Photo printing

The first photo printing technology was implemented in 1841 by the Englishman William Henry Fox Talbot . The calotype he invented was the first negative-positive process in the world suitable for replicating photographs ^[1] . From the paper negative obtained in the camera obscura , a positive method was printed on the same paper impregnated with silver chloride using a contact method . Talbot was the first in the world to establish a short-run issue of photo albums, where prints on “salted paper” were glued in ^{[2] [3]} . However, this method of replication, as well as all subsequent positive photoprocesses , was too expensive for publishing , and later gave up this niche to photomechanical processes, such as phototype, heliogravure, and offset printing ^{[4] [5]} .

Daily Photo

The image of the calotype appeared on paper during printing, since in the exposed areas silver was reduced from a halide to a metallic form directly under the action of light. At the end of the process, the image was fixed in a hyposulphite solution, which removed unexposed silver salts, was washed and dried. Due to the flaws and low image quality, the calotype did not spread, and the albumin print came to replace it. A new photographic paper was prepared on the basis of egg proteins that bind photosensitive silver chloride ^[6] . Albumin paper was also “daytime”, that is, it manifested itself directly in the process of exposure, which lasted 25-30 minutes ^[7] . The appearance of albumen printing in 1850 coincided with the invention of the wet collodion process , which produced negatives on glass photographic plates . A sheet of photographic paper was covered on top of the negative, which pressed tightly against the emulsion, and in this form was exposed to sunlight. For close contact, special copying frames with a clamp were used. The high quality of the collodion negatives was combined with the subtle tonal transitions of albumin photographic paper, giving an excellent imprint ^[8] .

Such printing technology dominated for about 30 years, until the main deficiency of papers with egg white was eliminated: the completion of the manufacturing process was carried out by the photographers themselves, since the photosensitivity remained for only a few hours ^[6] . More perfect werecelloidin photo paper, made on the basis of collodion , which replaced albumin ^[9] . Such photographic materials retained photosensitivity for up to 3 months, excluding the sensitization procedure before printing ^[10] . However, both celloidinic and later aristotype photographic papers were suitable only for “daily” printing, which does not require chemical manifestation, which occurred during the exposure ^[11] . The correct exposure was determined visually by the degree of darkening of the photo paper. The seal ended with fixation, which removed the unexposed halide residues, as in the very first “salt” papers in calotyping. Albumin and celloidin photo paper required additional toning with salts of gold and platinum , which improved the tint of the image and increased the light fastness of short-lived prints ^[12] . Usually, this process was combined with fixation of fixation in solution ^[13] .

Photo paper with manifestation

Because of the low sensitivity in the visible region of the spectrum, daytime photo papers were only suitable for contact printing, which took place in ultraviolet- rich sunlight, usually in the yard of a photo studio. At the same time, the size of the printout always coincided with the size of the negative, and shooting on the same photographic plate was necessary to obtain large format images ^[14] . Projection printing, allowing for an increase, became possible only with the advent of modern silver-gelatinous photo paper with a display with sufficient photosensitivity ^[15] .

Production of the first photographic paper of this type was established by Joseph Swan in 1879 , but their wide distribution began only in the second decade of the 20th century ^[16] . Up to this point, the market was dominated by platinum printing , which was also carried out by contact ^[17] . The dominance of photographic papers of this type put an end to the First World War , which led to a sharp increase in the cost of platinum. Attempts to replace it with cheaper palladium were unsuccessful, which led to the spread of silver print ^[18] . Optical zoom on such papers made it possible not only to obtain large-size pictures from small negatives, but also to crop the image, correcting the mistakes made at the time of the shooting. In contrast to the low-sensitivity “daytime” photographic papers, which allow manipulation under weak diffused light, the silver gelatin required a complete darkening of the photographic laboratories and their non-actinic lighting ^[19] . The so-called “gas-printed” photographic paper with chlorine-silver-silver emulsion got its name due to the possibility of processing with yellow gas light that was widespread at the end of the 19th century ^{[20] [21]} .

