Polarizing filters look like gray neutral density filters. However, their effect becomes apparent when you look at the blue sky through a polarizing filter while rotating it. As you rotate the filter, the sky appears to get darker, then lighter.

Polarizing filters are used in black-and-white and color photography for the following reasons:

1. Reduction or elimination of unwanted reflections (glare) from nonmetallic surfaces, such as glass and water

2. Exposure control, similar to ND filters

3. Reducing the effects of haze

4. Darkening the blue-sky image in both black-and-white and color photography

5. Increasing color saturation in a color photograph without altering the hues of image colors

As discussed in chapter 1 of this training manual, the term polarize refers to a property of light that cannot be seen-the direction in which light rays vibrate. Unpolarized light rays vibrate in all directions at right angles to the ray itself. A light ray is polarized when vibrations are in one direction only.

Any synthetic material that polarizes light may be called a polarizer, or polarizing device. A polarizing screen is a polarizer in sheet form.

There are a number of different polarizing filters. However, there are only two main types: one type fits over the camera lens and the other is designed to be used over a light source. Since they do not affect color, polarizing filters and screens may be used for both black-and-white and color photography. A polarizing device used over the camera lens may have small posts (known as indicator handles) projecting from the rim for aligning the axis of the polarizing grid.

The polarizing filter may be thought of as a screen, with an optical grid or slots, that stops all light that is not vibrating in a plane parallel to the axis of the grid lines.

As the filter is rotated, the amount of polarized light can be controlled. When the rodlike crystals are perpendicular to the vibration direction of the light, the polarized light is greatly absorbed. When the rodlike crystals are parallel to the vibration direction of the polarized light, the polarized light is almost totally transmitted.

Because polarizing filters are colorless, they can be used as neutral density filters. Even when polarized light is not present in a scene, polarizing filters can be used to reduce the intensity of light. When two polarizing filters are used, their combined densities can be varied considerably.

In color photography, the only way you can reproduce the sky darker without affecting the other colors in the scene is to use a polarizing filter. You can achieve various effects from light sky to dark sky by rotating the fiter to various positions. You can see this effect by viewing the scene through the viewfinder of a single-lens reflex (SLR) camera or by viewing the scene through the ground glass of a view camera. To see how much reflection control you are getting, rotate the filter as you are viewing the scene.

Getting the maximum effect with a polarizing filter depends on your angle to the subject as well as the rotation of the filter. When the reflection cannot be completely eliminated, try changing your camera angle to the subject. The maximum control of unwanted surface reflections and greatest reduction of light intensity occurs when two polarizing filters are used with their optical grids perpendicular to each other. This arrangement can be either two filters in tandem in front of the camera lens or one filter in front of the light source and another filter in front of the camera lens. You cannot control reflections from bare metal surfaces because the reflected light is not polarized.

SKYLIGHT FILTERS

By absorbing ultraviolet radiation, a skylight (1A) filter adds warmth to a scene recorded on a color transparency film. It does this by reducing the bluish cast prevalent in distant scenes and in scenes photographed on heavily overcast days or in open shade. A skylight filter is used primarily with daylight color reversal film exposed under the above conditions. A skylight filter is light pink in color.