Use quality filters on quality lenses to avoid degradation of the image. One of the biggest problems is vignetting (darkening of the corners, not always visible through your viewfinder). A polarizer on a wide-angle lens, two filters on a lens or a filter coupled with a lens hood can cause vignetting. You can use filters larger than the lens' screw-in thread. You can use 62mm filters for your 58mm lenses and attach them with a step-up ring, thus saving some money.
Cokin and other manufacturers make a wide variety of acetate filters for many kinds of special photographic effects. Photographers fall into two camps regarding these specialized filters: those who want their photos to look as realistic as possible, and those who enjoy pushing the boundaries of creativity and reality.
Filters come in several formats. You can purchase glass filters that screw or clamp onto the front of your lens. These are the sturdiest, but since lenses come in many different front-width sizes, this can become costly. You can purchase fragile polyester and gelatin filters, but generally you should be used these only for those impossible lens widths and when you only occasionally use the filter. They can be cumbersome to work with.
These filters' primary function is to protect the lens. They include clear, skylight 1A, skylight 1B, UV and haze. Skylight 1A and 1B filters reduce the blue tendencies of films under certain atmospheric conditions. They have a slight pink cast and absorb a little bit of UV. The UV filters absorb ultraviolet radiation that isn't visible to the human eye but does show up to a slight degree on photographic films as a blue haze. Haze filters are essentially UV filters that also absorb ultraviolet rays.
Warming and Cooling Filters
Daylight film correctly records color at 5500 degrees Kelvin. However, in natural light situations, colors are often bluer or more amber than this color balance. Often, excessive warmth is desirable. Warm/cool colors can be a personal preference.
Overcast skies produce excessive blue light; excessive blue also occurs at high altitudes, distant scenics, backlit situations, shady snow and marine environments. Reciprocity failure (performance of film under long [1/8 of a second or more] or short exposures) can result in color balance problems.
Basic warming filters: 81A, 81C. An 81A is a good all-around filter. You could replace your skylight with an 81A for all except sunrise/sunset situations. An 81C is good under moderate overcast. An 81EF is good in heavy overcast.
Cooling filters start with the numbers 80 and 82 (yes, strange but true!). They are numbered in order #82, 82A, 82B and 82C in increasing coolness.
A polarizer consists of two disks of dark, neutral-colored glass in separate rings. These rings rotate independently of one another and form a screen that blocks varying degrees of polarization.
Some basic physics: Light rays travel in electromagnetic radiation best described as waves. Direct rays from the sun travel in straight lines, but vibrate in all directions perpendicular to their path. When they strike a non-metallic object, perhaps a rock and the surface of a lake, some of the reflected light vibrates in a single plane that's vertical, or polar. The crystals in a polarizing filter absorb these while allowing the diffused light to be transmitted. By selective blocking-of polarized light only-glare is partially or completely removed. This allows for a better view of the subject without the unwanted interference. Metallic surfaces, such as chrome or a mirror, reflect primarily non-polarized light, so the filter is of little consequence with these.
The glare cutting effect is progressive, depending on the rotation angle of the filter, relative to the plane of the light's vibration. In practical terms, a turn of the outer ring controls the amount of polarized light that's blocked. A polarizer has no effect on a cloudy sky. On a clear day, the darkening effect is most pronounced when shooting with the sun to one side. If this is desirable, point the lens at a 90-degree angle, instead of toward (or away from) the light source. At high noon in central or lower latitudes, it's impossible to point at a 90-degree angle to the sun. Still, the polarizer produces some effect in any direction at mid-day, particularly near the horizon. With an SLR, the results are clearly visible in the viewfinder.
This filter serves several purposes, including darkening a blue sky, cutting glare or reflections in foliage, rocks and water, adding to color saturation and cutting down haze. A polarizing filter can also be used like a neutral-density filter, since you lose approximately 1 1/2 stops of light. Remember to buy a polarizer of good quality.
When shooting with a wide-angle lens, all sections of sky may not be darkened evenly. Try a longer focal length or mask part of the sky with overhanging foliage or a building. Also, when focusing and changing focal length of zoom lenses, the front element frequently rotates on some models, altering the filter's effect; make a habit of setting focus and focal length BEFORE adjusting the filter ring.
Circular Polarizers for Autofocus
Make sure you use a circular polarizer for autofocus cameras. In autofocus cameras, some of the light entering the lens is diverted to the metering cell by a mirror, with the rest directed to the viewing screen. This mirror polarizes some of the light rays, irrelevant in most shooting. Mount a linear polarizer, however, and the combination interacts to absorb some of the light. Hence, the meter receives less than the actual amount, producing an incorrect exposure reading. By the same token, SLRs that use a beam splitter for the AF system can't ensure accurate focus when a linear polarizer is used.
