This guide compares linear & circular polarizers to assist in determining your needs, or to help identify a filter that you already have. It covers 3 topics: PL/CPL Identification, Use and Technical Info.

Identification: A significant percentage of linear polarizers listed on eBay are described as circular, apparently because they are round, with rotating mounts. However, shape & mount are the same for both types. Linear PL’s are generally marked as PL or Polarizer, while CPL’s are marked Circular Polarizer (sometimes abbreviated Cir.), PL Cir., or CPL. If in doubt about a filter, find some glare to polarize & view it from the male threaded side of the filter while rotating. Now try flipping the filter over & look from the female threaded side (or unthreaded side w/slim designs). Linear PL’s work the same both directions, but CPL’s must be properly oriented. A circular model has no polarizing effect when the filter is reversed from it's normal camera orientation. (Another check is to hold the filter in contact with a mirror. A linear PL will appear gray, & so will a CPL with front threads contacting the mirror, but when the CPL is reversed it will appear black).

Only 1 of these polarizers is circular.   

Use: Both polarizer types control reflections, sky density, specular highlights & color saturation, & work equally well for color, b&w, film or digital. However, the linear design can interfere with metering &/or AF in newer generation SLR & DSLR’s. (When testing for this with a linear PL on a Kodak Pro SLRn I didn’t encounter a situation where the AF failed to work properly, but exposures tended to be a bit light).

A more expensive circular polarizer is usually recommended for auto-focus cameras, but the emphasis is a bit exaggerated. It depends on the type of auto-focus your camera uses. AF SLR’s use beam-splitting mirrors that require circular. Point & shoot AF cameras, including 98% of consumer digital models, don’t use beam-splitters, & work equally well with linear or circular polarizing filters.  If you’re a digicam user, you can probably use a cheaper PL. If you don't believe it, just go to a local store; ask for a PL & mount it on your camera. Rotate the PL to see if the AF system always works & the meter delivers proper exposures. If it does, you’re good to go. (I used a linear PL on a Nikon Coolpix 5000 for about a year w/o encountering any issues). Theoretically, a PL is optically cleaner than a CPL because an additional layer of polarizing film is required for circular polarization.

Technical: The rest of this article is a technical explanation for people who want to know more about how these filters interact with a camera.

Let’s assume you have a modern SLR with a beam-splitting mirror in the camera body. The purpose of a beam-splitter in a camera is 3-fold. First, a portion of the incoming light will be split to the viewfinder so you can view the scene. Second, a small portion is split to the AF system so that the camera can determine focus. Third, yet another portion of the incoming light is split to the meter system for determining exposure. This split (viewfinder, AF system, meter system) has to be constant, say 60%, 20%, 20% (although we don't really know the exact values).

A linear PL is comprised of a single layer of polarizing film between 2 layers of glass. With a simple PL, phase & angle change with rotation cause the split ratio to vary. Sometimes 20% is split to the metering system & another time the metering system may receive 15%. In this case the metering system will be inaccurate, because it doesn’t know the % of light received. It’s possible for the discrepancy to get rather large.

The AF system faces a similar problem. Most AF system require f/5.6 or faster to operate properly. If 20% of the incoming light is just enough for the AF system to work, & the AF system sometimes only receives 10% of the incoming light, then the AF system may fail because there isn’t enough light for it to operate properly.

A CPL solves this problem with an additional layer of polarizing film known as a quarter-wave plate. It transforms linearly polarized light passing though the 1st layer into circularly polarized light. Circular polarization eliminates phase & angle variations, and interacts with a beam-splitter in the same way as unpolarized light.