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This is the second installment of my mini-series on filters.  In case you missed it, Part I introduced the series and covered protective filters.

Today we’re going to talk about the (nearly) universally most used filter in photography—the polarizing filter.  A polarizer is a screw-on filter that attaches to the end of your lens.  (Rectangular polarizers are available for some filter holder systems as well.)  The end of the filter is then rotated to set the polarizing effect.  For cameras with modern metering and autofocus systems, it’s critical to use a circular polarizing filter, rather than the linear variety.

What A Polarizing Filter Does

To one degree or another, all light is polarized.  The formal definition of polarized light deals with the electromagnetic properties of light and the physical direction of the light’s wave-like properties.  For the practical purposes of photography, a properly used polarizing filter performs two visual tasks:

  1. it reduces or eliminates reflections from some non-metallic surfaces
  2. it darkens skies (and can enhance cloud contrast, if clouds are present of course); it can also have the effect of minimizing the light scattering effect of haze

When it comes to nature photography, the “eliminating reflections” aspect of polarizer use is most emphatic when used around water—flowing water, pools, damp rocks…all of these things tend to emit an annoying glare under certain lighting conditions.  The use of a polarizing filter can minimize or eliminate that glare.  It can also cut glare off of sunlit or wet foliage.  The effect can be quite dramatic.

In the case of sky darkening, the effect is most pronounced when the camera is at a 90 degree angle relative to the sun.  Note that, when using a polarizer on a wide angle lens, the effect of the polarization on the sky may be variable.  This can be fixed in post-processing, but it’s a pain and is best avoided if possible.  Additionally, particularly at high altitudes, it’s easy to overpolarize the sky and have it appear nearly jet black in the final image.  Watch out for this.  A polarizer is a variable thing; it’s not simply a case of using it full force or not using it at all.  The filter can be used partially, which enables the photographer to avoid overusing it.

This article shows some examples of the same scene polarized and unpolarized.

In addition to its primary uses, a polarizer can function as something of a neutral density filter.  If you want to slow your shutter speed down without closing down the aperture of the lens (or lowering the ISO), a polarizing filter will provide you with up to two EVs worth of reduction.

Why Not Use a Polarizer All the Time?

With the exception of sky darkening, is there any reason why a photographer shouldn’t simply leave a polarizer on his lens at all times?  The answer is yes.  A polarizer reduces the amount of light reaching the camera’s sensor (or film) by one to two stops (depending upon how fully it’s employed).  This can create a real problem when a minimum shutter speed is needed.  Furthermore, as discussed in the previous installment on filters, any piece of glass placed in front of the lens causes some denigration of your image.  As a practical rule, unless you have a positive reason to use a filter, you shouldn’t attach it your lens.

How Do I Know If a Polarizer Will Aid My Image?

The easiest way to know if a polarizing filter is called for in a given situation is to simply hold it up in front of your eye (make sure that the filter is oriented in the same manner that it would be on the end of your lens), look through the filter at the scene you wish to photograph, and physically rotate the filter.  If you can see a significant difference in the appearance of the scene, the filter may be worth employing.  You don’t have to attach the filter to your lens to see if it will have an impact.  With experience, you’ll begin to intuitively recognize the kinds of scenes that will benefit from a polarizer, but it’s always helpful to take a quick look with the filter in front of the naked eye.

Watch Out For…

vignetting with a standard polarizer on a wide angle lens.  To avoid this problem, purchase a thin polarizing filter.  They’re more expensive than a standard width filter, but they’re far less likely to vignette.  This is rarely an issue above 24mm on a full-frame SLR.  Wider than 24mm it becomes a problem in a hurry and the wider you go the more likely it is to be an issue.

Also, be sure to remember: for scenes where you want to retain a reflection, a polarizer isn’t your friend.

Cash-Saving Tip

If you have lenses with different filter sizes you can save yourself some money by purchasing a single polarizing filter—one that fits the lens you own with the widest front element—and purchasing step-up rings that will allow you to use that filter on your smaller lenses.  For instance, if your largest lens accommodates 77mm filters, but you have other lenses that use (for instance) 62mm and 72mm filters, you can buy rings that will allow you to use the 77mm filter on the lenses with the 62mm and 72mm filter threads, with little or no loss in functionality.  Step up rings are much cheaper than filters.

The Bottom Line

Caveats aside, the polarizing filter is by far the most widely used filter in my bag.  I literally always have a polarizer with me when I’m in the field and—since its effects are difficult if not impossible to reproduce in the digital darkroom, unlike many other traditional filters—it remains an indispensable part of the discerning photographer’s kit.

Thursday Tips is written by Kerry Mark Leibowitz, a guest blogger on 1001 Scribbles, and appears every other Thursday.  To read more of his thoughts on photography, please visit his blog:  Lightscapes Nature Photography.