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Choosing The Right Filters

Filters are those glass or gelatin disks or squares that are affixed to the front of your cameras lens, changing the light that passes through the lens in some way. Filters were really popular before the advent of digital photography because some of the effects you could get with them werent possible (or easy) to achieve in the darkroom.
Glass and gelatin filters are less used today among casual photographers because you can achieve many of the effects they provide within Photoshop or other photo editing software. Yet, not all filtration effects are possible in the digital darkroom. Heres a description of the most essential filter add-ons:
Infrared:
Many digital SLR (single lens reflex) cameras are capable of taking photographs by using only infrared illumination, which produces a spectacular effect outdoors. You can get dark skies, vivid clouds, and ghostly white trees in your landscape shots and strange, pale complexions with your photographs of humans. However, you must buy an infrared filter that blocks visible light, and be prepared to shoot at slow shutter speeds (because very little light is left for the exposure). Also, you cant preview your shot because an SLR viewfinder turns black when an infrared filter is mounted.

  

Polarizers:
Polarizing filters, like the one shown in Figure 1, can reduce the glare bouncing off shiny surfaces in your photos. Simply attach the filter and rotate it until the glare disappears. These filters can also help deepen the contrast of the sky from certain angles.


 
Be certain to buy a circular polarizer rather than a linear polarizer for your dSLR. (All polarizers are round; circular refers to the way in which the filter handles light.) Circular polarizers wont interfere with the autoexposure mechanism of your camera as linear polarizers can.

Neutral density:
The third kind of filter that every digital SLR photographer should own is a neutral density (ND) filter, so called because it blocks light but is neutral in color. ND filters come with various assigned filter factors, such as 2X (reduces the light by 1 f-stop), 4X (reduces light 2 f-stops) and 8X (cuts down 3 f-stops). As with most filters, they can be stacked to combine the effects of more than one, as long as the additional filters dont become visible in the image, thereby cutting off corners. Neutral density filters also come in a split variety - the top half (or bottom half or one side, if you rotate it) has neutral density, and the other half is clear.
You can do a lot of amazing things with a neutral density filter, including rendering objects invisible! All ND filters decrease the amount of light reaching the sensor, so you need to use a longer exposure than you would without the filter (or let your camera adjust the exposure for you automatically).
Special Effects:
Lots of different filters produce special effects, including star-like points on highlights, prisms, special colors, and so forth.

Neutral Clear Filters:
These filters only used to prortect your lens from damaging, scratches and dirts,

Skylight & UV Filters:
Remember that films and the CCDs behind your camera lens are sensitive to ultra-violet (i.e.. UV) rays, although they are invisible to our eyes. If you allow UV light to be recorded on the CCD, it is likely that your image will look bluish and the color casts may not be acceptable. Moreover, UV and skylight type filters can also eliminate the lack of sharpness caused by UV radiation and reduces distant haze. UV and skylight filters are design to remove UV light (to some extend) and the bluish cast. UV filters are colorless and skylight filters are pinkish. Both filters can effectively remove UV light; however, skylight filters may produce warmish images because of their pinkish cast.
Excessive bluishness frequently occurs in outdoor photography, especially in open shade under a clear, blue sky and near water front. The following shows an example. The left image was taken without a UV filter, and, the right one is taken with a UV filter.

UV filters absorb UV light and are clear glasses. Skylight filters are colored glasses and usually have a touch of pink. In addition to absorbing UV light, the pinkish color of a Skylight filter can also counter the excessive blue color of the sky in a sunny day, and produce a warmish tone.
Haze is caused by dust particles in the air. These particles reflect shorter wavelengths more than the longer ones. Thus, UV is affected the most, followed by blue, followed by green and red, and the recorded images will be not very sharp and look hazy. Haze filters are designed to reduce haze and are yellowish to counter the excessive blue. For example, Tiffen Haze 1 reduces excess blue and transmits 29% at 400mm wavelength, while Tiffen Haze 2A provides greater ultra-violet correction than Haze 1 and transmits 0% at 400mm wavelength. However, for color photography, polarizers are frequently used. Note that although polarizers many not cut through haze as effectively as Haze filters do, polarizer can increase the contrast of the scene by eliminating reflection! 

Comparison between UV & Skylight filters:
The following is a simple comparison of various UV, Skylight and Haze filters. All images were taken in a heavy hazy day. The original image is shown below. Then, for each filter, we take an image and the indicated rectangular portion is cut without editing.

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