Filters are those glass or gelatin disks or squares that are affixed to the front of your camera's 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 weren't 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. Here's 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 can't 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.
Figure 1: Polarizing filters can reduce glare in many situations.
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 won't 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 don't 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.
