خطوات عملية الإظهار الرفرسال ٢-a .. كتاب التحميض والطبع والتكبير

Especially if the solution is concentrated - it is possible that the sensitivity of the remaining silver in the gelatin will decrease to some extent, and the density will decrease in the “middle tone” regions, which is something that would increase the contrast in the image. It is also considered wrong to reduce the duration of film treatment in this solution, because if it is shorter than the appropriate limit, it is certain that all metallic silver deposited in gelatin will not be removed.

Fourth: bathroom cleaning and clarification of the film:

As a result of treating the film in the bleaching bath previously explained (i.e. in the reversal bath), some of the particles of the previous chemical reactions accumulate on the surface of the solution, which affects the degree of transparency of the film. Therefore, work must be done to remove them. The removal process is done by treating the film in a solution known as Clearing Bath, which consists of a sodium sulfite solution that converts the previous reaction products into easily soluble salts, which are disposed of either by fire, this solution, or during the washing with water that follows this stage. The film is treated in this solution for a specified period. It is a mistake to increase the period beyond what the manufacturer specifies, as it is then possible that some of the remaining silver salts will dissolve in the gelatin due to the ability of the concentrated sodium sulfite solution to dissolve silver halide, which results in a decrease in the general density in the final positive form. Therefore, when using some films, it may be necessary to reduce the concentration of this solution (as is the case with Kodak 3 Clearing Bath CB solution), or another method may be followed, which is adding a few grams of potassium bromide to each liter of the solution.

On the other hand, it also results from the shortening of the film treatment time in this solution (or from not treating it with it at all) that clear spots appear in the final image after the completion of all exposure processes (as the reaction products tend to blacken after the second exposure process) and the These spots are clear, especially in the transparent parts of the positive image (the parts that represent the high-light areas of the photographed subject).

The film must be washed with water well between the two phases of bleaching and clearing the film, as the washing process removes the residues of the bleaching solution attached to it.

And if it is theoretically permissible to replace the clearing bath by washing it with water thoroughly in a strong jet of water with continuous stirring, but it is more practical that we do not do that, but a sodium sulfite solution is used to perform this purpose as we mentioned previously.

Two ways to produce a positive image:

There are two basic ways to produce a positive image in the display operations, and we summarize them as follows:

(a) The film should be exposed to light so that the remaining silver salts (after the first exposure process) become blackish (reducible) if it is treated with a suitable brightening solution.

Therefore, this method is summarized in the necessity of exposing the film to light for an appropriate period, then it was re-exposed in a second exposure solution.

(b) The film should be treated (after subsequent washing in the cleaning bath) in a solution of a special kind that makes the rest of the silver salts blackish without being exposed to light, by creating a chemical mist on the film that increases its density to an appropriate proportion. The sensitive layer of (non-reducing) silver salts. For this reason, this type of solutions is known as “fogging developer solutions.” These solutions reduce silver salts to black metallic silver by converting it into another compound. From silver compounds such as silver sulphide, its color intensity (or the extent of its blackness) is equal to what we would have obtained if the film had been exposed and then shown according to the first method.

It has been proven in practice that the first method (exposing to light and then showing it again) ensures that we get the best results. As for the second method, it is less valid because the transparent areas in the image are not completely transparent. In the following, we give an example of the composition of the chemical haze-reducing highlighting solutions.

Fifth: Exposing the film to light: Re-Exposure

The exposure process can begin any time between the stage when the film is in the clearing bath and the start of the second exposure stage.

It is also permissible to start the exposure when the film is in the bleaching bath, but this is not recommended due to what is said about the low sensitivity of the film at this stage.

During the exposure process, the silver salts that were not reduced to metallic silver during the first exposure process are affected, and these salts become reducible in the second manifestation process, which will be discussed below.