In the early years, in order to increase the negatives, the camera was most often used, whose personnel window was lit by a projection lamp or simply by daylight, guided by a reflector through a hole in the wall of a darkened photo lab ^[22] . The negative was placed on the inside of the frosted glass, and was projected by the lens onto photographic paper fixed to the wall. Sometimes, two cameras of different formats, mounted opposite each other, were used for magnification. The lens of one of them was taken out, and photographic paper was charged into the cassette, onto which a negative camera was projected by another camera ^[23] . With careful light insulation of the joint between the objective boards of the cameras, such an installation made it possible to print in a darkened room with the light from the window reflected from the diffuser. Some firms produced ready-made enlargers of a similar design ^[24] . In large photo studios installed horizontal projectors with a coal arc lamp . Vertical photographic enlargers with incandescent lamps , familiar to modern photographers, appeared only in the early 1930s, and Ernst Leitz began to produce such devices as an option for small-format cameras “ Leica ” ^[25] .

Projection printing in those years was still considered too difficult for amateur photographers who preferred “daytime” photo paper with visual exposure control in a simple copy frame. However, with the advent of compact photographic equipment and fine-grained roller photographic films , optical printing quickly displaced contact from most areas, except for technical photography. In the studio studio large - format shooting, followed by contact copying also remained in demand for several decades. This technology allowed us to preserve the retouching of the negative, which was considered an integral part of portrait photography ^[26] . In addition, the contact print is not as critical to the quality of the negative as the projection, which reveals the smallest defects and grain of the film, both due to the increase and due to the directed nature of the copy light. In Soviet photo studios, contact printing remained standard for a studio portrait until the late 1980s . For example, the most common portrait format 9 × 12 centimeters was filmed on the same negative cameras such as “ FC ” with the mandatory retouching. Projection printing was used for negatives, shot on location by more convenient cameras of medium and small formats. In photojournalism since the late 1930s , optical printing has almost completely supplanted contact print ^[16] .

Color Photo

The first color multilayer photo paper of the chromogenic type for printing from slides , released in 1941 , required a new color photo printing technology, replacing the pigment and hydrotype ^[27] . At the same time, besides the exposure control, the need arose for color correction of the imprint tone, which is particularly relevant in the negative-positive color process that appeared a year later ^[28] . One of the first to develop this technology was the German company Agfa , which put on the market sets of corrective light filters for subtractive printing ^[29] . Color mixers of magnifiers with adjustment of the spectral composition of the printing light with the help of moving light filters became further development. The first color heads were developed by the end of the 1940s . Three adjustment knobs of such heads were graduated in the same density units of yellow, magenta and cyan colors as the filters in Agfa sets, but infinitely variable ^[29] . In the USSR and Russia , the photographic enlargers “Krokus GFA 69” and “Don-7002Ts”, equipped with such heads, became famous ^[30] . The Meohrom head developed in Czechoslovakia by the enterprise Meopta had a similar device ^[31] . An alternative to the color heads of the enlargers were lenses with the same light filters, moving into the light flux near the plane of the aperture diaphragm ^[32] .

High demand for photo printing of color amateur photographs led to the emergence of automatic printers for mass printing on roll photo paper. Exposure adjustment and color correction in such printers were carried out with high accuracy using a photocell system and additional coding on photographic films. The first automatic photo printer for color printing “Eastman Kodak 1598 Printer”, released in the USA in 1942, performed color correction in a subtractive way ^[33] . The following model of 1946, the Eastman Kodak 1599, was already designed for additive printing with sequential exposure of three primary colors dichroic filters ^{[34] [35]} . This technology allowed us to minimize color separation errors due to inaccuracies in the spectral sensitization of zone-sensitive layers of photographic paper. However, subtractive printing is more widely used due to higher productivity. The improvement of color television made it possible to equip automatons with electronic video color correctors with a monitor , using which the operator can correct the color of the future print ^[36] . The image of the negative is displayed on the monitor of such a color corrector in a positive form, and the controls are adjusted in such a way that the normal color balance on the screen corresponds to an exact print correction. Gradually, complexes consisting of a printer and a developing machine for photographic paper appeared, called mini-photo labs ^[37] .

