Concepts: Camera Filters

Introduction:
Camera filters are transparent or translucent optical elements that alter the properties of light entering the camera lens for the purpose of improving the image being recorded.  Filters can affect contrast, sharpness, highlight flare, color, and light intensity, either individually, or in various combinations.  They can also create a variety of "special effects."  It is important to recognize that, even though there are many possibly confusing variations and applications, all filters behave a reasonably predictable way when their properties are understood and experienced.  Most of these properties related similarly to filter use in both film and video imaging. 


Ultraviolet Filters:
Film, as well as video, often exhibits a greater sensitivity to what is to us invisible, ultraviolet light.  This is most often outdoors, especially at high altitudes, where the UV-absorbing atmosphere is thinner; and over long distances, such as marine scenes.  It can show up as a bluish color cast with color film, or it can cause a low-contrast haze that diminishes details, especially when viewing far-away objects, in either color or black-and-white.  Ultraviolet filters absorb UV light generally without affecting light in the visible region.


Infra-Red Filters
Certain special situations call for the use of black-and-white or color infra-red sensitive films.  For aerial haze penetration, recording heat effects, and other purposes they are invaluable. Their color and tonal renditions are very different, however, from other film types (consult film manufacturers for further details).  Various filters are used to reduce unwanted visible light.  Red, orange, and yellow filters, as used for panchromatic black-and-white film can enhance contrast and alter color. Total visible light absorption transmitting only infra-red, as with the Wratten #87 or #89 series of filters, can also be useful.  The results will vary with film type and other factors.  Prior testing for most situations is a must.


Neutral Density Filters
When it is desirable to maintain a particular lens opening for sharpness or depth-of-field purposes, or simply to obtain proper exposure when confronted with too much light intensity, use a neutral density (ND) filter.  This will absorb light evenly throughout the visible spectrum, effectively altering exposure without requiring a change in lens opening and without introducing a color shift.

Polarizing Filters

Polarizers allow color and contrast enhancement, as well as reflection control, using optical principles different from any other filter types.  Most light that we record is reflected light that takes on its color and intensity from the objects we are looking at.  White light, as from the sun, reflecting off a blue object, appears blue because all other colors are absorbed by that object.  A small portion of the reflected light bounces off the object without being absorbed and colored, retaining the original (often white) color of its source.  With sufficient light intensity, such as outdoor sunlight, this reflected "glare" has the effect of washing out the color saturation of the object.  It happens that, for many surfaces, the  the reflected glare we don't want is polarized while the colored reflection we do want isn't.