The title of this post is somewhat misleading. When the G9 is fitted with a polarizing filter, the focusing process does not change. Also, either linear or circular polarizing filters can be used on the G9. In fact, this is the entire point of this post.
Although filters were extensively used in film based photography, digital editing has significantly reduced the need and use of filters. Even so, a polarizing filter can be a great asset. This filter can pump up the colors, make the sky more blue, knock out the glare, etc. Many believe that a polarizing filter provides the only effect that cannot be simulated in the digital editing process. In a previous postings, the effects of a polarizing filter, graduated neutral density filter and various post processing effects, including HDR, were compared.
A polarizing filter was the second filter I ever bought (the first one was a clear, protective filter). Although I didn’t know it at the time, that polarizing filter was a linear type. Years later, my auto focusing (film) SLR required a circular polarizer. That was the first I’d heard of a circular polarizer. For a long time, I thought all auto focusing cameras needed a circular polarizer. A few years ago, I bought a circular polarizer for my Canon G3. For my G9, I still use that circular polarizer but I’ve since learned that a linear polarizer also works on the G9.
Once again, Wikipedia to the rescue for the details about polarization but be sure to scroll towards the end of the highly technical article. Also, that article does not really cover linear vs circular for photography.
It turns out that DSLR cameras and digicams like the G9 use different focusing systems. As mentioned previously, the G9 (and, I think, most digicams) use contrast detection as the basis for focusing whereas DSLRs use phase detection. As a result, the G9 can use a linear polarizer but DSLRs require a circular polarizer. This differentiation is often confused. The linear polarizer is less expensive but if you have, or intend to buy, a DSLR, the circular polarizer is more versatile.
To fit any filter to the G9, an adapter is needed (well, you can actually just hold the filter in front of the lens). In addition to selling the adapter, Lensmate sells polarizing filters and notes that either linear or circular types will work with the G9.
As final proof, I compared my 40 year old linear polarizing filter to my newer circular polarizing filter and found no obvious differences. The G9 worked fine with either. Nothing like a simple test to prove something to yourself!