The development of digital photography has led to the rejection of direct optical printing from a negative. The latest generations of minilabs involve digitizing a negative with the help of an integrated film scanner , the subsequent processing of the received files in a graphic editor, and photo output by a laser beam. At the same time, printing is possible both from the negative, and from the slide , as well as from the customer’s finished files. The advantages of this method are the possibility of digital processing that can sharpen and mask the grain of the image ^[38] . Currently, mass printing of amateur photographs from a color negative is carried out only with this technology. The next step was the final rejection of photo printing in favor of inkjet printers, leaving a high-quality color print on plain paper. Compared with the previous technology, which requires laboratory processing of exposed images in expensive reagents, the cost of inkjet printing includes only replaceable cartridges with dyes or pigments . In addition, an inkjet printer is much easier to maintain than a mini-lab ^[39] . Most gallery owners, however, consider inkjet unsuitable for the manufacture of originals of artistic photography, preferring the classic photo processes ^[40] .

Printer Profession

In professional photography, photography and photography began to divide the negative-positive technology from the first years, thus increasing the productivity of large photo studios. The printing of pictures was most often entrusted to the students of the photographer or apprentices, as a less demanding operation that does not require creative skills. Later, the profession of a photolabrant appeared, whose duties besides printing included the preparation of photoreactive materials and the laboratory processing of intermediate copies of the image: negatives and counter-marks . In large news photo agencies it was considered generally accepted to entrust all laboratory processes, including photo printing, to photographic laboratories. Thus, the efficiency of reporters freed from technical operations increased. For example, in the Soviet “ TASS Photo Chronicles ”, one of the largest photo labs in Moscow was involved in the manifestation of negatives and printing ^[41] .

Many large photo masters had a weak understanding of photo printing technology, since they were made by specialists for them. For example, Henri Cartier-Bresson’s “father of the photo report” believed that printing was not decisive for the picture that was created at the time of the shooting ^{[42] [43]} . Photographers of this school considered framing in print to be unacceptable, as it distorts the author's vision. However, there was also an opposite point of view, according to which the main content of a photo is created during photo printing, especially if its special technologies are used. Such an approach became widespread among photographic artists, some of whom in the late 1970s became fascinated by the image transformation during printing and the varieties of photographs , including literary printing , pseudosolarization , isopolichromy, and photo collage ^[44] .

At present (2018), traditional manual photo printing by optical or contact method from a negative is practically not used in mass photography. In modern analog photography, the negative obtained on photographic film, in most cases, is not printed in the traditional way due to the high cost of the content and complexity of the equipment of the photographic laboratory. After digitizing, images, including black-and-white, are printed on inkjet printers or on color photo paper using a digital photo output, and most often are placed on social networks or photo hosting sites without a hard copy. The pictures made in the technique of silver photo printing , belong to the author's exclusive works, as well as those made using alternative processes . The disappearance of the market for halogen-silver photography and the drop in demand for professional hand-made photo printing leads to the extinction of the profession of a printer that requires high qualifications ^{[45] [46]} .

Printing can be performed on positive photographic materials of various types: photographic plates, photographic film and photographic paper ^[47] . In the first two cases, as a result of printing, transparencies are obtained that are suitable for viewing in transmitted light, that is, projections on the screen and design of light boxes . The most widely used printing on photographic paper, the images on which do not require a special backlight and are convenient for viewing in reflected light. There are manual and machine printing. With manual printing, cutting of photographic material, its exposure and development are made by a photolab technician without automatic adaptations. Laboratory processing of photographic paper in this case is carried out in cuvettes , less often in semi-automatic drum-type processors ^[48] .

