Part II

Filters and Lens Attachments

Polarizing Filters

Contrast Control Filters

Although the best of the latest generation of professional video cameras is capable of capturing contrast or brightness ranges up to 700:1, most home television sets and viewing conditions limit that range to about 30:1. This means the brightest element in a scene can't be more than 30 times brighter than the darkest element -- with any hope of seeing the detail in each. (Digital/HDTV receivers do considerably better, but until everyone has a digital set, we must play it safe.)

"Real world scenes" often contain collections of elements that exceed the 30:1 brightness range. Although in the studio we might be able to control this with lighting, things become a bit more challenging outside. For critical exterior scenes, the videographer must often consider ways to reduce the brightness range. One way is with a contrast control filter.

Look at the scene on the left below, taken in a setting with contrasty lighting. The use of a contrast control (or low contrast or contrast reduction) filter resulted in the image on the right.

There are three types of these filters: low contrast, soft contrast, and the Tiffen Ultra Contrast. It's possible to use one of these, together with fog and mist filters, to simulate the "film look" in video.

Filters for "The Film Look"

Compared to film, some people feel digital video can look a bit harsh, overly sharp, and even brassy. Studies have shown that audiences have gotten used to and seem to prefer the slightly grainy effect of film -- leading some post-production houses to add electronic "grain" to video.

Since some directors of photography (DPs) feel people are more comfortable with the "film look,"  this link provides more information on achieving that.

At the same time, others feel video is a unique medium that should not try to take on the characteristics of film.

Day-For-Night

A common special effect, especially in the days of black-and-white film and television, was the night scene shot in broad daylight -- a so-called day-for-night. (In those days, film stocks and video cameras were not very sensitive to light, and you couldn't shoot at night.) 

With black-and-white film or video, you could place a deep red filter over the lens to turn blue skies dark, even black. (A red filter subtracts blue.) That, together with three or four f-stops of underexposure, completed the illusion.

Although not quite as easy to pull off in color today, you can simulate the effect by underexposing the camera by at least two f-stops and either using a blue filter or creating a bluish effect when you white balance your camera. (We cover this in a section called "lying to your camera" in Module 18.)

A contrast control filter and careful control of lighting (including avoiding the sky in scenes) adds to the effect. Embellishments you can add during postproduction make the night-time effect even more convincing.

With the sensitivity of professional cameras now down to one foot-candle (a few lux), "night-for-night" scenes are now possible. Whatever approach you use, experiment using a high quality color monitor as a reference.

Color Conversion Filters

Color conversion filters correct the sizable difference in color temperature between incandescent light and sunlight -- a shift of about 2,000K. Although the differences in color temperature among light sources will make more sense after we examine it in a later module, we need to at least mention it in this section on filters.

Even though professional cameras take care of minor color balancing electronically, colored filters are best for major shifts, such as the difference between indoor and outdoor lighting..

Two series of filters exist in this category: the Wratten #80 series, which are blue and convert incandescent light to the color temperature of sunlight, and the Wratten #84 series, which are amber and convert daylight to the color temperature of tungsten light.

Since video cameras are optimized for one color temperature, the cinematographer will generally use these filters to make the necessary ballpark adjustment. The rest is done electronically with camera color balancing.  

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 may need to experiment with a fluorescent light filter.

We say "experiment" because dozens of fluorescent tubes exist, 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 "smooth over" these gaps when it views things firsthand, but film and video cameras have problems.

Some other sources of light are even worse -- in particular the metal halide lights often used in gymnasiums and for street lighting. We discuss this in more detail in the lighting module on color temperature. Although the public may accept these lighting aberrations in news and documentary footage, it's a different story when it comes to most commercials and dramas.

As we will see, some color-balanced fluorescent lamps are not a problem, because manufacturers design them specifically for TV and film work. But don't expect to find them in schools, offices, or boardrooms.

Special Effect Filters

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

Star Filters - You've undoubtedly seen scenes in which "fingers of light" project out from the sides of shiny objects -- especially bright lights. The camera operator creates this effect 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 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 - To create a dreamy, soft focus effect, you can use a soft focus filter or a diffusion filter (on the right above). These filters, available in various levels of intensity, were often used in early cinema to hide aging signs in actors. (Some stars even wrote this requirement into their contracts.)

You can achieve a similar effect by shooting through either a fine screen wire placed close to the lens or a single thickness of nylon stocking. The f-stop you choose will greatly affect the level of diffusion. It's important to white balance your camera with these items in place.

Fog Filters - You can add a certain amount of "atmosphere" to dramatic locations by suggesting a foggy morning or evening. Without relying on nature or artificial fog machines, fog filters can create somewhat the same effect. (Note the photo on the right.)   

General Considerations

In Using Filters

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

Unlike electronic special effects that an editor creates during postproduction, the optical effects a cinematographer creates while recording a scene can't be undone. To reduce the chance of unpleasant surprises, carefully check the results as you shoot with the help of a high quality color monitor.     

Camera Filter Wheels

As we've noted, professional video cameras have filter wheels behind their lenses that can hold a number of filters. You can rotate individual filters on each wheel into the lens light path as needed.

Note the two filter wheels in the photo on the right. One is labeled 1 through 4 and the other A through D. Some of the options are noted in the chart on the right of the photo. For example, 2-B would be a 1/4 ND (neutral density) filter, along with a 3,200K (standard incandescent light) color correction filter.

Filter wheels might also contain the following:

Although the filters shown are located behind the lens, to be most effective you must mount some filters, such as polarizing filters, in front of the lens.    

Matte Boxes

A matte box is a device mounted on the front of the camera that acts both as an adjustable lens hood and a way of holding square or rectangular gelatin filters. These are much cheaper than the round, glass filters discussed earlier.

Matte boxes can also hold small cutout patterns or masks. For example, you could use a keyhole-shaped pattern cut from a piece of cardboard to give the illusion of shooting through a keyhole (although, unlike earlier days, we can now see through very few keyholes).  

Most of the effects that matte boxes formerly created can now be more easily and predictably achieved electronically with a special-effects generator.

Periscope Lens

A "bug's eye" view of subject matter is possible with a periscope/probe system.

This low angle is useful when actors are electronically keyed into realistic or fantasy miniature models. We can enhance the effect with the wide-angle views of the four lenses that come with the system.

In the photo on the right, the camera operator uses a lens probe to film a miniature prehistoric setting that will later come to life in a full-scale effect. Although this is a film camera, it has a video viewfinder to provide immediate feedback on the image captured on film. (Note the video monitor.)

In the next section, we conclude the discussion of lenses and lens attachments.