In the days when film reigned supreme, any serious photographer carried a stack of filters wherever they went: UV, polarizer, orange, red, yellow, blue, green… the list goes on. Before Photoshop, photographers relied on these filters for color correction and contrast control, as well as for a myriad of other special effects. Digital photography changed most of that, but you can still buy filters in camera stores today. The question is: do you need to?
Unlike with film, there are really only three standard filters that apply to digital photography: ultraviolet/protector (UV), circular polarizer (CPL), and neutral density (ND).
An ultraviolet filter was used on film cameras to cut back on atmospheric haze, helping to keep images sharp and clear. The effect was not huge, but it was noticeable. Digital cameras actually have a UV filter built in over the image sensor, so there is no benefit to image quality by using a UV filter on a digital SLR. Hence the term “protector.” UV filters now serve simply as an insurance policy for your lens. If you ever happen to drop your lens, the filter will help absorb the brunt of the impact. And if it shatters, at least it’s a relatively inexpensive replacement. When it comes to whether you need one or not, there are two competing camps of photographers on this. One claims that no matter how good the filter, adding another piece of glass in front of your lens can only make it worse. (In our tests, this is certainly true, though the effect is barely noticeable on high-end filters, and only in certain situations). The other says that any negligible decline in image quality is a worthy sacrifice for protecting the front element of your lens. I say, definitely use a UV filter if you’re going to be shooting at the beach, a rodeo, or inside a hurricane. If you’re shooting landscapes on a tripod, and your gear is secure, there’s no real point to having a UV filter on your lens. If you do opt for a protective filter, make sure you get one that matches the quality of your lens. There is no point in spending $2,000 on lens to only put a cheap piece of glass in front of it. Higher-end UV filters are multicoated to reduce flare and cut back on internal reflections, minimizing the effect they have on your images.
A circular polarizer functions by limiting the amount of polarized light that passes into the lens. Light becomes polarized when it bounces off of other objects, so this filter can be used to reduce reflections off of shiny surfaces, like glass and water. Polarizers will allow you to “see through” the reflections in a window, for example. They can also increase contrast in the sky by cutting out light polarized by electrons in the air. This latter use is of particular importance to landscape photographers, but the level of polarized light in the sky depends on your angle to the sun, so results can very. Be sure to experiment with different camera angles to notice how the effect increases and decreases depending on your orientation to the sun. If you initially find you’re not getting what you want, it may be as simple as changing your position.
Neutral density filters are like sunglasses for your lens: they make things darker. There is little use for an ND filter for general photography as even entry-level SLRs can shoot up to 1/4000 of a second, plenty fast enough to take pictures in bright daylight. However, there are two key areas where they are very important. The first is for capturing motion. Everyone has seen the classic waterfall shot, where the water looks like a cloud of mist. This is the result of using a dark ND filter to allow for a very slow shutter speed. As the water continues to fall over the duration of the shutter being open, it blurs into a “cloud,” but the surrounding ground—which is not moving—remains sharp. Keep in mind, the camera must be on a tripod or secured in some way for the effect to work. The other key use of ND filters is for shooting video when a shallow depth of field is desired and a 1/50th of a second shutter speed is needed to mimic the look of film. Without an ND filter, either you would have to shoot at a very high shutter speed (ruining the film aesthetic) or with a very small aperture (increasing depth of field). Obviously, the amount of light present in your scene determines the strength of ND filter required—or if one is required at all.
There are a few variations on the ND filter, such as the graduated ND which is darker on the top and lighter on the bottom and useful for shooting landscapes with bright skies. Variable ND filters allow the density (darkness) of the filter to be changed and are used most often for video shooting.
While all the special color and effects filters are still out there, the three mentioned above are the primary ones to think about if you’re shooting digitally. A filter can be an insurance policy, or a great tool. What you need depends entirely on how you plan to use your camera.