In the case of machine printing, all operations are performed by a special printer automatically, and the exposed rolled positive photographic material (usually photographic paper) is fed to a developing machine, where it is chemically treated. In the USSR, produced automatic photo printer for black-and-white photo printing "UPF-1" ^{[49] [50]} . The Pentacon-201 complex, produced in the GDR and consisting of an automatic printer, a developing machine, and a cutter, also became widespread in large Soviet photolabs ^[51] . The design of the miniphotolaboratory allows for its installation in a non-darkened room, requiring only reloading of replaceable cassettes with photographic paper in complete darkness ^[52] . Since positive photographic materials can only be processed in the dark or under nonactinic lighting, manual photo printing requires a separate room, isolated from daylight. Laboratory lights with protective light filters of the corresponding colors are used to illuminate workplaces. Most varieties of black-and-white photographic paper are not subjected to spectral sensitization, and non-actinic for them is yellow-green light, obtained using a light filter No. 113 or No. 118 ^{[47] [53]} . However, the most widely used laboratory lights are red, safe also for orthochromatic photographic materials: positive photographic films, slide photoplates and photographic films . For color positive photographic materials, a dark green light filter No. 166 ^{[54] is used} .

Contact stamp

In automatic printers designed mainly for small-format negatives, contact printing is not used. A photographic plate or a sheet photographic film with a negative is superimposed by an emulsion layer on photographic paper, the back side of which is protected from exposure to light ^[55] . The light that has passed through the negative exhibits a photo emulsion with an intensity that is proportional to the transparency of the negative image. In the case of using photo paper with daytime manifestation after exposure, the imprint on which the finished positive is clearly visible is separated from the negative and immersed in fixer solution ^[13] .

For classic photographic paper or positive photographic film, the manifestation begins after the end of the exposure, which during contact printing is regulated only by exposure. For the "day" photographic paper produced copying frames, in which the sheet fit under the negative. On highly sensitive papers with the manifestation of the contact printing was carried out in special machines, such as the Soviet "KP-8M" or "KP-10", with a glass or film negative pressed by an inflatable rubber cushion ^[56] . In the semi-automatic contact device “PKP-1” the clamping was carried out with a cover with an elastic polyurethane foam pad ^[57] . In large photo agencies , contact prints were made for small-format and medium-format negatives, intended for the primary selection of material by bildredactors. In most cases, they could be printed under an enlarger, where film segments were laid on a sheet of photo paper, and then pressed against glass ^{[58] [59]} .

Projection Print

This type of printing can be performed by both automatic printers and photo enlargers . In the printer, the image of the negative is projected by the lens into the personnel window, in which during the exposure a section of the roll paper is still located. Magnification can be changed in steps by changing lenses with a fixed focal length or using a zoom lens , allowing you to print pictures of different sizes on interchangeable rolls of paper ^{[60] [61]} .

With manual printing, the magnification changes smoothly by moving the enlarger relative to the table. The necessary negative is manually inserted into the negative of the enlarger, which with the help of an incandescent lamp projects its enlarged (less often - reduced) image onto the table where the framing frame with photo paper is placed ^[62] . Exposure during optical printing is adjusted by the lens aperture and the duration of the lamp's glow. Sometimes the exposure is interrupted by turning the red light filter located under the lens and overlapping light actinic for photographic paper. In printers, shutter speed is controlled by the shutter ^[52] .

After exposure, the sheet is removed from the framing frame and its laboratory processing begins. The main difference of contact printing from the projection is the nature of the illumination of the negative. With contact printing, with rare exceptions, diffused light is used to mask the mechanical damage of the negative and its graininess. For the same reason, negatives intended for contact printing should have a higher contrast than those taken for optical copying ^[63] . Projection printing allows to obtain both enlarged and reduced images of a negative, as well as to perform an optical transformation, for example, to eliminate perspective distortions ^{[64] [65]} .

One of the most important advantages of manual optical printing is the ease of adjusting the exposure of individual parts of a picture using masks ^[66] . For example, a face too dense on a negative can be “printed” with the help of an additional shutter speed in the corresponding section through a hole in a sheet of black paper. On the contrary, an excessively deep shadow of a print can be lightened by temporarily blocking the light of the enlarger with a piece of paper on a wire ^[67] . In everyday practice, photographers often used hands as a mask, blocking off part of the light flux ^[68] . With machine printing, masking of certain sections of the negative is impossible ^{[* 1]} . The techniques of highlighting areas of the image with hands and a round mask are reproduced in graphic editors under the names “Dodge” and “Burn” with the corresponding symbols ^[70] .

