Filters for black and white photography. Changing Tones with Filters The green filter, used to take the black and white picture above at right, blocks out much red light and allows green to reach the black and white film, making the red apples darker than the leaves, a natural appearance. Without the filter the tones seem confusingly similar. To make up for light absorbed by the filter, the aperture is opened 2 2/3 stops above the normal opening. Because black-and-white film records the world in tones of gray, dramatic contrasts in nature's colors are often almost indiscernible in a black and white photograph. To the normal human eye, red apples are easily distinguished against a back¬ground of dark green leaves, but when photographed in black and white the apples and the leaves are reproduced in almost exactly the same gray shades. Color filters are used to get a tonal separation that approximates the color contrast in nature. Objects acquire color because they reflect part of "white" light-a mixture of all colors and absorb the rest. Filters also absorb some of the colors in light, passing others. When the apples and leaves are black and white photographed through a green filter (above, right), the apples are considerably darker- i.e., less exposed-than the leaves because the filter has absorbed much of the red light the apples reflect but let the green reflection of the leaves reach the black and white film. Green filters are often used when a comparatively natural tonal separation is needed because they alter the exposure of black and white film to red, making the relative difference between the two colors closely match those sensed by the human eye. To control the exposure of bluish colors, yellow or red filters are used, particularly for adjusting the tone of the sky. Black and white film is so sensitive to the sky's blue and ultraviolet waves that even a dark blue sky causes about as much exposure as white would, and the sky often becomes nearly indistinguishable from clouds when black and white photo¬graphed without a filter. By using a yellow filter, which absorbs much blue, the sky is darkened and the smoke clouds become visible (top, right). The contrast between sky and clouds can be made even more marked if a red filter, which blocks nearly all blue light (bottom right), is used. Eliminating Tones with Filters - Reflections from glass or water (below, left) are tones a black and white photographer may wish to avoid in his black and white picture. They can be eliminated very simply because they are made up of light that is unusual in that it is polarized; i.e., the waves are oriented at one angle rather than many angles (pages 20-21). This makes it possible to control them with a polar¬izing filter, which can block light orient¬ed at one angle and thus also block the reflections (below, right). A polarizing filter looks transparent but contains submicroscopic crystals lined up like parallel slats. Light waves that are parallel to the crystals pass between them; waves oriented at other angles are obstructed by the crystals, as indicated in the diagram at right. Since the polarized light is all at the same angle, the filter can be turned to block it. This also blocks some waves in the general scene light, but only those oriented like polarized light; the rest get through. To find the orientation that will block polarized light, the black and white photographer looks through the filter and rotates it until the unwanted reflection vanishes. The filter is then placed in the same position over the lens. (With a single-lens reflex black and white camera, the filter can be adjusted while it is in place over the lens.) Because of the partial blockage of light by the filter, the aperture must be opened 1 1/3 stops above the normal exposure. A polarizing filter is adjusted here to permit passage of only those desired light waves (black) oriented parallel to its picket like aligned crystals and to screen out all other light waves (gray) angled across the pickets.