Polarizing filter enhances picture quality by blocking harmful reflected light. Use it to reduce polarized light reflections from glass and water surfaces or to improve color saturation. Simple to use, circular polarizing filters (such as Canon's PL-C) … see full wiki
Polarizing filters are used to eliminate glare on non-metallic surfaces like glass or water. They can also be used to increase the contrast and color saturation of your image. I use my polarizer not only to eliminate glare, but also to make skies a nice shade of blue or to get some "pop" out of clouds.
There are so many options to consider when purchasing the Polarizing filter for your needs. For instance, linear vs. circular, film or digital versions, or quality vs. price.
If you are making use of a TTL or auto-focusing system, Circular Polarizers are designed to work with those features. Linear Polarizers has their uses, but they are manufactured differently and may not work. Unless you are certain, just get a Circular Polarizing filter.
There are some digital circular polarizers on the market. I am not sure what makes a cir-pl lens better for digital as opposed to film. I use a Canon 20D Digital SLR and see no problems caused by using a supposed non-digital filter on a digital camera.
Most of the time, when it comes to filters; you get what you pay for. Does that mean you need an expensive polarizer? Of course not, just be sure to get the right filter for your needs. What ever polarizing filter you get, make sure it is color neutral to prevent unwanted tinting. Spending more on a color balanced filter will save time in post. If you need multiple filter sizes, consider saving money by using step down rings.
Currently I use the Hoya Pro1 filters, as the Canon polarizing filters I previously owned were without question bulkier and heavier. The weight might not seem like much, but when you stick all your items in a backpack all the extra bits of weight add up quickly. I can spend a good amount of time with my equipment on my back so this is a big concern for me. When it came down to the images, at times I would see a browning tint on my photos. The tint is easy to clean up in Photoshop CS2, but I don't see that problem with the Hoya Pro1 filter.
Overall, I would recommend against the Canon polarizing filters. I originally thought, "it is Canon, it must be quality." Since then I have replaced all my Canon filters. This filter works, but comes at a cost of disadvantages to other similarly priced filters.
PROS: Can be used for film or digital cameras Eliminates internal reflections and ghosting
CONS: Bulky and heavy Not neutral color balanced
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