Hereâ€™s a primer about optical filters from Photo-Mentor.com, describing what they are, why we use them and when.
An optical filter is a piece of glass attached to a camera lens to selectively transmit light of a particular wavelength while blocking the others. It conditions the light entering the camera in favorable ways â€“ often in ways that post-processing can’t easily duplicate. While this function can be emulated by post-processing in Photoshop, there is nothing quite like the real thing.
Polarizers, neutral density (ND) filters, IR pass and UV pass filters and UV cut filters are discussed in this post. There is also a discussion on the pros and cons of filters, questions like why you would use an extra layer of glass that could subtract from the amount of light entering your lens.