The circular type appears identical, but contains a second element that causes the light to vibrate in a spiral (or circular) direction. The light reaching the sensors now appears to be non-polarized, so the polarizing effect of the splitter isn't compounded. Consequently, exposure and autofocus operation will be more accurate and reliable. Circular polarizers, although more expensive, work nicely with both manual and autofocus cameras.
Graduated Neutral Density Filters
These filters are dark on the top half and clear on the bottom half. They're primarily used to hold down contrast in a scene, allowing you to capture on slide film what would otherwise be outside the film's tonal range (dark foreground subjects against a very light sky or a foreground subject in a shadowed area with a brightly lit background). Keep in mind that the range of slide film is about a stop and a half in either direction from your optimum exposure.
An example: when you're photographing a backlit landscape, where the sky meters several stops brighter than the foreground. Without the filter, you can record a normal-looking sky with a black, silhouetted foreground or a properly exposed foreground with a white, blown-out sky. You can't capture detail and color in both, except with a neutral-density filter.
Graduated neutral density filters are available in various densities and usually come in one and two stops. They're available in the (more expensive) glass and (less expensive) acrylic, which is easily scratched. You can choose ND filters that feature a soft, gradual change or an abrupt change in density. They can be round filters that screw into the lens or rectangular sheets of glass that slide into a special holder. You can get them color-neutral or slightly tinted. A good starter ND filter is a two-stop gradual neutral-density rectangular filter. Use your depth of field preview button to see exactly where the area of density lines up and rotate the filter if necessary. One trick is to slide the filter down until it looks right, and then physically push the filter up about half an inch. Meter the foreground (or shadow area) and manually set your exposure before attaching the filter.
Cheaper ND filters aren't truly neutral, sometimes producing a magenta or brown shift. If you're fussy, you'll likely spend more than $50.00 for a good ND filter. Singh-Ray rectangular grads are one high-quality option.
Soft Focus Filters
Many manufacturers make them, including Tiffen, Hoya, Ambico and Cokin. Such filters are popular for certain greeting-card effects, and especially for taking flattering portraits. These filters usually come in a variety of strengths. You can make your own low-tech, almost free version by putting a fine but open mesh over the front of the lens, such as nylon stocking or cheesecloth. Crumpled acetate or cellophane can modify the image. Or smear Vaseline on the edges of a skylight filter (NOT on your lens!), leaving an open area in the middle of the glass. In cold weather, you can "huff" on the lens for a temporary softening effect. Or you can double-expose your image, making one soft and one in focus. (Remember that the two exposures have to add up to one correctly exposed image. Nikon makes a silver-coated filter (Soft 1, etc.) that softens only the highlight areas and leaves dark areas sharp. Very expensive, but great for portraits, especially of older people who don't want their wrinkles to show.
Filters for Fluorescent Light
Fluorescent light lends an ugly green cast to your pictures taken with daylight film. Professional photographers find such lighting to be the bane of many assignments because of the ubiquity of such light. The problem with fluorescent light is that it is not precisely predictable. The apparent color temperature of fluorescent tubes varies with make and age, with no standardization among manufacturers or type classification. A fairly strong magenta filter (20M, 30M or 40M) will more or less correct for the problem. Mixed lighting presents special problems, especially fluorescent and tungsten. You must try to favor one of the light sources and filter for that source.
When you're forced to photograph under fluorescent lights, you can sometimes use flash to overpower the fluorescents.
Outdoor photographers sometimes use an FLD filter for its magenta effect. It offers a different solution to sunrise and sunset shooting. When adding a warm orange cast feels ordinary or clichéd, the FLD option may be the better choice for a zingier sky.
Shooting Tungsten Light with Daylight Film
An 80B filter will balance a daylight film for 3400 degrees Kelvin (tungsten) lighting; an 80A filter gives stronger correction for 3200 Kelvin lighting; and 80A and 82C filters combined will render most domestic tungsten lighting to the equivalent of daylight.
Digital cameras offer many options for circumventing light balance problems. Typically, they adjust for difficult lighting situations, such as fluorescent lighting. Many cameras, in addition to automatic "white balance," offer over-rides, such as "cloudy day" settings, etc. Even so, digital cameras still cannot reproduce the effects gained from a polarizer or a split/graduated neutral-density filter.