المرشحات الملونة للتصوير بالأبيض والأسود .. كتاب فن التصوير الفوتوغرافي

Color filters for black and white photography

When we photograph a subject in black and white, all the colors of the visible spectrum that make up the white color are reflected on the sensitive layer in the film to achieve the image, but the different colors do not affect the sensitive layer with the same value; The blue color is the most influential, while the red color is the least. This difference in effect is irrelevant as long as we are photographing a white subject, but when we photograph a subject of different colors with black and white film, we must take the difference in the effect of colors into account. Suppose, for example, that we are shooting an outdoor scene with a blue sky with some clouds. Since the blue color strongly affects the film, the negative will show a dark sky, and the printed image will be very white, with no clouds appearing because it is also white in color.

Also, the eye does not respond to colors in the same way that the film responds to. The blue color appears dark to the eye, while it appears to the film to be very light.
The pistachio color looks very bright to the eye
While it appears to the film to be of medium brightness, while the red color appears to the eye to be medium dark and appears of the same value to the film.
These differences may be significant or insignificant, depending on the differences. Often the differences are simple and do not notice the circumstances of the image. Let us remember here that we photograph in black and white and that the difference in the impact of colors is translated by the image in gray of varying value, not in different colors.

But there may be situations and circumstances in which it is important to pay attention to accurately representing the degree of luster of the colors. To adjust the value of these colors relative to each other as seen by the film, colored filters are used.

The work of filters is summarized by blocking or absorbing rays of some colors and allowing others to pass through, and this definition does not apply to polarizing filters. And because the filter allows the color rays to pass through in a selective manner, it appears colored itself. The red filter looks red because it absorbs all colors except red, and the filter that absorbs only blue appears yellow because it allows the passage of red and green, which together make up yellow.

Because the filter absorbs some of the light, its use requires increasing the degree of exposure of the film, and this increase is determined by the coefficient of the filter used. A filter that absorbs, for example, half the amount of light requires an exposure twice as great as the exposure without this filter, and its coefficient in this case is 2. The coefficient of a filter depends on three factors: the color of the filter, the nature of the light falling on the subject, and the sensitivity of the film to colors.
(See Table 1).

Apply the filter parameter

The filter coefficient can be applied to one of the component elements of the film exposure, i.e. the coefficient can be applied to the exposure period, lens aperture, or film speed, and we choose the most appropriate one to suit the conditions. For example, if you are using a coefficient of 2 filter, it is easier to increase the aperture by one degree. But if you need to keep the aperture small to keep the depth of field to the maximum, reduce the shutter speed. For example, if the appropriate exposure without a filter is a shutter speed of 1/100th of a second, set the shutter speed to 1/50th of a second with a filter with a coefficient of 2.
But if you expect to take several photos with the same filter, it is easier to reduce the film speed index by the same value as the filter coefficient used, and read the scale in a normal way. For example, if the used film speed is 100 AZA without the filter, set the film speed index to 50 AZA when using the K2 filter.