Polarizing Filters

an extremely gradual blend is required, an "attenuator" is used. It changes density almost throughout its length.

The key to getting best results with a Color-Grad filter is to help the effect blend in as naturally as possible. Keep it close to the lens, to maximize transition softness. Avoid having objects in the image that extend across the transition in a way that would highlight the existence of the filter. Don't move the camera unless the transition can be maintained in proper alignment with the image throughout the move. Make all positioning judgments through a reflex viewfinder at the actual shooting aperture, as the apparent width of the gradation is affected by a change in aperture.

Color-Grad filters are best used in a square, or rectangular format, in a rotating, slidable position in a matte box. This will allow proper location of the transition within the image. They can be used in tandem, for example, with one affecting the upper half, the second affecting the lower half of the image. The center area can also be allowed to overlap, creating a stripe of the combination of effects in the middle, most effectively with gradated filers in colors (see section on "Color-Grad Gradated Color 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.

The waveform description of light defines non-polarized light as vibrating in a full 360 degree range of directions around its travel path. Polarized light is defined as vibrating in only one such direction. A polarizing filter passes light through in only one vibratory direction. It is generally used in a rotating mount to allow for alignment as needed. In our example above, if it is aligned perpendicularly to the plane of vibration of the polarized reflected glare, the glare will be absorbed. The rest of the light, the true-colored reflection, vibrating in all directions, will pass through no matter how the polarizing filter is turned. The result is that colors will be more strongly saturated, or darker. This effect varies as you rotate the polarizer through a quarter-turn, producing the complete variation of effect, from full to none.

Polarizers are most useful for increasing general outdoor color saturation and contrast. Polarizers can darken a blue sky, a key application, on color as well as on black- and-white film, but there are several factors to remember when doing this. To deepen a blue sky, it must be blue to start with, not white or hazy. Polarization is also angle-