Most people are familiar with the effect that polarized sunglasses have on reducing reflections and cutting down glare. Unlike sunglasses, the effect of most professional polarizing filters can be continuously varied—and, as a result, go much farther in their effect.

Polarizing filters can:

reduce glare and reflections
deepen blue skies
penetrate haze
saturate (intensify) colors

Note the difference in the two photos below.

Once its many applications are understood, a polarizing filter can become a videographer's most valuable image altering filter. Simple polarizing filters need between one and one-half to two extra f-stops of exposure.

As we've noted, the effect of some professional polarizing filters is adjustable. This is done by rotating the orientation of two polarizing filters used next to each other in the housing.

When doing critical copy work, such as photographing paintings with a shiny surface, polarizing filters can be used over the lights, as well as over the camera lens. By rotating one or more of these filters, objectionable reflections can be eliminated.

Filters for Fluorescent Light

Some lighting sources are difficult to correct.  A prime example, and one that videographers frequently run into, is fluorescent light. These lights seem to be everywhere, and they can be a problem.

Although in recent years camera manufacturers have tried to compensate for the greenish cast that fluorescent lights can create, when it comes to such things as getting true-to-life skin tones—and assuming you can't turn off the lights and set up your own incandescent lights—you might need to experiment with a fluorescent light filter.

We say "experiment" because there are dozens of fluorescent tubes, each with different color characteristics. But one characteristic they all have is a "broken spectrum," or gaps in the range of colors they emit. The eye can (more or less) compensate for this when it views things first-hand, but film and video cameras typically have problems.

There are other sources of light that are even worse—in particular the metal halide lights often used in gymnasiums and for street lighting. This is discussed in more detail in the lighting module on color temperature. Although the public accepts these lighting aberrations in news and documentary footage, when it comes to drama or commercials it's a different story.

As we will see, there are color balanced fluorescent lamps that are not a problem, because they have been especially designed for TV and film work. But, don't expect to find them in schools, offices, or board rooms.

Special Effect Filters

Although there are scores of special effect filters available, we'll just highlight four of the most popular: the star filter, the starburst filter, the diffusion or soft focus filter, and the fog filter.

Star Filters — You've undoubtedly seen scenes in which "fingers of light" projected out from the sides of shiny objects—especially bright lights. This effect is created with a glass star filter that has a microscopic grid of crossing parallel lines cut into its surface. Notice in the picture on the right that the four-point star filter used also slightly softens and diffuses the image.

Star filters can produce four, five, six, or eight-point stars, depending on the lines engraved on the surface of the glass. The star effect varies with the f-stop used.

A starburst filter (on the left, below) adds color to the diverging rays. Both star filters and starburst filters slightly reduce the overall sharpness of the image, which may or may not be desirable.

Soft Focus and Diffusion Filters — Sometimes you may want to create a dreamy, soft focus effect. This can be done by using a soft focus filter or a diffusion filter (on the right, above). These filters, which are available in various levels of intensity, were regularly used in the early cinema to give starlets a soft, dreamy appearance, while hiding signs of aging. (Some "over-30" starlets even had this requirement written into their contracts.)

A similar effect can be achieved by shooting through fine screen wire placed close to the lens, or by shooting through a single thickness of a nylon stocking. The f-stop used will greatly affect the level of diffusion. In the case of soft focus filters or diffusion materials it's important to white balance your camera with these items in place.

Fog Filters — A certain amount of "atmosphere" can be added to dramatic locations by suggesting a foggy morning or evening. Without having to rely on nature or artificial fog machines, fog filters can create somewhat of the same effect. (Note the photo on the right.)
   

General Considerations
in Using Filters

Whenever a filter is used with a video camera the black level of the video is raised slightly. This creates a slight graying effect. Because of this, it's advisable to readjust camera setup or black level (either automatically or manually) whenever a filter is used.

Unlike electronic special effects created during postproduction, the optical effects created by filters during the taping of a scene can't be undone. To make sure there are no unpleasant surprises, it's best to carefully check the results on location with the help of a high-quality color monitor.
   

Camera Filter Wheels

As we've noted, professional video cameras have one or two filter wheels located behind the lens that can hold a number of filters. Individual filters on each wheel can be rotated into the lens light path as needed.

Note in the photo on the right that the camera has two filter wheels. One labeled 1 - 4, and one A - D. Each wheel can be turned to rotate into position the various options noted on the right of the photo. For example 2-B would be a 1/4 ND (neutral density) filter, along with a 3,200K color correction filter.

Filter wheels might also contain -

Although filters shown are located behind the lens, it should be noted that some filters, such as polarizing filters, must be mounted in front of the camera lens to be most effective.
   

Matte Boxes

A matte box is a device mounted on the front of the camera that acts both as an adjustable lens hood and as a way of holding gelatin filters.

As we   noted earlier, instead of using circular glass filters, comparatively inexpensive square or rectangular colored gels (gelatin filters) can be inserted into the the matte box, just in front of the lens. Note photo on the left.

Matte boxes can also hold small cutout patterns or masks. For example, a keyhole-shaped pattern can be cut out of a piece of cardboard and used to give the illusion of shooting through a keyhole (although these days there are very few keyholes you can see through).  Most of the effects formerly created by matte boxes are now more easily and more predictably achieved electronically with a special-effects generator.