التباين .. كتاب الإضاءة والفلم

Chapter Seven
CONTRAST CONTRAST

If we put a white object like a piece of chalk in front of a black surface like a chalkboard, then we express the relationship between these two colors as contrasts, and if we draw black drawings. On white paper, they are contrasting, and if we put a yellow body in front of a blue surface, there is a contrast between their colors. The contrast when seeing with the eyes depends on the
Two factors:

(a) The difference in the amount of rays reflected from adjacent objects, surfaces, or spaces, as we have seen in the case of placing chalk in front of a black board.

(b) The difference between the color of the rays reflected from adjacent objects, i.e. in other words, the difference in the length of the light wave reflected from adjacent objects.

Whereas the colors of (non-coloured) photographs are no more than white, black, or gray, light or dark, so the degree of contrast in the (opaque) image is defined as the ratio between the degree of blackness of some of its parts to the degree of whiteness of its neighboring parts. If this percentage increases, we express the image as having high contrast (gh Contrast) (Fig. 48-a). Conversely, if the image is gradient from white to black and the colors between them (very light gray, then light gray - then medium gray - then straight gray - very straight gray), then we express the colors of the image as having low contrast, and it is sometimes said that they are « Soft (Fig. 48-c).

With regard to transparent images (negative or positive), what is meant by contrast is the ratio between the degree of transparency of some parts of the image and the degree of opacity of the adjacent parts, and the higher the ratio between transparency and opacity, the higher the degree of contrast.

Estimating the degree of contrast in the negative image is of great importance when printing the image or enlarging it in order to obtain a positive image from it. It is possible to fix that defect by using sensitive raw materials with low contrast.

There are factors that affect the degree of contrast in the image, which we briefly describe below, and we will come back to study them in detail in this book.

Factors affecting the degree of contrast in the image

1- The nature of the negative and positive sensitive layer:

Among the negative films manufactured to be highly contrasted, such as those called process films, are sensitive papers, which are also prepared for this purpose (hard paper). The speed of the sensitivity of the film also has an effect on the degree of contrast, so the contrast increases whenever the speed of the sensitivity decreases. Among the positive films prepared for printing transparent positive images, what was also made to lead to strong contrast.

2- The duration of exposure and exposure:

The contrast decreases if the film is underexposed or underdeveloped, and vice versa, the degree of contrast increases by increasing either of them. As an example, we suppose that the light conditions called for the use of an aperture of f A lens and a speed of 1 in 200 f of a second, so if we made the photography with an aperture of 28 and a speed or even with it, the degree of contrast increased slightly, but if the error increased a lot, we estimated the exposure speed by half a second, for example, the lens aperture remained. Constant as it is (28), the contrast will decrease and the Density will increase a lot.

3- Presentation solution: The composition of the revealing solution has a great effect on the degree of contrast, and the contrast increases with an increase in the percentage of some substances included in its composition, such as hydroquinone, sodium carbonate, or caustic soda, and the contrast increases with an increase in the degree of concentration of the solution or an increase in its temperature.

4- Using optical filters:

Optical filters play roles in increasing or decreasing the degree of contrast significantly, as they absorb some of the colors that make up the white rays and allow others to pass according to the color of the filter, which leads to a significant increase or decrease in contrast, and we will talk about that in detail later in this book.

5- The nature of visible objects, their color, and the illumination falling on them and reflecting from them:

There are other factors related to the visuals themselves and affect the degree of what is around them, whether when seeing with the eyes or when photographing them. This contrast with the factors are:

(a) The nature and color of the body: that is, the amount of radiation it reflects, absorbs, or transmits. Light bodies reflect a large amount of rays, while stellar bodies absorb a large amount of them, and transparent bodies transmit the largest part.

(b) The ratio between the amount of light rays falling on it and that falling on the surrounding medium, and as an explanation for this we assume a body of a certain color on which focused rays fall to a degree greater than those falling on the surrounding medium, such a body appears contrasting with the surrounding medium, and from It is easy to distinguish and perceive at first glance, unlike another of the same previous color, the intensity of its illumination is close to the illumination of the surrounding medium.