Exposure Management

Photo printing on modern silver gelatin photo paper with a manifestation, unlike “daytime” with a visual control of the positive density , requires an exact preliminary determination of the correct exposure. In automatic printers for this, the optical density of the negatives is measured by the integrated densitometer ^[61] . With manual photo printing, test prints are more often made for this ^[71] . One sheet of photographic paper from the pack is cut into several parts, each of which is used for test printing. A piece of sheet is placed on the plot important part of the image and exhibited. Sometimes a graded sample is made, in which different areas of photographic paper are exposed at different shutter speeds ^[67] . Due to the small photographic breadth of photographic paper, test print is considered the most reliable way to determine exposure ^[72] .

After the end of the laboratory processing, the image is taken out to daylight and the necessary correction of the exposure is determined by its density or a portion of the stepped sample with normal density is selected. When color printing on multilayer photo paper or positive films, the accuracy of exposure should be even higher than in black and white, because the color balance of the image depends on it. Adding corrective light filters reduces the luminous flux, requiring additional correction of the shutter speed after each color test ^[73] . For instrumental exposure measurement, the sensitivity of conventional exposure meters for photographing is insufficient, therefore, special photometers based on photoresistors ^[74] are used to accurately measure the illuminance of photographic paper. To measure the exposure during color printing, color analyzers were produced that can determine not only the correct exposure, but also the color balance. In the USSR, the device under the name "Tsvetan" was the most popular, and devices like "Soligor Melico", "Minolta Color Analyzer", "Rodenstock" and "Macbeth" ^{[75] were} most popular abroad.

In amateur practice, the shutter speed was often counted empirically, measuring the time between turning on the lamp of the enlarger and turning it off. For higher accuracy, special electronic time relays were manufactured. In professional photographic enlargers (for example, “Azov”), a time relay is built into the frame ^[76] . Some photometers and color analyzers were equipped with a built-in time relay. The Ros electronic framing frame contained a movable photoresistor in the housing under the photo paper and the screen, automatically working out the exposure depending on the light intensity ^[77] . A similar device possessed an automatic framing frame "AKR", correcting the exposure depending on the density of the negative ^[78] .

Fingerprint Finishes

After the main stages of laboratory processing, black and white photographs were sometimes subjected to virirovaniyu to give any color shade. Currently, the same effect is achieved on color photo paper when printing with a deviation from neutral color balance, and on black-and-white photo paper, toning is considered undesirable because it reduces the durability of the print. This is followed by flushing, after which both black-and-white and color prints are dried or glossy to produce saturated shadows. In the photo amateur practice for rolling used knurling finished pictures on skim window glass. The process required pre-tanning of the emulsion or treatment in baking soda solution, while unprepared prints could be rolled onto the plexiglass ^[79] . The process was significantly accelerated with the use of electro -glossers with polished chrome-plated plates ^[80] . In professional photo labs, semi-automatic drum glossers were installed instead ^[81] . In the USSR, for this purpose, devices "АПСО-5М" (Apparatus for semi-automatic drying of prints) and "АПСО-7" were manufactured with high productivity ^{[82] [83]} . Photo papers on a polyethylene substrate (type “RC” according to international classification) are dried without heating in a suspended state. Dried prints are cut with guillotine or roller photo cutters ^[84] .

In addition to direct printing on photo paper, art photography used printing of one or several intermediate counter - photographs on a phototechnical film to produce artistic effects, such as isohelium , pseudosolarization , isopolychromy (“color isohelia”), and many others ^[44] . Intermediate counterplay could also be used as a mask, shading the desired part of the image or allowing you to change the nature of the image. The selection of the exposure mode and the display of the counterplay allowed, using the “unsharp mask” and “filtering the details by manifestation” technologies (FDP-method), to increase the detail of the image, as well as to imprint a large grain as a creative technique. Most of these technologies were used by photographers within the framework of the photography genre and for exhibition printing ^[85] .

Another popular technology to emphasize the effects and increase the sharpness of prints, by the mid -1980s was the use of a point light source without a lens instead of a diffuser instead of a conventional frosted glass lamp above a condenser ^[86] . As the latter, low-voltage automobile lamps with compact filament were most often used. This required an additional transformer and a modified printing technique, excluding the lens aperture ^[87] . The point source of light makes it possible to get prints with underlined detailing, but reveals all mechanical defects, therefore it has found application mainly in a black-and-white photo that allows for the retouching of positive ^[88] .

