The principle of dark room or (b), certainly already known to the Athenian philosophers of the classical age, had a great development in the sixteenth century when artists and scientists put it into practice to faithfully reproduce reality. Shortly after the introduction of a lens, which consisted of convex spectacle lenses, he allowed a clear and sharp image to be projected on the focusing screen, facilitating the tracing of landscapes, buildings and still lifes.
The camera became a reality only possible in the nineteenth century with the discovery of compounds capable of reacting to light and said photosensitive. The photosensitive emulsion was spread on glass plates to be inserted in the camera, instead of the ground glass of focus, and exposed for the time necessary to the environmental conditions and then developed. If the control of the shutter speed required the construction of mechanical shutters, that of the brightness of the image made it essential to use the diaphragm.
Thanks to the fortunate marketing operation of the American George Eastman, founder of Kodak, photography abandoned the role of pastime for the elite and was transformed into the prince device able to capture the reality seen by the bourgeoisie. Eastman, in fact, knew how to combine the various technical elements, which were then available, with the psychology of potential customers, making sure that the demands and expectations of an increasingly mass audience were all met. In this regard, in 1886, he was able to produce a camera in which a spool of photosensitive jelly film was mounted on the paper, the advertising slogan was: "You press the button, we do the rest". Although instead of pressing the button it was necessary to pull a rope to load the shutter of the N ° 1 Kodak Camera, since then the experiments have not stopped and even today we try to miniaturize the components in order to ergonomically and improve the quality of the device .
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What does a camera do? Capture the subjects of reality. But how should the subjects be to be captured? They must have consistency and be illuminated. If you can not do much for the consistency of a subject, for its lighting we can use a light source, be it the sun, a lamp or a candle. Let us remember that "to photograph" means "to write with light", so its presence is essential in every shot, even if only a few light rays will pass through the lens to form the image.
Although there are several types of cameras, as we examine in detail in our articles, in this discussion we will take as an example the shot with a reflex, which in addition to using the same components of the other devices offers an excellent synthesis of the photographic process. Having identified the subject to be captured and chosen the position from which to take it up, subjective parameters that our discussion can not discuss here, we pay attention to the expository reading of the scene. The exposure meters consist of a silicon photocell built into the top or side of the lens and facing the subject. A portion of light beams, insignificant to the success of the shot, is diverted towards the exposure meter and here, by calculating the parameters set in the camera, offers a precise reading in the viewfinder. In SLR cameras, precisely because they are equipped with pentaprism, everything that is framed through the viewfinder, then taken by the lens, is faithfully reproduced in the shot: incomparable value that determines the absolute absence of any parallax error.
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The lens, in its simplest cylindrical shape, looks like a shiny glass disk thinner at the edges than in the center, with a convex shape. The balance between concave and convex lenses allows him to collect, converging, the light rays reflected by the subject, which will give a clear and sharp image during development only if all the components were focused during the shot. The use of composite lenses allows obtaining ever more perfect images. We define the focal length as the distance from the optical center, then from the lens, to the focal point, or where the image is formed on the film.
At this point the beam of light, which had not encountered any obstacles in the entire path through the lens, is filtered by the diaphragm a structural element of the camera that works exactly like the iris of the human eye: when there is little light open, when there is too much it closes leaving little to pass. By adjusting the aperture diameter, we adjust the flow of incoming light that will tickle the photosensitivity of the film or sensor, affecting image sharpness and depth of field. Most of the diaphragms are made of metal lamellae: by rotating a control ring, the diameter of the opening is precisely adjusted, ie the lens is closed, ensuring the light input recommended by the exposure meter. The control loop is calibrated in f / values, each of which, in succession, halves or doubles the brightness of the image.
The light, which up to this point has followed a horizontal direction, is vertically deflected by a 45 ° inclined mirror that introduces it into the pentaprism, a deceptive shaped crystal block that reflects the image captured on all its surfaces so that the image, overturned by the frosted glass, can be seen straight in the viewfinder. Pressing the shutter button causes the deflecting mirror to overturn and closes the diaphragm to the preset value. At this point, our beam of light, no longer being diverted, reaches the back of the camera leaving the viewfinder blind, a disconcerting disappearance of the image that many photographers include among the disadvantages of these types of machines.
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After crossing the entire length of the lens without obstacles, the light finally ends up meeting the large door that hides the photosensitive support: the shutter. This device opens for a variable time pre-set by the photographer according to the exact moment in which to take the picture. In the course of the history of photography we encounter two types: thin metal lamellae, inserted between the diaphragm and the lens, and a curtain, which allows the interchangeability of the objectives and allows the use of the reflex system for the image of the image traced by the objective at full aperture of the diaphragm.
Although it may change the media on which the photographs are saved, analog or digital, the physical and chemical process that leads to photo capture is the same and constant for any camera model. He is the photographer who, through experience and critical judgment, must combine all the aspects that revolve around the photograph to obtain the shots that best satisfy his aesthetic standards.
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