Polarizing filter

The indispensible filter for color & black & white

A polarizing filter (aka polarizer or polarizing screen) is an adjustable filter, with an inner ring that screws onto the lens and an outer ring that can be rotated. Turning the outer ring reduces or increases the filter’s effectiveness. This means the photographer can control the degree to which the filter works, and the amount of change can be seen when looking through the viewfinder of a TTL (through the lens) camera. (Note: In some filter systems, the polarizer may slide into a filter holder that itself rotates, rather than screws onto the lens.)

HOW DOES IT WORK?

Although the sun’s rays travel in straight lines, they deflect off of objects in their path. This stray reflected light will vibrate in a single plane off of certain reflective surfaces (a window, a road or a pond’s surface, for instance) and becomes known as glare. When you aim your camera at such a surface from the side (at an angle of between 30 to 60 degrees), the polarizing filter acts on the glare, absorbing as much of it as you set the filter to absorb by progressively turning the outer ring. Shooting at angles between 32 and 34-degrees to the reflecting surface seem to be most effective in reducing glare. Only light that is properly oriented to the filter will pass through it. A polarizer is not effective on all reflective surfaces because some, such as bare metal, chrome or mirrors, don’t change the light’s polarization. But, it's darned effective on just about every other reflection.

A POLARIZER DARKENS CLEAR BLUE SKY

A polarizing filter is very effective on the minuscule surfaces of water vapor, smoke and dust particles found in the atmosphere, but only at certain angles to the sun. Perhaps the number one problem encountered in aerial photography is haze and the number one filter for haze removal is the polarizing filter. A polarizer prevents stray reflected light, such as is found in haze, from reaching the film, thereby increasing the sky’s color saturation, often quite dramatically. And it does this without darkening other parts of the image (unless other parts of the image also reflect glare which the polarizer removes).

WHAT DO I HAVE TO KNOW TO USE IT MOST EFFECTIVELY?

The light from a clear blue sky is partially polarized. Clear sky can be rendered darker by using a polarizing filter in the area of the sky where polarization is strongest. And polarization is strongest over the arc of the sky that is 90º from the sun, and is weakest both close to the sun (0º) and farthest from the sun (180º).

So, when shooting a scene that includes the sky, whether an aerial image or one taken at ground level, the lens must be aimed at a 90-degree angle to the direction of the sunlight for maximum effect in eliminating reflections from atmospheric haze, therefore darkening the sky. This means that whenever the sun is directly on your left or right side as you aim the camera straight ahead, the polarizer will work best. If you take a picture at an angle other than 90º to the sun, only a portion of the sky may be polarized, resulting in a dark blue sky on one side of your picture and a lighter blue on the other.

Some polarizing filters have an indicator handle on the screen. When it is pointing at the sun, the sky will be its darkest.

If the foregoing sounded too technical, don't let it worry you, because you can actually see the polarizer's effect as you turn the filter and the sky becomes an increasingly deeper blue. Keep rotating it, and it will become lighter again. The change can be quite dramatic, increasing contrast and turning a pale sky into one that is deep blue (or a darker grey if using black and white film) by cutting through the light reflected from atmospheric particles. Take your picture when you feel the sky is appropriately dark.

NON-SLR CAMERAS

If you use a viewfinder camera, hold the polarizer up to your eye to view the subject through it, then gradually rotate the outer ring of the filter to gauge its effect on the scene. When the degree of glare reduction or sky darkening that you want is reached, place the filter in front of the lens in exactly the same orientation as you had been looking through it. Be careful not to accidentally turn it or the effect may change. Some polarizing filters are made with markings on the rim to assist you in maintaining its proper position when attaching the filter to a viewfinder camera's lens.

LENSES THAT ROTATE AS YOU FOCUS

You may have a lens in which the filter ring at the front of the lens barrel rotates as you focus or zoom. If you have a polarizer attached, it will turn, too, and change the polarizer's orientation as you focus or zoom. When this occurs, remember to readjust the filter for the polarizing effect you desire after you have changed focus or zoomed.

A HAZE FILTER IS NOT A POLARIZER

A polarizing filter is not to be confused with a haze filter, although it is probably more effective at haze reduction, particularly distant haze, than a haze filter is. Although a polarizer works with both color and black and white film to reduce haze, to increase contrast and to eliminate glare, a red filter will have a greater effect on haze reduction for black and white images.

REFLECTIONS BEGONE

The polarizing filter’s ability to reduce glare is invaluable in bringing out color that is concealed beneath unwanted reflections. A shiny leaf or a wet rock that to the naked eye glistens brightly with reflected light will suddenly lose its glare when a polarizing filter acts upon it, and its true colors will come through. Reflections in window glass can be made to seemingly disappear and the person you couldn’t see (or photograph) standing behind the window can suddenly become quite visible and quite photographable. You may not want to remove all reflection from a scene, since a fish in water may look like it is floating in the air if there is no reflection at all to indicate that it is in water.

