المرشحات الضوئية .. كتاب التصوير علم وفن

Chapter V

Optical filters

These are the colored glass disks that we see some photographers put in front of the lens. What's the use of it? This is what we will know now

As we know (see the chapter on light), white light consists of several colors. Young's theory of seeing colors, there are three main colors from which all other colors can be made, and these colors are red, green and blue.

As its name indicates, the filter filters the light that falls on it, allowing some light waves to pass through and preventing the passage of others, like a filter that allows the passage of a certain size and prevents the passage of larger sizes.

For example, a red filter prevents the passage of blue and green rays and allows the passage of red rays only. Thus, every filter prevents the passage of any rays except for the component of its color only. For example, the yellow filter allows red and green to pass through, but blue is not allowed.

Filters are used with all types of films. We will discuss each group in relation to the films that are suitable for it, and then we will discuss a group that has appeared in recent years, which is the group of special effects filters.
First: filters for black and white films

As we know, the sensitivity of the eye to light differs from the sensitivity of black and white films, so we find that orthochromatic film (currently no longer available) has little sensitivity to red and green colors and is very sensitive to blue, while panchromatic films (all black and white films currently available) are sensitive to all colors, although they are less sensitive than The eye is green.



From a practical point of view, when photographing a landscape with a blue sky and white clouds, we notice that the resulting image does not show clouds, but rather the sky appears in a very light gray color. This is because the sensitivity of the film to blue, ultraviolet, and white is very high, and they were all recorded on the film in one color tone, black on the negative and white or very light gray on the positive. If we prevent the entry of blue and ultraviolet rays with a yellow filter, for example, the difference will appear in the image and clouds will begin to appear. And if we completely prevent the entry of blue and ultraviolet rays with a dark red filter, the color of the sky in the pictures will become black, with the appearance of white clouds in a dramatic way.

We notice that by using a filter, we prevent part of the light from falling on the film, and in order to get an exact exposure, this deficiency must be compensated. This is done by increasing the exposure at fixed rates for each filter. We call this ratio the filter factor. If we say the filter factor is 2, for example, then the exposure must be doubled, either by using a wider pupil or by using a lower shutter speed, and to prevent prolongation, consult the following table: