Polarizing Filters

Polarizing filters are the most useful filter in my repertoire. These color neutral filters should appear gray and without colorcast. If there is more than the slightest greenish or bluish cast, pass it up for a better quality more neutral filter.

The standard screw in type of polarizing filter will have the normal mounting threads plus an attached rotating ring for orienting the polarization angle. The polarizer’s rotation ring adds extra depth or thickness to the filter. This makes it important to take care when using one with a very wide-angle lens to avoid vignetting of the image. There are special thin ring models made for very wide-angle lenses or you may use a filter two sizes larger diameter and connect it to the lens with a step-up ring. Most filters have some type of identification engraved on their mounting ring. You should look for polarizer, pola, c. pola, circ. pola, or something similar for proper identification.

Drop-in, slot style of filters, such as the Cokin P series, will have some means of filter rotation other than the twin rings.

In practice, there are two different types of polarizers: the older linear and the newer circular. The circular polarizer is required with camera systems where the built-in light measuring system uses light that has passed through the mirror via a beam splitter. In this case the light is already partially polarized, which is the case on all AF-cameras (auto-focus). Using linear polarizing filters on these types of cameras cause incorrect focusing and exposure measurements. It is recommend that you use a circular polarizer with all digital and AF cameras. Actually, I cannot think of a reason to use the older linear type polarizer any longer.

Exposure Compensation
As with any dense filter, some light is lost reaching the film or digital sensor as it passes through the filter. Every filter is assigned a filter factor which indicates how much exposure increase is needed to compensate for that particular filters density. The polarizer has a factor of approximately 3.4, which translates to 1½ stops. The best advice is to determine correct exposure and then add an additional 1.5 exposure units for the filters density. Many metering systems give incorrect exposure data when metering through a filter, due to its color or polarizing angle. Mounting a polarizer onto the lens and rotating it can easily show this effect. Often the exposure data will change with rotation, which would give an incorrect exposure. The density for the filter remained constant therefore any exposure variations should not be implemented. The same advice is given for those using a medium or large format camera and a hand held meter, determine correct exposure then add the 1½ stops necessary for the filter density then polarize as desired.

Regardless of the type of shooting you do, a polarizer is the most useful and versatile filter you can own. A polarizer may be more expensive than most other filters but well worth the investment. These filters are very visual to work this: as you rotate it in its mount, the effect are immediately noticeable. A polarizer is often used for pictures with blue skies. Without a polarizer, blue skies often appear a weak, in the picture; with a polarizing filter, the sky can be enhanced with a rich, deep color or a deeper gray tone in black and white pictures. A polarizing filter will deepen the color and contrast in a bright sky (the most intense effects are always 90° from the sun or light source), eliminate glare from wet or reflective surfaces and diminish the effects of atmospheric haze to increase clarity and contrast in a scene.

Useful Tip
To determine the useful direction for using a polarizing filter, make an "L" with your thumb and forefinger. Point your forefinger at the sun and your thumb will point in the direction that the polarizer will have its greatest effect. You can rotate your thumb around the axis of your forefinger; pointing the camera lens in any of these directions will take full advantage of the polarizing effect. This is possible because the effect of a polarizer is most pronounced when it used perpendicular to the light source (the sun usually). A Polarizer will have little or no effect when used in the same direction as the light source.

A Polaroid filter is able to polarize light because of the chemical composition of the filter material. The filter can be thought of as having long-chain molecules that are aligned within the filter in the same direction. During manufacture, long-chain molecules are used so that each molecule is (as much as possible) aligned in say the vertical direction. As un-polarized light strikes the filter, the portion of the waves vibrating in the vertical direction are absorbed by the filter. The alignment of the molecules gives the filter its polarization axis. This polarization axis extends across the length of the filter and only allows vibrations of the electromagnetic wave that are parallel to the axis to pass through.

You can figure out why polarization works in some cases and not in others. Using wide-angle lenses such as a 20mm lens with a polarizer yields undesirable effects. The angle of polarization covers about 358 degrees and the lens 948. This leaves a great blue wedge in between two pale wedges. Not really the kind of sky we would like. There are many more uses for the polarizer: reflections on autumn leaves to open up the color and boost contrast, ‘see through’ a reflection to reveal what’s beneath, select how much of a reflection or the intensity of the reflection to select. Now have fun finding these and many more.

^top