Filters and Lens
You're probably familiar with polarizing sunglasses that reduce reflections and cut down glare.
Unlike sunglasses, however, the effect of most professional polarizing filters can be continuously varied and, as a result, go much farther in their effect.
Polarizing filters can:
Note the difference in the two photos below.
Once you understand a polarizing filter's many applications, it can become one of your most valuable filters.
As noted, you can often adjust the degree of polarization. This is done by rotating the double glass elements in the filter.
To eliminate objectionable surface reflections when doing critical copy work, such as photographing paintings with a shiny surface, you can use large polarizing filters over the lights or the camera lens. This is one of the areas where post-filtration can't match the effect of optical filters.
Contrast Control Filters
Although the best of the latest generation of professional video cameras are 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 results in the image on the right.
There are three types of these filters: low contrast, soft contrast, and the Tiffen Ultra Contrast.
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 softer and grainy effect of film -- leading some post-production houses to electronically add these things to video.
Some directors of photography (DPs) people feel it's better to add these things as the video is shot. This link provides more information on achieving this "film look" with optical filters.
A common visual 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 as sensitive to light as they are today and you couldn't readily shoot at night.)
But with black-and-white film or video you could place a deep red filter over the lens to turn blue skies very 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, 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 careful control of lighting and avoiding the sky in scenes adds to the effect.
Embellishments you can add during post-production 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,
you need to check out the effect 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 can electronically take care of minor color correction, colored filters are best for major shifts, such as the difference between indoor and outdoor lighting.
Two series of filters have long been widely used in motion
picture production: 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
Since video cameras are optimized for one color temperature, videographers will generally use these filters to make the necessary "ballpark" adjustment. The "fine tuning" is then done electronically.
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 are everywhere, of course, 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 all standard fluorescent lamps 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.(Note photo above.)
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 above that the four-point star filter also 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 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, which are 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 above.)
In Using Filters
Using a filter with a video camera raises the black level of the video slightly. Because this 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 visual effects that an editor creates during postproduction, the optical effects a videographer creates while recording a scene can't be undone.
To reduce the chance of unpleasant surprises, you need to carefully check the results with the help of a high quality color monitor as you shoot.
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. Two filters can be used at once. 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.
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.
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 see through very few keyholes today).
Most of the effects that matte boxes formerly created can now be more easily and predictably achieved electronically with a visual effects generator.
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.
Video Projects Revision Information
Issues Forum Author's Blog/E-Mail Associated Readings Bibliography
Index for Modules To Home Page Tell a Friend Tests/Crosswords/Matching