Last time I talked about protection filters here, but there are so many different types of filters on the market that can be confusing what filters to use. In most cases, the effect of filters can be duplicated by software equivalent, such as those filters found in Photoshop. There is at least one filter that software can not duplicate -- the polarizing filter.
A polarizing filter has many benefits. It can act as a Neutral Density filter, colour/contrast enhancer, and finally, a polarizing filter, which can reduce or even remove reflections from water and glass (but not metal). Polarizing filters come in two types: Linear and circular.
Early polarizing filters were all of the the linear type. It's function is identical to that of the circular type, except one important difference: Linear polarizing filters can fool the camera's metering and auto focus system, due to the way the light is polarized. Linear polarizing filters are more effective and easier to make, thus much less expensive than circular type, but they should only be used on manual focus cameras with manual metering, or digital cameras with live view.
Circular polarizing filters are much more common now a days. This type of polarizing filter allows the camera to meter and auto focus, otherwise both types does the same function.
How can you tell them apart if it's not specifically written on the filter? The trick is actually quite easy. All you need is a mirror. Hold the filter in front of you and look through the filter and at the mirror, now switch it to the other side, and do the same thing. If you can see your eye when you look through the filter both sides, it's a linear polarizing filter. If one side is dark, it's a circular polarizing filter.
William -- Panasonic G1 & JML 50mm f0.95 TV Lens. Larger Picture.