In black-and-white photography, colored filters can provide separation between colors that otherwise would appear as about the same shade of gray. Photograph a lovely spring display of red wildflowers without a filter, and you'll get gray flowers against gray leaves because both reflect about the same amount of light. Put a red filter on the lens, and you'll get lighter flowers against darker leaves because the red filter transmits a lot of red wavelengths, and few green and blue ones. Ansel Adams often used colored filters to brighten portions of an image; he used a yellow filter to lighten the leaves in his famous "Aspens, Northern New Mexico, 1958" image.
Black-and-white landscape photographers also use yellow and red filters to darken a blue sky so white cloud formations really stand out. You can use colored filters to add drama to black-and-white images or to suit your artistic vision—a blue filter to increase the effects of haze, for example. The key to success is understanding that a colored filter will cause objects of its own and similar colors to appear lighter in a black-and-white photo, and subjects of complementary color, darker, than they would in an unfiltered photo.
Colored filters absorb a portion of the light from the scene, so you have to increase exposure to prevent underexposure. The filter factor tells you how much to increase exposure when using the filter so that neutral ("colorless") objects will reproduce the same tone as they would in a shot made without the filter. In a properly exposed photo made using a colored filter, neutral objects would appear "normal," objects the color of the filter would appear brighter than "normal" and objects of the filter's complementary color would appear darker. A filter factor of 2X means you should double the exposure (increase exposure one stop) when using the filter, a factor of 4X means give 4X the exposure (2-stop increase), a factor of 8X means 8X the exposure (a 3-stop increase) over the "normal" unfiltered exposure. TTL metering will sort of compensate for the filter, but exposure meters and film and digital sensors don't all have the same color sensitivities, so it's best to meter without the colored filter, lock in that exposure in manual mode, then put the filter on the lens for the photo. This also provides you with a brighter viewfinder image for composing and focusing.
In color photography, a colored filter will lend its cast to the entire image, although it won't turn everything its own color. Objects that reflect the filter's color (red, yellow, orange and neutral objects, when using a red filter) will take on a red cast, while objects that don't reflect the light of the filter's color (cyan, deep blues and greens) will just appear darker. Colored filters are thus more effective for black-and-white photography, but feel free to use whatever filter suits your creative vision (bearing in mind the cooking analogy at the start of this article).
You can't use a red filter to darken a blue sky in a color photo because the photo will take on a red cast. But you can use a polarizing filter. Polarizing filters transmit light waves vibrating in one direction while blocking vibrations in other directions. Thus, a polarizer can deepen a blue sky, by blocking much of the light from it, without altering the colors in the scene. The polarizer is most effective when used with the sun off to one side; it has little effect when shooting directly toward or away from the sun. You can combine a polarizer with a red filter to really darken the sky in a black-and-white photo.
Polarizers can also reduce or eliminate unwanted reflections from nonmetallic surfaces, in both color and black-and-white photos. The exact effect of the filter depends on the angle between the filter and the light source, but it's easy to figure out: Just look at the scene through the filter and rotate the filter until you see the effect you want. Then mount the filter on the lens in the same orientation. With a DSLR, you can just rotate the filter as you look through the viewfinder, but note that DSLR metering and AF systems polarize light, so they can't be used with conventional "linear" polarizers: If you want to use TTL metering and AF, you'll need a more costly "circular" polarizer.
Polarizers have a filter factor of around 3 to 4 (increase exposure by 1.5 to 2 stops to get a "normal"-looking exposure), but it's a good idea to bracket exposures until you get a feel for how your filter works with your expectations.
Neutral-density (ND) filters reduce the amount of light reaching the image sensor or film without otherwise affecting it. ND filters are handy when you want to make a long exposure in bright light (as to blur a waterfall) or shoot wide open for a selective-focus effect with minimal depth of field. Variable ND filters let you adjust the strength using a single filter—handy, and relatively economical, as a whole set of ND filters can be costly. Note that some "ND" filters aren't truly neutral, but if you stick with the major brands, you should be okay.
Graduated ND Filters
Graduated ND filters contain a clear portion and a portion that has density. These are handy for landscapes where an exposure for the dark foreground would blow out the bright sky, while exposing for the bright sky would render the foreground too dark. Position the dark portion of the filter over the bright sky, and you can knock the scene's contrast down to something film can handle. Of course, with digital, you can use HDR techniques to solve this problem, but some landscape specialists will like to use graduated filters to capture the image in a single shot. You can also get colored grad filters, which add a warm or cool cast to the sky.
You can buy filters that screw onto the front of your lens(es) or a filter system consisting of a filter holder and rectangular filters that slip into it. If you have lenses of different diameter, with a system, you just buy a holder plus an adapter ring for each lens size and use the same filters for all your lenses. With screw-in filters, you need filters to fit each lens (or you can buy filters to fit your largest-diameter lens and use step-down mounting rings to mount them on your smaller lenses). The screw-in filters are more secure when shooting in a strong wind, but the system holders allow you to position the dividing line of a graduated filter where you want it in the frame. There are professional photographers using both types of filters; choose the type that suits your own preference.
B + W
German-based B + W has been manufacturing quality filters since 1947, and offers a full range of high-quality screw-on filters using Schott glass, including colored, polarizers (including Kaesemann), ND and variable ND. www.schneideroptics.com
French filter-maker Cokin introduced a filter system with a universal holder back in 1978, and today offers four different holder sizes and a wide range of filters. Cokin also recently introduced the superthin PURE Harmonie 4.5mm-thick Circular Polarizer and 9.5mm-thick Variable Density Neutral Gray screw-in filters in a wide range of diameters. www.cokin-filters.com
Another German manufacturer dating back to the 1940s, Heliopan produces quality screw-in filters featuring Schott glass and brass mounting rings, including polarizing and neutral density. Heliopan also offers square filters. www.macgroupus.com
Japan's Hoya produces its own quality glass and offers a huge selection of filters in a wide range of sizes at cost-efficient prices. Among the notable are the EVO series with dust-repellent antistatic coating, super-rugged HD2 circular polarizer, 1.5- to 9-stop Variable Density, and the classic colored filters of black-and-white photography (25A red, K2 yellow, G orange, X1 green). www.kenkotokinausa.com
Widely known for its teleconverters, Japan-based Kenko also produces filters, including the superthin Zeta ZR EX circular polarizer and Variable NDX 2.5-1000 10-stop variable ND filter (practical limit 8.5 stops/450X). www.kenkotokinausa.com
Originally gaining fame for its theatrical lighting filters, Lee has long offered a holder-and-resin filter system for photography, as well—each filter handmade and individually inspected. The standard 100mm System features a wide range of adapter rings to fit most lenses and can hold up to four filters, while the Seven5 Micro Filter System was designed especially for compact system (mirrorless) cameras. The Lee system contains a wide range of filters, including the Big Stopper and Little Stopper 10X and 6X ND filters. www.leefilters.com
Besides B + W filters, Schneider Optics (the U.S. distributor for B + W) offers its own line of high-end MPTV (motion picture and television) filters, diamond cut from crystal-clear water-white optical glass. They include color, polarizing, ND, grad ND and effects filters. www.schneideroptics.com
Based in Florida, Singh-Ray has been providing unique filter options since 1962. The Vari-N-Trio filter combines 3- to 8-stop variable ND, polarizing and color-enhancement filters in a single unit. www.singh-ray.com
Long a favorite supplier of filters to the motion picture industry, New York-based Tiffen offers an extensive range of quality colored, ND, variable-ND and polarizing filters (and much more) in a wide range of sizes. tiffen.com