The spread of digital photography has pushed aside traditional photo printing technology. Among photo artists, alternative photo processes such as cyanotype , gum printing, and lit printing have become popular. These processes, besides the unusual nature of the image, have a low repeatability, which increases the value of each print that is unique. In most cases, the same equipment is used for printing as in the case of traditional technology: a photographic enlarger, a framing frame, etc. In addition to the classical technology on photo paper with a manifestation, processes on chrome-plated colloids are known, such as bromoil and pigment photo printing . Images obtained in this way are even more durable, but photographic materials in this case must be made independently. In addition, the cost of such processes is much higher than gelatin-silver, even taking into account the current prices for photographic paper: for example, a sheet of silver bromide paper “Ilfobrom” format 11 × 14 inches reaches a price of almost 2 dollars ^[89] .

Manual black-and-white photo printing does not require time-consuming laboratory processing of photo paper and complex color correction, and for these reasons it has become widespread not only in professional but also in amateur photography. In the latter, this type of printing was most prevalent in the USSR, where the network of minilabs was completely absent before Perestroika , widely branched in Western countries since the early 1950s. In professional photography, black-and-white printing was also the main one here, because until the early 1990s the proportion of color prints was significantly less than half. Currently, it is practically the only technology that exists in analog photography unchanged, because color prints are machine-made in miniphotolabs after digital color correction. The only exception is special color processes of the “ Sibahrom ” type, which require high-quality manual photo printing ^[90] .

Machine photo printing on classic silver-silver paper is not widespread, since the full recovery of silver is possible only from color photographic paper. Silver prints can be made manually by contact or projection methods. Their main advantage remains higher durability than copies on chromogenic photo paper or prints from inkjet printers. With careful fixation and washing of black and white photographs, their image, consisting of metallic silver , can be stored for more than a century even in the light. In the case of printing from a black and white negative by a printer, prints are made on color photo paper of a chromogenic type, the durability of which is inferior to gelatin-silver ^{[* 2]} . Therefore, images printed by hand on black-and-white photographic paper have a higher auction value and are appreciated by gallery owners. Currently, the release of classical photographic paper is discontinued by most manufacturers that support only the production of color photographic paper for automatic printers. The only company that still produces a large range of black and white photo paper is Ilford Photo ^[89] .

One of the main difficulties of black-and-white photo printing is the need to select photo paper for negative depending on the contrast of the latter ^[91] . Classic silver bromide photographic paper produced several gradations: "soft", "semi-soft", "normal", "contrast" and "special contrast" ^[92] . The listed names corresponded to numbers from 0 to 5, while the “normal” photographic paper was designated by the third number. Too much negative should be printed on soft or semi-soft photographic paper: otherwise the print will have too few semitones, and the light and shadow of the image will not be worked out. On the contrary, sluggish negatives require contrasting photographic papers, since on normal they turn out to be “gray” ^[93] .

The inevitable deviations of the contrast of negative films made photo laboratories have as many varieties of photographic paper as possible, which was not always available ^[94] . Thanks to the development and cheapening of the irrigation technology for multi-layer photoemulsions, so-called polycontrast photographic papers have become common, allowing the print contrast to be adjusted depending on the characteristics of the negative. Such photographic paper contains two or three black-and-white emulsion half-layers with different spectral sensitivity and contrast ratios ^[48] . Contrast is regulated by color filters of yellow and magenta colors. Installing one of these colors in front of a filter magnifier reduces exposure to contrasting or soft half-layers, giving an image of the desired gradation ^[95] .

Color photo printing is different from black and white complicated technology of laboratory processing of photo paper and the need for color correction. The latter is caused by the inevitable deviations of the spectral composition of the shooting light from that provided by the color balance of the negative film, as well as errors during its processing ^{[96] [97]} . By the way that color correction is performed, two types of color printing are distinguished: subtractive and additive. The subtractive method is technologically simpler, since it provides for a total exposure for all three layers of photo paper. For the same reason, photo printers built on the subtractive principle have higher productivity and are more common ^[98] .

