درجة التباين .. كتاب أسس التصوير الضوئي

CONTRAST contrast

It is one of the characteristics of the sensitive layer. In order to bring its interpretation closer to mind, we suppose the presence of a white object as a piece of chalk in front of a black surface. Likewise, if we plan drawings with a black fine
Like a blackboard, then we express the relationship between these two colors as contrasts
On white paper, the two colors are contrasting. And if we put a yellow object in front of a blue surface, this is a contrast between their colors. The contrast when seeing objects with the eye depends on two factors: the first is the difference in the amount of rays reflected from both, as we saw in the case of placing chalk in front of a blackboard. The second factor is the difference in the color of the rays reflected from converging rays.

Following the previous explanation: If we want to explain the meaning of contrast in a positive image, we find that the degree of contrast depends on the ratio between similar colors in a single image, and given that the colors of reproductive images are nothing but black and white (considering that we are not talking about recorded images), therefore The degree of contrast in a positive image is defined as the ratio between the degree of blackness in some areas to the degree of whiteness in the adjacent areas.. The greater the difference between these two colors, the more we deem the image to be of high contrast (Fig. 67).

Conversely, if the colors are seen graduating from white to very light light gray - then light gray - then standing gray then very standing gray - black, i.e. the gradation is light, we express the image as having low contrast. Law Contrast. Or it is sometimes expressed as soft (Fig. 67-1). As for the study of the properties of negative transparent sensitive films. (or sometimes positive), so we find that what is meant by contrast is the ratio between the degree of transparency of some areas
And between the degree of opacity of the adjacent areas, the greater the ratio between transparency and opacity, the greater the degree of contrast.

Estimating the degree of contrast in the negative image is of great importance when the image is printed or enlarged to obtain a positive image from it. Delicate papers with low contrast. And she expressed the lack or increase in contrast as a defect in the pictures, as the picture is considered successful if the degree of contrast in it is medium, unless the increase or decrease in contrast is intentional for an artistic purpose.
In future chapters, we will come back to talking about measuring the degree of contrast in the image. I intended not to expand now until you know something about the display process, because the contrast is closely related to the display process. There are factors that affect the degree of contrast in the image, which we briefly describe as follows:

Factors affecting the degree of contrast in the image:

1- The nature of the negative and positive sensitive layer:
This is due to the method of manufacture. There are films that are prepared to reach a high degree of contrast, such as those called Process Films, and sensitive papers that are equipped for this purpose, such as Hard Paper. Also, the speed of the sensitive layer has an effect on the degree of contrast that can be obtained, so the lower the sensitivity speed in A type of film, the greater the degree of contrast that can be obtained.

2 - The amount of exposure to light and the time of exposure:

If the time decreases in these two cases, the degree of contrast decreases, and vice versa, if the time increases to a certain extent, the degree of contrast increases, and if the time increases incorrectly, the contrast decreases. So if we make photography with an aperture of 8 and speed! Or the degree of contrast increases - but if the error increases a lot and we estimate the speed as one second, for example, while the lens aperture remains constant, the degree of contrast decreases very much - and on the other hand, you want the overall density in the image Densit


3- Display process:

The composition of the show solution has a great effect on the degree of contrast, as by increasing some of the substances included in the show solution, such as hydroquinone, the degree of contrast increases.


The contrast in the image - as well as by increasing the concentration of the temperature of the solution, the degree of contrast increases and the contrast increases with the increase in the display time also.

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. sensitive.

5- The nature of the body, its color, and the incident and reflected lighting from it:

There are factors that affect the degree of contrast of the visuals when viewing them with the eye, and as we mentioned, we impose before us a group of a certain color on which a certain amount of rays fall, focused on it, and to a greater degree than the rays that fall on the surrounding medium. Such a body appears clearly to us and we can easily distinguish it from the surrounding environment. Another body of a certain color, similar in color and in the light falling on it, reflects the surrounding medium. There is no doubt that the first body appears to us and we can distinguish it easily due to the great contrast between it and the medium, and this contrast resulted from the difference in its color and the amount of rays reflected from it. From the previous observations, we can show some factors that affect the degree of
The contrast in the image is:

​​​​​​The nature of the body: that is, the amount it reflects, absorbs, or runs out of, noticing that light bodies reflect a large amount of absorbs a large amount, and transparent bodies run out of the largest part