Giovanni Battista Benedetti, a patrician of Venice, 1530–90, published at Turin a book called Diversarum Speculationum Mathematicarum et Physicarum Liber, “A Book of Various Mathematical and Physical Speculations”, in which was included the first complete and clear description of the camera obscura equipped with a lens. The date of the volume was 1585, four years before Porta published his revised edition.
Benedetti used a double convex lens. His first knowledge of optics came from a study of Archimedes, whom he admired greatly. But his learning was not confined to optics. He influenced the great Descartes in geostatics, studying the laws of inertia and making the contribution of the path taken by a body going off from a revolving circle, i.e., tangent. In 1553 he reported that bodies in a vacuum fall with the same velocity.
Benedetti’s description of the camera obscura included details on how to make the images appear upright. The material is contained in a printed letter to Pierro de Arzonis. First Benedetti discusses light and the fact that a greater light overshadows a smaller, “just as by day the stars cannot be seen.” He then pointed out that if the light were controlled in a camera the outside images could be seen, but if the rays of the sun were allowed to enter (as by making the opening hole too large) then the images would “more or less vanish according to the strength or weakness of the solar rays.”
Benedetti continued:
I do not wish to keep any remarkable effect of this system a secret from you ... the round opening the size of one small mirror may be filled in with one of those spectacles which are made for old people (but not the kind for those of short sight), but one whose both surfaces are convex, not concave. Then set up a white sheet of paper (as the screen), so far back from the opening that the objects on the outside may appear on it. And if indeed these outside objects are illuminated by the sun they will be seen so clearly and distinctly that nothing will seem to be more beautiful or more delightful. The only objection is that the objects will appear inverted. But if we wish to see those objects upright, this can be done best by interposing another plane mirror.
In the revised and expanded edition of his Natural Magic, Porta gave a more complete description of the uses of the camera. Part of the text was identical with the earlier accounts; part was new.
Ars Magna Lucis et Umbrae, 1646
CAMERA OBSCURA, the natural room camera, was accidentally discovered in antiquity, probably in the Far East. Here is shown an improved version by Giovanni Battista della Porta, 16th Century Neopolitan writer, scientist and showman. A translucent sheet was the screen. The images were upside down and indistinct as no lenses were used. Artists and entertainers found the apparatus of value.
Porta had learned how to make his opening in the single window better by this time—“make the opening a palm’s size in width and breadth and glue over this a sheet of lead or bronze which has in the middle an opening about the size of a finger.” He next pointed out that the outside objects can be seen clearer and sharper if a crystalline lens is put in the opening of the camera as suggested by Barbaro and Benedetti. Porta also mentioned that the insertion of another mirror in the system would make the images appear upright instead of upside down.