Subtractive print

Color correction when printing by the subtractive method occurs by filtering the light that illuminates the negative. To do this, the absorption light filters of complementary colors — yellow, magenta, or blue — are added to the luminous flux of the lamp ^[99] . The light filter of each of these colors directly affects the exposure obtained by the corresponding zone-sensitive layer of photographic paper. For example, a yellow light filter subtracts the blue component from the white light of the lamp, reducing the exposure obtained by the blue-sensitive layer. As a result, with its manifestation, the yield of yellow dye in this layer will decrease, eliminating undesirable color ^{[* 3]} . Two other filters act in a similar way, reducing the dye yield of its own colors ^[100] . With machine printing, the color and density of the light filters are determined by the built-in densitometer, and with manual printing by means of test prints (strip tests) or a color analyzer ^[101] .

After determining the exposure and the end of the laboratory processing of the sample, it is brought to daylight, where the prevailing color tone is visually assessed ^[102] . In accordance with this selected filters of the same color to print the finished image. The greater the color deviation, the more dense the light filter should be used with a corresponding increase in exposure ^[73] . Light filters of only one or two colors can be used at the same time: the third color combined with the other two gives an additional gray density that does not affect the tone of the print, but increases shutter speed ^[103] .

On the contrary, the predominance of any color can be compensated by reducing the density of the light filter to an additional shade. For example, an undesirable green color can be corrected by adding yellow and blue, and removing a purple filter with an increase in the exposure time in the first, and its decrease in the second case. Manual color correction can be accelerated with the help of mosaic filters and step proofing ^{[104] [97]} . Corrective light filters are usually placed in a sliding tray between the lamp and the condenser provided in most photo enlargers. More advanced enlargers, designed specifically for subtractive printing, are equipped with a special color-mixing head, operating according to the same principle ^[102] . Retractable head filters continuously adjust the color balance of the copy light ^[30] . The most modern models of color heads instead of gelatin are equipped with interference filters of complementary colors with improved filtration quality and increased durability ^[105] .

Similarly, special lenses with an integrated color mixer act ^[102] . Near the aperture of such a lens there are two movable light filters with transparent and colored zones, moved with the help of special handles. Compared to color mixing heads, such lenses have a higher durability of light filters, which are much less susceptible to heat exposure from a light source. However, placing the filters in the lens reduces the sharpness of the resulting image. In the USSR and the CMEA countries , the Yanpol-Color lens, produced in Poland in the 1970s and 1980s, was best known ^[106] . The domestic analogue of "Vega-22UC" had a similar principle of operation ^[107] . With certain limitations, color mixing heads and lenses can also be used for black and white printing on multi-contrast photo paper ^[108] .

Additive printing

The additive color printing method is based on the separate exposure of positive photographic material through three interference light filters of primary colors with a very narrow spectral transmittance range ^[109] . The selectivity of such filters is much higher than that of each of the zone-sensitive layers of positive photographic material, which virtually eliminates color separation errors, which are unavoidable due to the imperfection of the emulsion sensitization. The color balance of the image is determined by the ratio of the exposure for different light filters. Exposure can occur both sequentially and simultaneously. In the latter case, which is characteristic of automatic printers, three independent optical paths are required, the images of which are combined with the help of translucent prisms and mirrors ^[110] .

With manual printing, sequential exposure occurs through the light filters attached to the turret under the photographic enlarger ^[111] . To prevent the magnifier from shifting in the interval between exposures, sometimes a device is used to automatically change light filters. The exposure behind each of the zonal light filters can be adjusted both by the duration of exposure and the brightness of the lamp. The latter can be changed by a rheostat without affecting the print color, as the spectral composition of the radiation exposing each of the layers depends only on the transmission zones of the filters, and not on the degree of heat of the lamp. In the photo amateur and small photo studios, the additive method has not been applied because of the low productivity and considerable cost of the equipment, but it has been widely used in automated mini - photo laboratories equipped with automatic color analyzers ^[112] .

• Positive photo process
• Kinokopirovalny machine
• Certification seal
• Printing

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•   The process of manual photo printing