Tuesday, April 24, 2012
Gadget Bag: Anatomy Of The Polarizer
What makes one polarizer different from another?
Why do some 52mm polarizing filters cost nearly $200 and others less than $20? Maybe a better question is: Why do some photographers spend $200 for a polarizer when there are alternatives available for a fraction of that price?
Across the various brands, different models offer specific characteristics, which you may or may not need, depending on the type of photography you do. Although many brands and models look the same at first glance, there are at least 10 major ways in which they may differ—and those differences can have a powerful effect on the captured image. Once you understand the variations, you can decide what works best for you and your photography.
The great majority of polarizers in use today are circular polarizers. Most autofocus cameras and cameras that use a beamsplitter or semi-silvered mirror require a circular polarizer.
If you're not using autofocus, a less expensive linear polarizer will work for you, but for the vast majority of shooters today, the circular polarizer is best. While they're not interchangeable, both do the same thing, and both are available in plastic and glass versions.
As optical materials go, plastic has a bad connotation, but it does offer lighter weight, and that can be a plus. Glass is generally regarded as superior, not only because of intrinsic properties, but because sheets of glass can be more readily made plano-parallel. That means that the front and rear surfaces are exactly the same distance apart, no matter where they're measured. In other words, plano-parallel filters are perfectly flat on both sides, and the two surfaces are parallel at all points.
Of course, not all glass is created equal. Some high-quality filters are made from glass that's specially formulated to provide certain performance characteristics. In the same way that glass elements in camera lenses have differing refractive indices, the optical properties of glass used to make filters can vary widely. It's important to remember that when you place a filter in front of a lens, it becomes part of the optical formula for that lens. No light reaches the sensor or film without passing through it. Quality is of paramount importance.
Filter dimensions matter, and not just the diameter. Filter thickness influences performance in several ways. A polarizer that's too thick overall and that sits too high in the filter ring can cause vignetting, especially with wide-angle lenses and kit zooms. Filter thickness can sometimes affect density, as well. Increased density means less light passes through—and that means increased exposure time. Not all polarizing filters have the same exposure-compensation factor.
Manufacturers typically make the exposure-compensation factors available so it's easy to compare the specs of one versus another.
The composition of the threaded ring is also important. It can be made of steel, brass, plastic or other materials. Dissimilar metals tend not to get stuck when screwed together, so brass is the material of choice for attachment to the steel threads found on lens barrels. Also, brass is a very stable metal that doesn't readily contract or expand like an alloy. Because polarizers are rotated during use, extra stress is placed on the threads. It's easy to overtighten a polarizer, so many high-quality filters are made with brass bodies.
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