What Are Filters?

With traditional photography, filters are typically circles of glass that fit in front of a lens and change or attenuate the light passing through in some way. A few lenses with very large front diameters accept filters in a slot at the rear of the lens, to reduce the expense. My old 7.5mm fisheye and 16mm semi-fisheye lenses have a few filters built in which can be changed by rotating a wheel or dial. Filters also are available as inexpensive square sheets of gelatin that fit in holders that attach to the front of the lens.

Digital cameras, too, can use this kind of filter. Many digital camera lenses have threads on the front edge that accept conventional filters. Some cameras require a special adapter to let you mount filters.

They all work in much the same way. Some have a tint and partially or completely block the light of other colors, as when a red filter is used with black-and-white film to darken the blue of the sky and the green of foliage. Others may remove certain types of light to reduce glare, or break up an image into multiple fragments, as if your subject were viewed through an insect's eye. Filters can blur parts of your photo, or add star-like twinkles to bright highlights. Figure 8.1 shows a popular effect that can be achieved with a split filter, orange on top and blue on the bottom.

Figure 8.1. A split filter produces one color on top and another color on the bottom.

Figure 8.1. A split filter produces one color on top and another color on the bottom.

In Photoshop, filters can perform even more amazing magic tricks with your images. They can transform a dull image into an Old Masters painting with delicate brush strokes, or create stunning, garish color variations in a mundane photograph. Blast apart your images into a cascade of sparkling pixels, or simply add some subtle sharpness or contrast to dull or blurred areas. Plug-in image processing accessories have the power to affect a complete makeover on all or parts of a scanned photo or bitmapped painting you created from scratch. You can also use these addons to produce undetectable changes that make a good image even better.

Figure 8.2 shows a variation on the flag picture used in Chapter 1. There, the intent was to show how Photoshop could duplicate traditional photographic effects. In this illustration, however, you can see what the same image looks like with six different filters applied in a deliberate attempt to create a more highly "processed" appearance. While Photoshop can duplicate many traditional camera effects, it can go far beyond them, too.

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