Star filters turn point-light sources and specular highlights in a scene into flared stars with (depending on the filter used) 2-16 points, the 4-, 6- and 8-point filters being the most popular varieties. Star filters require no exposure compensation, but with some the effect changes at different apertures, so it's a good idea to use your camera's depth-of-field preview to check the effect at the aperture you intend to use. A cautionary note: many star filters also reduce image sharpness.
Fog filters make a scene look foggy. They contain particles and patterns that scatter the light, just as real fog does, thus softening the image, lowering contrast, muting colors and producing halos around lights in the scene. Fog filters come in several strengths. You can also vary the effect of a given filter through exposure (more exposure increases the effect) and choice of lens aperture (wide apertures also increase the effect).
Some fog filters are graduated (similar to the graduated ND filters mentioned earlier) from a heavy fog effect in the top portion to clear in the bottom. Thus nearby areas of the scene (exposed through the clear bottom portion of the filter) will show little fogging, while more-distant portions of the scene (exposed through the foggier top portion of the filter) will show more fogging. This creates a picture that has a more realistic fog effect.
Diffusion filters produce soft images by diffracting (bending) some of the light rays. Some light rays go straight through the filter, producing a sharp image, while other rays are bent by the filter, producing unsharp secondary images overlying the sharp primary one. This combination of sharp and unsharp images produces the diffusion filter's soft, glowing effect, with light areas spreading into darker ones. Commercial diffusion filters are available in several strengths to produce varying degrees of diffusion.
Some diffusion filters really reduce contrast; these are best used with strong, contrasty light (unless flat, gray images are what you want). And some cheaper diffusion filters just reduce overall image sharpness. With some diffusion filters, the effect tends to disappear at small apertures, so use your depth-of-field preview to see what effect you'll get at a given aperture.
Note: On-camera diffusion produces a different effect than diffusion used when printing the negative. If you use a diffuser under the enlarger lens, it will spread the bright areas of the negative into the dark areas, resulting in a print in which the dark areas spread into the light ones—an unnatural looking effect.
Certain diffusers, like Tiffen's Soft/FX filters, soften fine details like wrinkles and blemishes, while retaining an overall sharp appearance—ideal for use in portrait photography. Fine-mesh net filters (ladies' stockings can be used in a pinch) produce a pleasing diffusion effect for portraits—finer mesh produces a stronger diffusion effect than coarser mesh, and black mesh produces greater contrast than light-colored mesh.
As their name suggests, multiple-image filters produce multiple images of a subject. These filters come in many varieties, and are quite sensitive to changes in aperture, lens focal length and camera-to-subject distance. Normal lenses generally produce the best results—longer lenses produce a large main image with the secondary images cropped partially out of frame, while wide-angle lenses can produce vignetting. Moving closer to the subject causes the images to move together; moving farther away causes the images to spread apart—possibly clear out of frame. Wide apertures blur the peripheral images; small apertures can bring the filter's segments into partial focus.
Popular multi-image filter varieties include those that surround an almost-sharp central image with three, five or more unsharp surrounding images, and the linear type that produces a parallel series of secondary images next to the main image.
Diffraction gratings turn white point-light sources into rainbow-colored slashes by breaking the white light up into its spectrum. Holographic (laser-produced) diffraction filters do the same thing, but produce patterns of colors rather than just straight slashes. Diffraction grating films are available from Edmund Scientific (800/728-6999; www.ScientificsOnline.com) in 12x6-inch sheets (Stock No. CR30402-67); holographic diffraction filters are available from Edmund Scientific (12x6 sheets, Stock No. CR30545-09) and photo-filter manufacturers such as B+W, Cokin, Heliopan, Hoya, and ProOptic. No exposure compensation is required, although with some filters, the aperture alters the effect.