The purpose of a filter is to selectively modify the light used for exposure. Filters are regularly used by professional photographers. They are an indispensable means of controlling the variations in light the photographer is likely to encounter. However, when capturing RAW file images it is possible to undertake color correction in post production.

Studio

In the studio the photographer is able to create images with a consistent quality using few, if any filters. The photographer must simply ensure the film stock or image sensor white balance is carefully matched to the light source being used. The studio photographer has the option of filtering the light source, the camera lens or placing the filters between the lens and the image plane. If the photographer is using a mixture of light sources the photographer should ideally filter the light at its source. Any filter attached to the camera lens must be of premium quality – preferably glass.

Location

On location where ambient light is the primary light source or cannot be eliminated from the overall exposure the necessity to carry and use a broad range of filters increases. If choosing to purchase filters for lenses the photographer needs to be aware of the thread size on the front of each lens intending to be used. Filter sizes for fixed focal length, wide angle and standard lenses on 35mm and DSLR cameras are usually between 48 and 55mm. Medium-format, telephoto and zoom lenses may have thread sizes much larger. Purchasing every filter for every lens would be an expensive operation, so filters have to be selected carefully through actual rather than perceived need. Plastic filters are available from manufacturers which can be adapted to fit a range of different lens diameters. The initial outlay to filter all lenses can be greatly reduced, but their working life may be far less due to the greater risk of damage.

Basic color theory

To be comfortable with filtration it helps to understand basic color theory. The broad spectrum of visible light is divided into three primary colors and three secondary colors. The primary colors of light are red, green and blue (RGB).

The secondary colors are yellow, cyan and magenta. When used in the printing industry to create images black is added (CMYK or four color printing).

Each secondary color is a combination of two primary colors and is complementary to the third primary.

• Yellow is complementary to blue
• Cyan is complementary to red
• Magenta is complementary to green

Filter categories

Filters can be divided into five main categories:

• Neutral filters
• Black and white
• Color conversion
• Color balancing
• Effects

Neutral Filters

A range of filters are manufactured that have little or no color. These include:

• UV (haze) and “skylight” filters (clear)
• Neutral density filters (grey)
• Polarizing filters (grey)

UV and skylight filters

UV radiation present in the spectrum of light is invisible to human vision but adds to the overall exposure of the image. It is most noticeable with landscape images taken at high altitude and seascapes. To ensure the problem is eliminated, UV filters can be attached to all lenses used on location. If the optical quality of the filter is good the filters may be left permanently attached to the lenses. The added benefit of this practice is the front lens element of each lens will be protected from scratching.

A skylight filter can also eliminate UV light and has a color compensating effect. Shadows filled by skylight have a blue cast and the slightly pink filter helps to create shadows with a neutral color cast. Skylight filters are identified as a 1A and a stronger 1B.

Neutral density filters

With manufacturers going to great lengths to create fast lenses with wide maximum apertures it may seem strange to find a range of filters available which reduce the amount of light at any given aperture. These are neutral density filters and are available in a range of densities. If only one is purchased the photographer should consider one that can reduce the light by at least two stops. Neutral density filters are used for two reasons:

• Reducing depth of field
• Increasing duration of exposure

Reducing depth of field

Very shallow depth of field is not always possible when working on location if the ambient light is very bright. If the photographer is using an ISO setting of 100 and direct sunlight to illuminate the subject the photographer may only be able to select f5.6 or f8 as their maximum aperture, if the maximum shutter speed available on the camera being used is 1/1000 or 1/500 second. This aperture may not be enough to sufficiently isolate the subject. If the aperture was increased further overexposure would result. The problem can be solved by using a lower ISO, a camera with a focal plane shutter capable of exceeding 1/1000 second, or a neutral density filter.

Increasing duration of exposure

Long exposures are not always possible when working on location if the ambient light is bright. If the photographer is using a film or image sensor at 100 ISO and direct sunlight to illuminate the subject the photographer may only be able to select 1/60 or 1/30 second as their slowest shutter speed, if the minimum aperture on the camera lens is f22 or f32. This shutter speed may not be sufficient to give the movement blur required. The problem can be solved by using a lower ISO, a lens with a minimum aperture smaller than f32 or a neutral density filter.

Polarizing filters

Polarized light is the light reflected from non-metallic surfaces and parts of the blue sky. A polarizing filter can reduce this polarized light and the effects are visible when viewing the image in the camera.

A polarizing filter is grey in appearance and when sold for camera use consists of the actual filter mounted onto a second ring, thus allowing it to rotate when attached to the lens. The filter is simply turned until the desired effect is achieved.

The polarizing filter is used for the following reasons:

• Reduces or removes reflections from surfaces
• Darkens blue skies at a 90 degree angle to the sun
• Increases color saturation

Possible problems

The filter should be removed when the effect is not required. IF not removed the photographer will lose two stops and reduce the ability to achieve overall focus.

