When you use a polarizing filter on a single-lens-reflex camera, you can see its effects through the camera viewfinder as you rotate the filter. But remember, if you are using a lens in which the filter ring on the front of the lens barrel turns as you focus, the polarizing filter will also turn and alter the effect if you change focus. Before you take the picture, be sure to check that the filter is in the proper orientation. With non-SLR cameras, hold the filter and look through it as you rotate it; when you see the effect you want, keep the filter in exactly the same orientation as you slip it over the camera lens.

You can also use a polarizing screen to reduce the appearance of atmospheric haze in distant views.

A polarizing screen has another application you may find useful. Since you have to increase exposure with a polarizer over the lens, you can use a larger lens opening to reduce depth of field, such as when you want the background out of focus to concentrate attention on the subject. This is especially helpful when you take pictures on a high-speed film in bright sunlight and you want less depth of field.

When you use a polarizing screen, you must compensate for the light it absorbs. For a normal front lighted subject, increase the exposure by 1 1/2 stops. This exposure increase applies regardless of the rotation of the polarizing screen. If your subject is sidelight or top lighted, as it will be when you are using the screen to get a dark-sky effect, give it 1/2 stop more exposure, or a total of 2 stops more than the exposure you would use without the polarizing screen. The extra 1/2 stop usually is not necessary with a distant subject or a landscape where shadows are only a tiny part of the scene.

When you determine the exposure for subjects with bright reflections, remember that removing the reflections with a polarizing screen will make your subject appear darker than it was before. So in addition to the 1 1/2 stops more exposure required to compensate for the polarizing screen and any exposure increase necessary for the direction of the lighting, increase the exposure by another 1/2 stop when bright reflections will be removed from the subject.

Calculating the exposure increase to compensate for a polarizing screen on the camera lens is more effective in obtaining correct exposure than using an exposure meter to make the reading through the polarizing screen. A reading made through a polarizing screen with a built-in meter or with a separate exposure meter varies with the rotation of the polarizer. Such a meter reading can indicate the wrong exposure because the exposure increase required for a polarizing screen is the same regardless of the rotation of the polarizer. For example, when you use a polarizing screen to darken a blue sky, the meter reading would be affected by the darkened sky; however, you really don't want to make the adjustment the meter indicates because you want the sky to be darker than normal. 

Also, because of the semi-silvered mirrors used in auto focus (and some auto-exposure) cameras, the normal "linear" polarizing filters used for manual-focus cameras can cause problems with the exposure systems and even with autofocusing. With most auto focus cameras, you should use a circular polarizing filter-one that has its crystals arranged in a circular rather than linear pattern. See which type of filter your manual recommends before buying a polarizing filter.

Since colored filters absorb some of the light that would normally reach the film, you must compensate for the light lost by using a larger lens opening or a longer exposure time. This extra exposure, which is different for each type of filter, is based on a filter factor. A filter factor is a number that tells you how much to increase the exposure when you use that filter.

In practice, however, you probably won't have to concern yourself with filter factors too often, because most cameras with built-in TTL meters will measure the light through a lens-mounted filter. Simply set the speed for the film you're using and, put the filter on the lens; the meter will automatically make the adjustment for the light loss.

If you want to use a filter factor, take your meter reading through the lens without the filter (or use a handheld meter), calculate the extra exposure needed, set the adjusted exposure on the camera, and then mount the filter. For example, a filter with a factor of 2 means that you need twice as much or one stop more exposure. After you make your meter reading, you can either open the lens by one stop or use an exposure time twice as long (the next slower speed). A filter with a factor of 4 would mean you need four times as much light-or two stops. (Just remember that each time the factor doubles, you have to add another stop.).

