One of the most overlooked—and occasionally most useful—accessories you can buy for your camera is a good filter. Filters screw onto the front of your lenses, changing the characteristics of your shot in key ways. But many filters that used to be crucial for film shooters—such as color filters—are obsolete in an age where everyone has access to software post-processing tools.
Even so, there are a few that still have a place in the digital age: UV filters, neutral density filters, and circular polarizers. There are a lot of widespread assumptions about the practicality, functionality, and usefulness of these filters. Some of are spot on, while others are total bunk. Here's a breakdown of the three types and what they're actually good for.
The common wisdom about UV filters is that you should just have one on all the time. This was sage advice in the film days, as lots of film stock (especially black-and-white) was sensitive to UV light, which could add a haze to your shots. Digital cameras aren't nearly as sensitive, so why bother?
Most people these days use UV filters because they're generally cheap and they sit in front of the lens, keeping dirt, dust, moisture, and scratchy things away. There's also always someone's friend-of-a-friend who knows a guy whose gear was saved when he dropped his camera, shattering the cheap filter and saving his precious lens. These stories are frequently told by people that—surprise!—also happen to have a UV filter to sell you. Weird.
To try and clear things up, we reached out to Roger Cicala, owner of popular rental house LensRentals.com, who sees more damaged gear than anyone this side of a manufacturer service center. Here's his take on whether UV filters will save your lenses:
"Are you protecting against minor scratches? If so then yes, I believe a filter can prevent grit and sand from getting on the front element and causing a scratch with cleaning. Also a filtered lens gets cleaned less and I believe most minor scratches occur during cleaning. Are you protecting against a drop? Absolutely not. The only time a filter would help there is if you happen to drop it right on the angle of the filter, so the metal ring deforms and absorbs impact. (Of course, then you'll be going to some extremes to cut the now jammed filter off.)"
Roger was also quick to point out that most minor scratches are impossible to see in a final shot, and he'd mostly just consider using a UV filter if there was lots of blowing sand around. Owners should also be aware that filters can lead to ghosting, flare, and internal reflections when shooting at night. With that in mind, we'd suggest only using them when you feel it's necessary for physical protection, as even small scratches can detract from the resale value of a lens.
Neutral Density Filters
Neutral density filters are another interesting holdover from film days. While the function of a "UV filter" is obvious, the term "neutral density" is a little more opaque. It's complicated, but the gist is that an ND filter blocks a percentage of light coming into your camera.
So if you're using a one-stop ND filter, it will cut the amount of light entering the camera in half. Why on Earth would you want to do that? Because reducing the incoming light will allow you to use wider apertures or slower shutter speeds when those settings would otherwise result in a blown-out image. These filters are often used with video cameras (some even have them built in) and by stills shooters when they want to capture a long exposure of something like a waterfall on a bright day.
For example, a typical exposure outdoors might be f/8 for 1/1000th of a second, which for a waterfall will freeze the water droplets in place. If you were to use an 8-stop ND filter, you'd be able to shoot the same scene at f/8, but for 1/4th of a second. Your shot would then have the same brightness level, but the moving water would create a dreamy blurred effect.
Landscape shooters should keep any eye out for graduated neutral density filters (GNDs). These filters are designed to darken half of the image while leaving the other half untouched. This can be useful when shooting a dark foreground against a bright sky. At the beach near sunset, for instance, a good GND can help you preserve the highlights in the clouds around the sun without making the waves irretrievably dark.
There are lots of different types of ND filters, so what to get will depend on your particular needs and gear. One thing to remember: Most ND filters—even very expensive ones from name brands—give images a slight pink/magenta color cast. The good news is that in most cases this can be easily removed in post, especially if you shoot RAW. (You may even find you like the effect on sunset shots.)
Polarizing filters are far more complex than the other two mentioned here. To explain how they work in detail would involve all sorts of fancy terms like "quarter-wave plate" and "birefringence" and something called "Brewster's Angle," all of which gives me a headache and disappointingly has nothing to do with Richard Pryor and John Candy.
Simply stated, polarizing filters work just like sunglasses in that they block certain kinds of light, especially glare coming off of surfaces such as water or glass. They come in two flavors: linear and circular. Both types have the same ultimate effect on your shot, but linear polarizers can mess up your camera's autofocus system.
For photography purposes, the most useful aspect of a polarizing filter is the ability to control or eliminate reflections and glare. When photographing a lake, for example, you often will get a reflection of the sky in the water. Since the light coming off the water is polarized, you can cut out this reflection with a polarizer; turning the polarizer to the correct angle blocks this reflected light, revealing what is under the water. Polarizers can also be used to great effect when capturing shots of vegetation, glass/plastic surfaces, buildings with lots of windows, and the sky—often increasing contrast and saturation.
It's important to note that polarizers only block certain angles of polarized light at a time, so you'll have to rotate the filter to get the effect you want. The effects should be visible through your viewfinder or live view, so this is pretty easy. Just make sure you do this after you focus on your scene, since many lenses (especially cheaper ones) rotate the front ring when focusing, changing the polarizer's orientation.
While there are plenty of other types of filters on the market, these three are the basic ones that every photographer should understand. Photo editing programs can do a lot of the heavy lifting for you, but some things you've just got to get right in the camera.
[Hero image: Flickr user "_belial"]