Filters are used in all the various steps of the photographic process. Though often neglected in the shooting stage, the use of filters can tremendously enhance the final product in both black-and-white and color photography.

PURPOSE OF PHOTOGRAPHIC FILTERS

The purpose of photographic filters is to alter the characteristics of light that reaches the light-sensitive emulsion As light is transmitted through a filter, at least one of the following alterations occurs:

1. The color of light is modified.

2. The amount of light is reduced.

3. The vibration direction of the light rays is limited.

The two most important reasons for using photographic filters are to create an effect with an emulsion and to control the exposure of an emulsion. Interlocked with the use of filters are characteristics of light and characteristics of photographic emulsions. The effectiveness of a filter depends upon the ability of an emulsion to respond to the color of light transmitted by the filter.

Colored filters modify the way colors are recorded. Without the use of filters, black-and-white panchromatic film records colors as gray tones. These gray tones correspond roughly to the tonal range as seen by the human eye. Colored filters selectively brighten or darken these tones. In color photography, colored filters are used to correct or distort color balance.

Filters of a specific color transmit most of the light of that color and partially or completely absorb light of all other colors. For example, a red filter transmits red light and may partially or completely absorb blue and green light, depending on the deepness or purity of its color (fig. 3-1). Likewise, a yellow filter transmits red and green light and partially or completely absorbs blue light. Remember, a secondary color of light is produced by combining two primary colors of light. Red and green equal yellow; thus a yellow filter passes red and green light.

Figure 3-1. Red and yellow filters.

Filters are available in three forms: optical glass disks bound with metal rims, lacquered gelatin film squares, and glass squares. Glass disk filters are the most practical for general use. They are available in different sizes called series numbers, such as Series 4,5, and 6 or in millimeter sizes, such as 52mm and 59mm. Glass disk filters attach to a camera lens in two ways. Some have threads and screw directly into the lens barrel, and the others are held on the lens barrel by an adapter ring. Gelatin filters and square filters made of glass are either inserted into special filter holders that are part of the camera, or they are held on the camera by a square filter holder.