Good for outdoor portraits, but may exaggerate freckles. Corrects response of panchromatic film to approximate to that of

    human eye.

    x 2 + 1

    Blue

    Lightens blue skies. Turns red stains black.

    x 6 + 3

    Table 1: Colour Filter Characteristics

    It will be noted that each filter has an associated filter-factor. This factor is the increase in

    exposure which will be required if a hand-held meter is used. If a through-the-lens (TTL)

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    metering system is used, it is important that the filter to be used is attached when calculating

    exposure. The filter factors given in the table are those recommended for Cokin filters

    [Cokin, n.d., p99]. Manufacturers technical descriptions should be checked to determine the

    filter-factor for the filter used.

    Obviously, filters must be placed in front of the lens in order to affect the light reaching the

    film. Filters are available in three main forms: thin sheets of dyed gelatin, glass screw-mountfilters, and filter systems. The choice of which form to use depends on how photographers

    use their camera system. Gelatin filters are cheap but fragile. They are either held over the

    lens or cut to size and mounted in a holder. Screw mount filters are more robust and also

    more expensive. Since they screw directly onto the end of the lens, they are normally only

    used with a single lens diameter (although adapters are available).

    Filter systems are made by a number of manufacturers including Cokin, Hoya, Jessops and

    Boots. Filter holders are attached to the lens with a screw-fit adapter. A wide range of filters

    can then be fitted into the holder. Common filter sizes for filter systems are 67x67mm,

    75x75mm, and 84x84mm. Each requires the appropriate holder. The size should be chosen to

    avoid vignetting with the particular camera/lens combination being used. There are times

    when the effect that the photographer wishes to achieve requires something more than simplecolour correction filters. Several manufacturers produce a range of effects filters: far too many

    to be exhaustively covered here! Most effects filters can be used with either colour or black

    and white photography. Some examples will suffice to illustrate what is available.

    * Graduated filters are only coloured on only part (typically half) of thefilter. Thus the effect will only be seen on that part of the image framed

    against the coloured half of the filter.

    * Star filters have a pattern of lines etched on the surface which causelight to flare along the lines. The pattern determines the number of rays

    (commonly 2, 4, 8, or 16). With these filters a high-intensity light sourceappears as a star. The size of the star depends on aperture.

    * Diffusion filters produce some degree of soft-focus. Portrait-softdiffusion filters can be used to soften facial lines and highlights in portrait

    work. The effect of diffusion filters varies with focal length and aperture.

    * Spot filters have a clear central area and a diffused surround. They areoften used in portrait photography.

    The final category of filters, colourless filters, are also suitable for use with either black and

    white or colour photography. The three main types of colourless filters used in black and

    white photography are ultra-violet (UV) filters, neutral-density (ND) filters, and polarisingfilters.

    Unlike the human eye, film is sensitive to ultra-violet light. Scattered UV shows as a misty

    haze on distance shots, especially in mountains or near the sea. A filter which absorbs UV

    eliminates this haze and helps record the image as it appears to the eye. If the lens is also used

    for colour photography the skylight filter may be used in place of a standard UV filter. This

    has a slightly pink colour to correct the blue colour cast which accompanies high ambient UV

    conditions. Many photographers fit a screw-mounted UV or skylight filter to every lens. This

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    not only eliminates UV but also offers relatively inexpensive protection against damage to the

    lens itself.

    Neutral density (ND) filters affect all wavelengths of light equally. Thus they simply reduce

    the amount of light reaching the film. This can be important in a number of circumstances.

    For example, if one needed to take pictures in bright sunlight on a high-speed film, or a wide

    aperture was required in order to limit the depth of field. Alternatively, a ND filter may berequired to allow use of a slow shutter speed to create blur on moving objects. Neutral density

    filters are grey in appearance and are available in a range of densities. Table 2 shows the light

    transmission, aperture increase and time increase offered by a range of neutral-density filters.

    FILTER TRANSMISSION APERTURE

    (stops)

    TIME

    ND0.1 79.4% x 1.25

    ND0.2 63.1% 1.50

    ND0.3 50.0% 1 2.00

    ND0.4 39.8% 1 2.50

    ND0.5 31.6% 1 3.00