All black-and-white photographic filters (yellow, orange, red, green, and blue) allow light waves to pass through that are the same color as the filter itself. Other colors are impeded to a greater or lesser extent depending on two things: the strength or depth of hue of the filter and the “oppositeness” of the color of the light wave from the filter. Thus, a red filter allows red light to pass through unimpeded, but it impedes yellow, green, blue, and other colors to varying degrees, depending on how close they lie to red on the color sphere (Figure 6-6).
In black-and-white, all colors are seen in terms of their gray equivalents on the axis of the color sphere. When two objects of different colors have the same gray tonal value, it may be difficult to separate them in black-and-white. The classic example of the red apple and green leaves comes to mind. There is no problem separating them in color, but in black-and-white they both translate to a middle gray tonality. Textural differences help separate them, but only through filters can their tonal values be changed to create a visual separation of the tones.
Let’s say you use a red filter during exposure of the red apple and green leaves. The predominant red rays of the apple pass through the filter, but the predominant green rays of the leaves are stopped. Because the green rays are stopped, they don’t reach the negative and the leaves appear darker on the final print. The leaves don’t become black, however, because other colors reflect off their surfaces, and those colors pass through the filter to sensitize the negative. Some of the colors reflected off the apple are stopped by the filter, but at a far smaller percentage than those radiating off the leaves. Thus, the apple appears lighter and the leaves darker. If a green filter were used, the opposite effect would occur: the apple would be darker and the leaves lighter.
Which filter would be most appropriate? This question brings us into the realm of interpretation and creativity. The green filter is the more “realistic” choice because the darkened apple appears heavier. Psychologically, we ascribe greater physical weight to dark objects. The green filter would be more in keeping with this fact, while the red filter would oppose it. Of course, there may be a good reason to purposely fight the natural, comfortable feeling. You may want to create a feeling of strangeness or a slight off balance. It’s a matter of individual preference and creativity.
When you understand what various filters do, you can use them as interpretive and expressive tools. This is crucial for creative photography. Full knowledge of your equipment is immaterial unless you use it to further your interpretive goals. And, of course, you cannot further your goals unless you first define them.
If two filters have the same color, the darker or deeper filter is the stronger one, and allows a smaller percentage of other colors to pass through it. Thus, a deep red filter allows little but red radiation to pass through and blocks a greater portion of other colors than a medium red filter. A yellow filter, which is lighter-hued than red, allows substantial amounts of colors other than yellow to pass through. For example, the #12 yellow (or “minus blue”) filter cuts out blue light almost completely. For this reason, the deeper-hued filters are considered stronger filters, as they have a more pronounced effect on the relative intensities of light passing through them.
All filters require an increase in exposure to compensate for the amount of light being stopped. Stronger filters require substantial increases in exposure. This exposure increase is known as the “filter factor”. A #25 red filter, for instance, requires a three-stop exposure increase over the unfiltered exposure because only
Because the red filter prevents
Note
Full knowledge of your equipment is immaterial unless you use it to further your interpretive goals. And you cannot further your goals unless you first define them.
A yellow filter requires only a ½ or 1 stop exposure increase, as it is weaker than a red filter. It should be pointed out that even with the appropriate filter factor for correct exposure, a red filter will darken blue far more than a yellow filter because it impedes blue more thoroughly than the weaker yellow filter.
The filter factor is correct only if the scene contains a broad spectrum of color. For example, if a scene is dominated by red and a red filter is used, little exposure increase is necessary because all red wavelengths would pass through the filter unimpeded. However, if a red filter is used on a scene that is dominated by green, an exposure increase of more than three stops would be necessary because a high percentage of the wavelengths would be stopped by the filter. Thus, the filter factor represents a starting point for the average scene. It must be increased or decreased as appropriate if the scene diverges greatly from the theoretical standard.
Examples with a Hypothetical Landscape
Walk with me into an idyllic landscape and consider the effect of various filters on it. The foreground is a rolling pasture of luxuriant green grass with a background of low hills and distant mountains. A red barn is off to one side, large orange poppies dot the foreground grass, and a deep blue sky with fleecy white clouds tops the scene. It is a hypothetical scene that possesses just about every color, so playing with filters can be an interesting exercise. Following are some of the possible effects of using (or not using) filters on such as seen:
Using no filter with any of the common panchromatic films (which have a higher sensitivity to blue light than our own eyes), the sky would come out as light gray. The clouds would not stand out strongly against the light tonalities of the sky, and the distant mountains would also be rather light due to the bluish atmospheric haze—however slight that haze may appear—and therefore would not be terribly outstanding. The barn would be dark, as would the grass, while the poppies would stand out as light gray dots.
With a light yellow filter (#8 or K2), the sky would be darkened to the gray value that your eye would tend to expect. The clouds would be more visible against the darker gray, and the distant mountains would also be somewhat more visible due to the reduction in blue haze. The grass would be slightly lightened and the poppies would be lightened even more, while the barn would be slightly darkened, separating it somewhat from the grass tones. A deeper #12 yellow filter would darken the sky considerably, making the clouds pop out more strongly. Again, the grass would be lightened to about the same extent as with the K2 filter, but the poppies would be made even a bit brighter against the grass. The barn would be darker than with the K2 filter.
A #21 orange filter would darken the sky considerably, bringing out the clouds quite strongly, and the mountains would also be darkened. The orange poppies would be nearly white against darkened grass, and the barn would be slightly lighter than the grass.