When the polarizing filter is used in conjunction with a wide angle lens, any filter already in place should be removed. This will eliminate the problem of tunnel vision or clipped corners in the final image. Photographing landscapes when the sun is lower in the sky can result in an unnatural gradation, ranging from a deep sky on one side of the frame to a lighter blue sky on the other.

Black and white filters

When the photographer is using black and white film there is the option of controlling tonal values and contrast by using colored filters, e.g. a green apple and an orange may record on black and white film with the same tonal value or shade of grey. The use of an orange filter would result in the orange recording lighter and the apple darker; using a green filter would result in the apple recording lighter and the orange darker. Filters lighten their own color. In this way the tones are made different.

Many photographers using black and white film use a yellow/green filter as standard to correct then bias of the film towards the blue end of the spectrum.

The basic rule when using colored filters with black and white film is:

• Adjacent colors are lightened.
• Opposite colors are darkened.
• Color conversion filters

The definition of a color conversion filter is one used to convert the color temperature of various light sources to match the color balance of the film being used. When capturing RAW file images it is possible to undertake color correction in post production.

Tungsten light

The filters used for this purpose are the 80 series including the 80A and 80B. These filters are blue in color and are used to balance daylight film with tungsten light sources. The 80B is used mainly in a studio situation to balance daylight film with photoflood bulbs specifically made for photographic lighting purposes.

When using film a location photographer should have an 80A filter available. This filter allows daylight film to be used with tungsten-halogen lamps and record with approximately neutral tones. If daylight film is used in conjunction with an 80A filter and ordinary household light bulbs the resulting transparencies will still have a slightly warm cast.

A problem associated with using an 80A filter is the loss of two stops. The photographer prepared for this will usually carry a 400 ISO film which they are prepared to push if necessary.

Fluorescent light

It is very difficult to assess the color cast that will result when fluorescent lights are illuminating the subject. Most images record with a green cast and most conversion filters are predominantly magents colored with additional yellow or cyan filtration. An FLD filter is marketed for use with Fluorescent lights and daylight film but it usually only improves rather than rectifies the situation. With six main types of fluorescent lights commercially available all requiring different color conversions the best advice to a photographer is to:

Switch fluorescent lights off if possible.

Photographers leaving fluorescent lights on run the risk of a heavy color cast. This is often an oversight or the photographer may think the fluorescent lights are contributing little to the overall illumination and knowingly leave them on. The result is often devastating. Individuals with fair hair appear with bright green sprouting tufts from the tops of their heads and the tubes if in the frame appear as bright green.

Color balancing filters

Color balancing filters are used to produce more subtle changes in the color balance of the final image. They are particularly useful when working on location using color transparency film. The most common light balancing filters used are the 81 and 82 series filters. The 81 series are warm in color (yellow) and the 82 series are cool in color (blue). The subtlest changes are made with the straight 81 or 82 filters, the A, B and C becoming progressively stronger. Exposure compensation is between one-third and two-thirds of one stop.

One or two filters from the 81 series are particularly useful for removing the blue cast recorded when photography in overcast conditions or in the shadows present in full sun.

Effects filters

Numerous special effects filters are available from camera stores. Most are a gimmick and once used are quickly discarded by the serious photographer. These include star-burst filters and graduated color filters.

Probably the most commercially viable special effects filter is the soft focus filter or diffuser. This is especially useful in close up portraiture where the photographer wants to create a flattering portrait but the sitter has a somewhat less than perfect complexion.

Effects with standard colored filters

Many special effects can be created with conventional color filters and a little imagination. These include using a colored filter over the light source and a complementary filter on the camera lens. The effect is a neutral toned foreground with a background that bears the color cast of the filter on the camera lens.

Another effect is obtained by mounting a camera on a tripod and taking three exposures of moving subjects on the same piece of film. If the photographer uses a different primary colored filter (25 red, 61 green and 38A blue) for each of the exposures the effect is a near neutral background with colored moving subjects. Take a meter reading without the filters attached and open up one stop for each of the three exposures.

Filter factors

Filter factors are indications of the increased exposure necessary to compensate for the density of the filter. A filter factor of two indicates the exposure should be double (an increase of one stop). A filter factor of four indicates the exposure should be increased four-fold (two stops) and eight requires three stops increased exposure.

Establishing an appropriate exposure

When using a camera with TTL metering the light meter is reading the reduced level of transmitted light and so theoretically the exposure should require no compensation by the photographer. In practice, when using the deeper color filters and TTL metering, it is advisable to take a meter reading before the filter is attached to the lens and then apply the filter factor. This leads to more accurate exposures being obtained as the meter can be misled by the colored light if metering through the filter.

Using more than one filter

Remove UV or skylight filters when using other filters. All filters provide UV filtration. Beware of vignetting or cropping the corners of the image when using multiple filter combinations. When using two filters the combined filter factors should be multiplied not added. If a neutral density filter with a filter factor of four is used in conjunction with an orange filter, also with a filter factor of four, the resulting filter factor would be sixteen, not eight. Adding the filter factors instead of multiplying them would lead to a one stop underexposure.