A POLARIZER REQUIRES ADDITIONAL EXPOSURE

Because of its light-absorbing qualities, additional exposure is required when using a polarizing filter, no matter how the filter is rotated – generally one-and-a-half to two stops.

This means you will have to either decrease the shutter speed or widen the aperture, or do both to lesser degrees. For example, an exposure setting of 1/500 sec at ƒ5.6 without a polarizer could be changed when using a polarizer (with a filter factor of one-and-a-half stops) to 1/250 sec and an aperture setting midway between ƒ4 and ƒ5.6 to obtain correct exposure.

If your subject is lit from the front, an exposure increase of one-and-a-half stops should do the trick. But if your subject is (1) lit from above or the side, and is relatively large in the viewfinder, or (2) if the polarizer is used to eliminate bright glare from your subject, then a two-stop exposure increase is called for. Far off landscapes or distant subjects that are side-lit or front-lit won't generally need the extra half-stop.

A built-in TTL (through the lens) meter is ideal in quickly making the necessary exposure adjustment since light measurement can be made while the filter is in place and being turned.

USE A POLARIZER AS A NEUTRAL DENSITY FILTER

Since exposure must be increased when using a polarizer and since a polarizer does not alter colors, it can be used in the same manner as a neutral density filter to reduce the amount of light, permitting you to use a slower shutter speed or a wider aperture for decreased depth of field.

POLARIZERS AREN'T THE ONLY FILTERS THAT DEEPEN BLUE SKIES

Other filters that can change a pale sky into a deep blue sky are the gradated neutral density (ND) filter and the gradated blue filter. If you have to choose between a gradated ND or blue filter and a polarizer, you will probably want to get a polarizer first, since it is very effective in making a clear sky turn blue and so useful in other ways. In black & white photography, yellow, orange and red are among the filters that darken skies. (See Colored filters for black and white photography.)

TWO TYPES OF POLARIZING FILTER

Linear and circular polarizers are available. The linear type, which has its crystals arranged in a line-like fashion, is made for use with manual-focus cameras, and can throw off the exposure systems and auto-focusing of fully-automatic cameras. The circular polarizing filter has its crystals arranged in a circular pattern, and is recommended for most auto-focus cameras.

BUYING A POLARIZER

Be sure to buy the best quality polarizer you can afford. (We recently purchased one at more than $120 to fit a large lens. They can be quite expensive.) An inexpensive polarizer may be poorly-made and unable to stand up to rigorous use, or may not be neutral, and can cause color shift in your images. As with other types of filters, type of glass used, its flatness, coatings or lack of coating, and other manufacturing characteristics determine the filter's quality.

If your camera has interchangeable lenses, you might want to consider a filter system that permits you to use the same filters for different sizes of lenses, so you then will not have to buy new filters for every lens.

Be sure to check your camera's manual to see if the manufacturer recommends a linear or circular polarizing filter before you buy a polarizer. If you are not sure, you can't go wrong buying one that is marked "circular" - although it will be more expensive - because it works on any camera.

If you are using a wide-angle lens, bring it with you (mounted on your camera) when you buy your polarizer or gradated filter, and try the filter on the lens before you buy it. Some filters, particularly polarizers, have frames (rims) so wide that they block light at the edges of a wide-angle lens, causing unwanted vignetting. There are special slim polarizers made to get around this problem, and they are usually more expensive. Sometimes called "thin mount" filters, they can be 3 mm wide versus 6 mm for a standard rim, and have no front threads, which means other filters cannot be attached to them and only a slip-on cap will hold in place. An example of a slim polarizer can be seen at the top of this page, next to the title. Note that it is actually identified by the word "slim" engraved on it. (We recommend you read our section on Filter quality before buying any filter.)

COLORED POLARIZERS

Some filter manufacturers make color polarizers that let you take polarized pictures with color, however they are most-commonly used in conjunction with a normal (neutral, or non-colored) polarizer. They can be turned just like a regular polarizer, but their effect is to gradually increase the amount of overall color in the image. They can provide nice effects with color film. The color of reflections can also be altered, or made to simply disappear as with a normal polarizer. Some color polarizers are made for use with black and white film and do not need to be used in conjunction with a normal polarizer.

POLARIZERS CAN BE USED WITH OTHER FILTERS

Polarizers and other filters, including color filters for black and white photography, can be used together, making it possible to control color rendition and glare at the same time. But keep the filter factors of both in mind when using a polarizing filter with another filter. The factor of one filter must be multiplied by the factor of the other - not added to it - to calculate their combined factor. The combined factor must then be multiplied by the exposure given by the meter.

Through-the-lens metering eliminates the need to make this calculation since it is done automatically with the filters attached to the lens.

THIS IS ONE FILTER YOU SHOULD HAVE

The polarizer’s effect can be almost magical since it changes perception in ways the human eye can’t, and the results of those changes can actually be seen through the viewfinder of an SLR and accurately recorded on film. This is one filter we recommend for every photographer who wants to increase color saturation and achieve a more dramatic look in his or her pictures. Once you become used to the changes that a polarizer can bring to your pictures, you'll wonder how you ever did without it before.