It is sufficient to expose the film to a lamp (60 watts) at a distance of 12 to 18 inches from it, or to a lamp of 100 watts at a distance of half a meter, to verify that it has been exposed to all the light in a uniform manner. In the second case, it is sufficient to move the film in front of the lamp for a minute and a half. . The correct reference in each case is what is determined by the manufacturer producing the film in the statement of the stages of display. It does not harm the film much if the exposure is increased or decreased slightly by (twice). However, if the exposure is greatly increased, then it is certain that the density will increase, just as if the exposure is decreased a lot, then it is also certain that the general density will decrease. It is customary in the laboratories for showing advanced cinematic films to prepare an automatic means for adjusting the necessary exposure process after the first exposure, in a way known as Automatic Exposure compensation. In this method, a photocell is used, which is affected by the amount of light that permeates the film. This photocell is connected to the light source and controls the light energy it emits. Thus, the amount of light required for exposure depends on the degree of density of the negative images.

Sixth: The second display process:

After the completion of the previous processes, the remaining silver salts in the sensitive layer are capable of being reduced to metallic silver if treated in an appropriate appearance attempt, so a transparent positive image is formed, in which the tonal values ​​are reversed. Reversed (i.e. the opposite of what it was in the negative image), so the parts that were dense in the negative image (i.e. that were black in it) leave behind only very few silver salts, so they become less dense in the positive image, and vice versa.
From the foregoing, it is clear to us that the degree of density of the transparent positive image that you get at the end depends on the amount of silver that remained after the negative image was removed. Formation of the positive image, so it is expected that the final positive image will be of little density (Light Image) and vice versa if the first negative image is sparse. This indicates that a large amount of silver salt remains, which leads to an increase in the density of the positive image. Accordingly, it is required - in order to obtain a good image - that there is a balance between the negative image (which was formed after the first exposure process) and the remaining amount of silver salt (which is for the positive image later on). Accurate estimate, without cutting or adding to any of them. It is expected that the positive form will be in the best condition, as only half of the amount of silver salts was reduced during the first manifestation stage, so that the second half remains to form the positive image.
As the contrast of the first negative image increases, the contrast of the positive image increases as well.

It is possible to control to some extent the intensity and contrast of the final image by manipulating the duration of the second display. If the first negative image was of low intensity, it is expected that the second positive image would be very dense if it received the correct exposure. While the intensity of the positive image can be reduced slightly if we decrease the duration of the second display. However, the image at that time may be criticized for its lack of contrast as a result of the short duration of the second exposure. Therefore, it is not recommended to follow this method unless the (first) negative image is of low density and high contrast, which are the conditions that may result primarily from a lack of exposure while photographing highly contrasting subjects in nature. Therefore, it is certain that the final positive image suffers from a lack of detail in the shadow areas.

In the second exposure process, one of the contrast solutions is usually used, such as Kodak solutions (D-19), (D-72), and (D-88). It is advisable to add a few antifog solutions to it.

It is a grave mistake to use a solution of the same type previously used in the first manifestation process in the second manifestation process, because it carries salts that have a great ability to dissolve alfalfa salts such as potassium thiocyanate. The general density in the positive image is noticeably reduced.

Likewise, it is also not desirable to use a solution for the second display from the category of solutions that lead to low contrast. Soft developers, under the pretext of wanting to reduce the contrast of the final positive image, or to correct one of the errors that occurred in the previous stages.

Seventh: the installation process:

If you perform all the steps of the Reversal development process very accurately, then the installation process is considered - at least from the theoretical point of view - an unnecessary procedure. As for the practical point of view, it is done as a precaution as a means of dissolving any surplus of silver salts that were not reduced in the two stages of exposure, which would damage the shape of the image. Increasing the hardness of the gelatin, but if it is necessary to finish the second exposure before all the remaining silver salts in the sensitive layer are reduced, then the image must be fixed to remove the remaining silver bromide in it.

And after fixing the film, it should be washed for a period of not less than a quarter of an hour in running water, then dried in the usual ways.