Polarizers 101

Most people use clear protective filters on their lenses, but surprisingly few take advantage of the power of polarizing filters. With a quality circular polarizing filter (CPL for short), you can eliminate reflections, emphasize clouds with darker skies, shoot through windows as if the glass weren’t there, and more. What’s more, it is one of the few effects that is almost impossible to replicate with post-processing. Have you tried one yet?

How polarizers work

An easy way to visualize the movement of light is to pluck a guitar string (or a rubber band stratched between your fingers). The direction of the wave of the vibration depends on how the string was plucked, and most likely is not in one precise line, but rotates around the string as it vibrates. Light, too, is a wave, and normally bounces in more or less random directions. “Polarized light” is light consisting of waves that are in one precise form. Linearly polarized light behaves like the two-dimensional sine waves that form the wavy backbone of trigonometry classes; circularly polarized light is more like a spring.

Polarizing filters work by absorbing light which is polarized at a certain angle and passing the rest after polarizing it in a different dirtection. Fortunately for photographers, light becomes somewhat polarized when it is reflected off of a shiny surface. Thus, when a polarizer is carefully oriented, most of the light from reflections can be selectively removed from the photo. There is some light-loss (normally 1-3 stops), but the tradeoff is often well worthwhile. For examples of ways to use this effect, see below!

Be sure to use a circular polarizer

This nearly-magical effect has a major defect: linearly polarized light can play havoc with automatic focusing and metering systems. Once more, however, the scientists came through for the photographers. The artistic effect is always accomplished by a linear polarizer (that way it is possible to rotate the filter and target certain light), but most modern filters have an additional circular polarizer directly behind the linear filter, passing the light to the camera as a spiral, fully compatible with the camera’s analysis systems. If you are working fully manually, a linear polarizer will be fine, but for the rest of us, only a CPL will do.

In the wild: real-world usage examples

The best way to communicate the functionality of a polarizer, of course, is not with a description of the physics but by showing photos with and without. Note that it can be hard to predict how well a given surface will polarize light—not every window can be made completely clear and not every sky can be made dramatic—but when a CPL works, it works jolly-well.

Water does an excellent job polarizing light, so a polarizer can almost totally eliminate reflections on its surface. If you can’t quite capture the true color of a lake, for example, a polarizer is exactly the tool you’re looking for.
Window displays are famously frustrating but, similarly, polarize light very well. Wipe the plate-glass clean with a carefully-adjusted CPL.

 

This works just as well inside for framed photos and items behind glass, as in museums.
Not all reflections are quite so obvious. Statues and stone buildings are surprisingly reflective, and removing the harsher light can make a massive difference in revealing detail.
Not only did the CPL significantly reduce the glare on this clock and on the flags next to it, you can see how the brightest light reflecting in the atmosphere is also filtered out, leaving a darker, more saturated sky.
Darkening and flattening harsh light works similarly to normalization in music, allowing you to rescue details from the shadows without completely blowing out the brighter parts of your photo.

All polarizers are not equally well-made

One final word of advice: no matter how excited you are about getting your first polarizer, remember that quality is extremely important when choosing a filter. There are many varieties available, but I’ll use the three tiers of ProMaster filters as an example.

  1. The basic filters—marked “Standard“—will get the job done. For kit lenses and artistic uses that do not require the highest standard of technical clarity, these will be just fine. A polarizer will polarize.
  2. Digital HD, level two, increases the quality of the anti-reflective coating. Rather than a single layer, Digital HD filters are given 12 layers of anti-reflective coating. The edge of the glass is black, to reduce reflections, and the filter itself is thinner than a Standard would be.
  3. At the professional level, Digital HGX filters are designed to do their job and nothing else. The HGX filters are finished with a 16-layer anti-reflective coating and an exclusive Repellamax™ treatment to repel moisture, fingerprints, and all the other schmutz that accumulates over time. The glass is also stronger and more scratch-resistant than that used in the less-expensive levels.

Using lens filters (of any kind, including clear protective filters) means risking introducing aberrations to your images, making the quality of your lens irrelevant to the final product. Why ruin the photos you take using a $1200 lens by trying to save $50? This is not to say that more inexpensive filters are necessarily a waste, but rather that the quality of your accessory glass should match the quality of the “glass” itself.

If you have additional questions, feel free to leave a comment below or come visit us in any Mike’s Camera location. We would be happy to help you along your photographic quest!

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