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Technical Documents - Documentos Técnicos: Minilabs. Making a photograph from camera to finished print
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If you are the owner or an operator of a minilab (color photograph film processor and paper processor ) , here you will find the basics about how it works . To make good quality prints in your minilab, you need to know something about how photography works. You will learn more about how to apply this knowledge as you gain experience with the equipment used in your own minilab. Making a photograph from camera to finished print - How is a photograph made ? Exposing the Film (Taking the Picture) When film (light sensitive material) is exposed in a camera (a lighttight box), an invisible (latent) image is formed on the film. Let's assume you're looking at a particular subject, such as a fast-moving sports scene. To lend permanence to that fleeting impression before your eyes, you press the shutter release of your camera. For a fraction of a second light falls into the camera, producing a small, weak image at the back of it. This image still isn't a photograph. In fact, it's totally inaccessible, because the only reason why it exists is that the camera is dark inside. As soon as the release is shut again, the image is irretrievably lost. To capture this fleeting image and to make it accessible to the viewer, it has to be recorded on a light-sensitive medium. An image consists of a large number of details of colour, brightness, etc, and the most efficient storage medium for optical information is a layer of photographic emulsion - a method which is still far superior even to the most modern electromagnetic storage medium. This extremely thin coat consists mainly of gelatine with evenly distributed light-sensitive compound salts of silver, so-called silver halide. (At this point we are still only concerned with B/ W emulsion. Later, we shall concentrate on other photographic materials as well as their production and basic structure). The B/ W emulsion then stores the image-forming light beams (i. e. the photographic information), depending on the different brightness values: if a portion of a picture is brighter - e. g. snow - then more silver halide crystals are exposed than if a portion is darker. Fast-moving scenes - such as our sports scene - or photographs at dawn or dusk require a particularly "sensitive" emulsion. Let's press the shutter release again, this time capturing the scene with the help of complex physical and chemical processes; on the silver halides in the film emulsion the exposure produces nuclei of metallic silver which are at first invisible to the eye - a faint "latent" image is created. So this is our first important chapter in the production of a photograph: once exposure has taken place, the picture is stored as a latent image in the emulsion. This is followed by two further steps which are equally important: 1. Making the latent image visible (development). 2. Stabilizing the developed image (fixation). This therefore answers the question put earlier: "How is a photograph created?" Before we go on to deal with some of the basic principles of photo-chemistry, let's ask it in a slightly different way: How can a photograph be created in the simplest possible way ? And here is the answer. In order to produce a photograph it is not absolutely necessary to have a complicated camera. A small lightproof box with a tiny hole in it which lets the light fall on a piece of film or photo-paper inside the box is all you need. A simple camera of this type is called a camera obscura or pinhole camera. Many photographs can be taken with such a simple cardboard device. The picture taker now has an exposed film, with a latent image present that represents the original scene. It is this film that the picture taker brings to your minilab.
Processing the Film (Making a Negative) . How does the latent image become visible and stable ? To become visible to the eye, the latent image has to be amplified thousands of times (i. e. developed) and then stabilized (i. e. fixed), so that a usable photograph can be obtained. The developer amplifies the latent image with the help of chemicals and in complete darkness. It "reduces" the silver halide (which is still invisible) to black metallic grains of silver, thus forming the visible structure of the black-and-white picture. (At this stage the photograph is a negative - more on this later.) However, even this metallic silver image is not stable enough. If light got onto the picture, then the unexposed silver halide (which is still there) would get exposed and then reduced (i. e. blackened) by the remaining developing agents that are still present in the emulsion.
Figure : Formation of the black-and-white negative: Light details of the original correspond with blackened details in the developed and fixed emulsion layer. To prevent this from happening, the "original" metallic silver image (still in complete darkness) is fixed in a special solution, known as a fixer. There comes a point when the development of the negative film is finished, i. e. when the important details of the photographic image have reached a sufficient degree of "blackness" (also called "density"). The unexposed - and therefore undeveloped - silver halide is converted by the fixer to a non-light-sensitive complex salt and rendered soluble (i. e. washable). The result is a fixed black-and-white negative which can now be viewed in broad daylight. The final wash then gets rid of any residual chemicals that might upset the picture. Once dried, it will serve as the starting point for producing a positive print. The invisible (latent) image is made visible and permanent by processing the exposed film in a sequence of chemical solutions. After film processing, you now have a negative strip. The visible image in this negative looks different from the original scene. The part of the image that was dark in the original scene is now light and the part that was light is now dark. The negative does not show the colors as they appeared in the original scene. Why is the developed picture a negative ? When light hits the emulsion, a latent image is formed which is, in a way, an accurate copy of the subject. You'll notice this if, say, you take a black-and-white photo of a chessboard. After processing (i. e. development and fixing), the film will show an exact copy of the chessboard, though with the fundamental difference that the white squares are now black and black ones white (or transparent, to be precise). Let's just go back to the stage before the development, when, after exposure, a latent image of the chessboard was formed on the emulsion: all the black squares were missing in this image, because no exposure took place. All the white squares, on the other hand, led to a large amount of silver halide being exposed, depending on the intensity of the light. The subsequent development was then a matter of blackening the exposed silver halide, so that the highlights of the subject came out as black (i. e. consisting of metallic silver), once the emulsion had been developed and fixed. Conversely, no blackening is caused on unexposed areas (i. e. black squares on the chessboard). Once developed and fixed, these portions are therefore transparent on a black-and-white negative. How does a negative produce a positive image on photographic paper ? So far we have only described the production of a negative image. However, this is not the final picture. The next stage involves contact-printing it onto black-and-white negative paper or - if a larger format is required - enlarging it. The copying light then produces a copy of the negative on photographic paper. However, the black (= originally white) squares of the chessboard absorb the copying light, so that no exposure - and therefore no latent image! - is formed on the photographic paper. Once the paper has been developed, these undeveloped squares show up as white again. Conversely, the transparent (= originally black) squares let through the copying light, and this light then produces a latent image on the emulsion of the photographic paper which is subsequently blackened at the paper development stage. By producing a positive via a negative we have come full circle, and the print is now a faithful rendering of the black and white chessboard. So far we have only concentrated on two specific densities - black and white. To give a correct rendering of such a small "tonal range", we only need a photographic paper with what is known as "steep gradation". The term means that this paper is suitable for rendering black and white but not for the more subtle intermediate shades of grey (i. e. the various densities or grey tones between black and white). However, the majority of photographic subjects have a large number of such shades, which need to be rendered on the photographic paper in the form of subtly differentiated grey values. To ensure that different subjects have different tonal ranges, there are B/ W papers with different contrast grades.
How does the colour get into the picture ? As with black-and-white, let's start with the negative film. For B/ W negative films we only needed one light-sensitive emulsion to produce a metallic silver image. Colour negative films, on the other hand, need three layers, one above the other. All three emulsions have one thing in common - a B/ W negative which consists of metallic silver and is formed in each layer by means of exposure and the subsequent development. However, this is where the parallel with B/ W ends, because each of these light-sensitive layers has an additional quality: it is also receptive to "coloured" light with a certain spectral composition: Each of the three layers has been made sensitive (i. e. "sensitized") to either blue, green or red light in a certain chemical process.
Figure : Formation of the complementary coloured negative .
Colour film developed into a film processor , following the four chemical steps , developer , bleach , fixer and stabilizer ending with the dry step using hot air flow . >> Next >> 1 - 2 - 3 |