For most color pictures, no filter is necessary when you use a film with the light source for which it is balanced. However, there is one filter that many photographers like to have on hand. This is the No. 1A, or skylight, filter. The skylight filter reduces the bluishness of scenes photographed on overcast days or in the shade. It also helps reduce the bluishness in shots of distant scenes and in aerial pictures. Use a skylight filter only with daylight-type, color-slide films. The printer will correct for excess bluishness in color prints made from color-negative films. No exposure compensation is necessary with the skylight filter.

When you use a color film with a light source other than the one for which it was designed, you should use a conversion filter over the camera lens. A conversion filter chart helps you adjust the color of the light source to match that for which the film is balanced.

Light rays which are reflected by any surface become polarized and polarizing filters are used to select which light rays enter your camera lens. PL (Linear Polarizing) and PL-CIR (Circular Polarizing) filters have the same effect, but it is important that you choose the correct version for your camera. They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc. They also enable colors to become more saturated and appear clearer, with better contrast. This effect is often used to increase the contrast and saturation in blue skies and white clouds. HOYA's polarizing filters do not affect the overall color balance of a shot. 

Pictures that include glass surfaces often show distracting reflections or glare. You may be able to remove or reduce the reflection or glare by using a polarizing screen. You can use a polarizing screen to reduce or eliminate reflections, except those from bare metal. If you look through a polarizing screen at reflections on a shiny surface and then rotate the screen, you'll see the reflections change. At one point they may be completely eliminated. The polarizing screen will have the same effect in your pictures when you use it over your camera lens.

When you're looking through the polarizing screen, if reflections from a shiny surface are not reduced enough, try a different viewpoint or camera angle. Sometimes this will make the polarizing screen more effective.

You'll obtain the maximum effect with a polarizing screen in reducing reflections when the camera angle is about 35 degrees from the reflecting surface, depending on the surface material. At other angles the polarizing screen is less effective. At 90 degrees a polarizing screen has no effect in controlling reflections.

We mentioned earlier that you can get some great shots with backlighting. But backlighting often creates bright reflections from the sun (or other light sources) on water, foliage, boat decks, etc. You can use a polarizing screen to reduce or eliminate these reflections in your pictures. And since reflections desaturate the colors of a subject, reducing the reflections allows the colors in the photograph to be more saturated.

You can make a blue sky darker in a color photograph without affecting the color rendition of the rest of the scene by using a polarizing screen. When you photograph the sky at right angles to the sun, you can control the depth of the blue, from normal to dark, simply by rotating the polarizing screen. The sky will look darkest when the indicator handle of the screen (if it has one) points at the sun. If your polarizing screen doesn't have a handle, you can determine the area of the sky the polarizer will darken by forming a right angle with your thumb and index finger. Point your index finger at the sun and as you rotate your hand, your thumb indicates the area in the sky that the polarizer can darken. With black-and-white film you can get night effects by using a red filter and a polarizing screen together. The sky effects created by a polarizing screen appear most striking with a very clear blue sky, but disappear completely with an overcast sky.

The two basic classes of filters for black-and-white photography are correction filters and contrast filters.

Panchromatic films respond to all the colors that the human eye can see but they don't reproduce them in the same tonal relationship that the eye sees. For example, although blue and violet normally look darker to the eye than green does, black-and-white panchromatic film is very sensitive to blue and violet. Consequently, these colors will be lighter than green in a black-and white print.

Fortunately, you can easily change the response of the film so that all colors are recorded with approximately the same tonal relationship that the eye sees simply by your using a correction filter over the camera lens. To get this natural tonal relationship with Kodak panchromatic films, use a No. 8 filter in daylight or a No. 11 filter in tungsten light.

A filter lightens its own color in a black-and-white print and darkens the complementary color. For example, a yellow filter lightens yellows and darkens blues.

You can use contrast filters to lighten or darken certain colors in a scene to create brightness differences between colors that would otherwise be reproduced as nearly the same shade of gray. For example, a red apple and green leaves that are equally bright would reproduce as about the same tone of gray in a print. To provide separation between the apples and leaves, you might shoot through a red filter. This would lighten the red apple in a print and darken the green leaves.

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