الأنواع المختلفة للأفلام الفوتوغرافية - ٢ - تجهيز المخبر .. كتاب التصوير الضوئي

Different types of photographic films - 2 - Preparing the laboratory .. Choosing and using materials .. Book of photography

Microfilm:

It is a slow contrast film used to copy documents that do not contain half-tones: It is sensitive to green and yellow radiation, but there are films sensitive to all colors of the visible spectrum for colored documents. Its grains are very weak and its clarity is excellent, but its speed is much slower than the speed of ordinary popular films.

- Light film:

It was invented for linear professions where it is used to make printing plates. The contrast is very high, especially if the film appears in a special detector containing formaldehyde, which gives the image very clear edges and borders. It is used in the laboratory to make intermediate negative and positive films.

- Positive film:

It is a film with a half-tone color, its light-sensitive surface is similar to the surface of the enlargement paper. It can therefore be used under light that has no chemical effect and can be used to draw transparent positive images (black and white magic lantern slides).

- Black and white reversible film:

Most slow speed black and white films can be reversible which gives a direct positive image on the film. Before the generalization of color there were reversible black and white films which were processed by the manufacturer to produce black and white strips for magic lantern display.

- Peelable film:

The light-sensitive mixture (Lite) is spread on an additional virgin layer of gelatin. When immersed in warm water, the gelatin becomes soft and the image separates from its substrate: it can then be transferred to another surface.

- Autoscreen film:

This is a very special film that immediately gives an interlaced image, i.e. a composition of black and white dots of different thicknesses, the juxtaposition of which creates a sense of continuous pattern. If we enlarge a negative image formed on autoscreen film, we obtain a positive image of dots whose size depends on the applied magnification ratio.

- Special films for color work:

- Infrared color film:
This film was created for military uses such as aerial photography camouflage explorations, but it has many scientific uses. Modifying the color properties of the film layers creates a yellow positive image in the green-sensitive layer, a red-positive image in the red-sensitive layer, and a cyan image in the infrared-sensitive layer. A yellow filter absorbs the violet-blue radiation that can To get to the film. You can never predict the final scene of the picture.

- Intermediate negative film:
This film was specially developed to produce intermediate-gradient negative images from color slides. This resulting negative image is then used to draw images on paper or on color film. The method of forming intermediate-gradient negative film is very complicated, so it is often preferable to draw the slides on reversible paper without intermediate-gradient negative film.

- Exchange errors:
The exchange law states that exposure is the product of working time × light intensity. Intensity and time are therefore related in a reciprocal relationship, and if the duration is doubled, the intensity must be halved in order for the exposure to remain constant.

Unfortunately, for very short stops (1/1000 sec and less) under strong light on the one hand, and for long stops (1 sec) under weak light on the other hand, the exchange law cannot be verified: this is what we call exchange errors.

This incident only becomes apparent after the outer limits between shots of 1 and 1/1000th of a second, and even in black and white, thanks to the freedom of manipulation of the shot time in this principle: it is rare to have to correct for exchange errors. For very short stops (such as those provided by the flash) or very long ones (several seconds or several minutes, a night shot for example), some correction in the exposure must be made. Depending on the case, the correction can be made either by adjusting the shot time or by means of the aperture. This correction varies according to the nature of the film used. This information is often given by the manufacturer.

The plate below gives some instructions on the value of the exposure correction to be taken for shots longer than a second.

For stops less than 1/1000th of a second, the exposure is increased by increasing the development time, and the value of this additional time can only be determined by experimentation. A value of 25% increase seems to us a good starting point.

- Swap errors in color films:

Color films contain three superimposed sensitive layers, each with its own characteristics. The balance of the three layers is applied to a series of stop times. Beyond these limits, the swap errors become significant and different for each layer: this not only represents a visible loss of speed but also leads to the appearance of a dominant color. In general, if possible, it is preferable to increase the illumination level or open the pupil so that the shot times remain at their normal values.

Manufacturers produce films (especially for professional use) specially weighted for long stops (L model) or short stops (S model) and they also specify the correct filters to use for the actual film speed when the shot duration exceeds the specified limits.

- The tangle image
Autoscreen film automatically produces tangle negative images. Here (above) is a very large part of the face and the shape is shown from adjacent points.