Camera obscura

A camera obscura is a dark room with a hole in the wall that is used as a metaphor for human perception and for the creation of images. If the dark room is the size of a box, one speaks of a pinhole camera .

Camera obscura (dark chamber), also referred to as pinhole image, is the natural optical phenomenon that occurs when an image of a scene at the other side of a screen (or, for instance, a wall) is projected through a small hole in that screen as a reversed and inverted image (left to right and upside down) on a surface opposite to the opening. The surroundings of the projected image have to be relatively dark for the image to be clear, so many historical camera obscura experiments were performed in dark rooms.

The term “camera obscura” also refers to constructions or devices that make use of the principle within a box, tent, or room. Camera obscuras with a lens in the opening have been used since the second half of the 16th century and became popular as an aid for drawing and painting. The camera obscura box was developed further into the photographic camera in the first half of the 19th century when camera obscura boxes were used to expose light-sensitive materials to the projected image.

While the technical principles of the pinhole camera were already known in antiquity, the use of the technical concept for the production of images with a linear perspective in paintings, drawings, maps, architectural implementations and later also photographs was only in the (or, cf. Erwin Panofsky , the) Renaissance (s) of European art and the scientific revolution of modern times applied. Among other things, Leonardo da Vinci used the camera obscura as an image of the eye, René Descartes for the interaction between eye and consciousness and John Locke began to use the principle as a metaphor of human consciousness itself. This modern use of camera obscura as an “epistemic machine” had important implications for the development of scientific thinking.

The camera obscura was used as a means to study eclipses without the risk of damaging the eyes by looking into the sun directly. As a drawing aid, the camera obscura allowed tracing the projected image to produce a highly accurate representation, especially appreciated as an easy way to achieve a proper graphical perspective.

Before the term “camera obscura” was first used in 1604, many others are attested: “cubiculum obscurum”, “cubiculum tenebricosum”, “conclave obscurum” and “locus obscurus”.

A camera obscura device without a lens but with a very small hole is sometimes referred to as a “pinhole camera”, although this more often refers to simple (home-made) lens-less cameras in which photographic film or photographic paper is used.

Physical explanation
Rays of light travel in straight lines and change when they are reflected and partly absorbed by an object, retaining information about the color and brightness of the surface of that object. Lit objects reflect rays of light in all directions. A small enough opening in a screen only lets through rays that travel directly from different points in the scene on the other side, and these rays form an image of that scene when they are collected on a surface opposite from the opening.

The human eye (as well as those of other animals including birds, fish, reptiles etc.) works much like a camera obscura with an opening (pupil), a biconvex lens and a surface where the image is formed (retina).

Principle
As light is reflected by objects in all directions according to their own qualities of absorption, reflection, diffusion, each point on the surface of a screen receives light rays from all the surrounding objects; these rays mix and combine ( additive synthesis ). The screen appears white (or a major shade lighting up the decor).

By restricting the external light so that its light rays, emanating from the decor, enter only by a single point in a dark room , the screen intercepting this light will receive, in each precise point of its surface, only the rays derived, in a straight line (principles of geometric optics) from a single point of the decoration placed opposite the wall with the hole. We will see the image inverted (left / right) and inverted (top / bottom) of the decor, outside the dark room, appear on the screen.

The projected image is real since it is received on a screen (whether the observer’s eye is present or not): the instrument is said to be “objective”.

The principle is described from Aristotle and is repeatedly mentioned by the authors of the 13th century ( Roger Bacon , William of Saint-Cloud ).

Technology
A camera obscura device consists of a box, tent, or room with a small hole in one side. Light from an external scene passes through the hole and strikes a surface inside, where the scene is reproduced, inverted, (thus upside-down) and reversed (left to right), but with color and perspective preserved.

In order to produce a reasonably clear projected image, the aperture has to be about 1/100th the distance to the screen, or less.

As the pinhole is made smaller, the image gets sharper, but the projected image becomes dimmer. With too small a pinhole, however, the sharpness worsens, due to diffraction.

In practice, camera obscuras use a lens rather than a pinhole (as in a pinhole camera) because it allows a larger aperture, giving a usable brightness while maintaining focus.

If the image is caught on a semi-transparent screen, it can be viewed from the back so that it is no longer reversed (but still upside-down).

Using mirrors it is possible to project a right-side-up image. The projection can also be diverted onto a horizontal surface (e.g., a table). The 18th-century overhead version in tents used mirrors inside a kind of periscope on the top of the tent.

The box-type camera obscura often has an angled mirror projecting an upright image onto tracing paper placed on the glass top. Although the image is viewed from the back, it is now reversed by the mirror.

Structure
A camera obscura consists of a light-tight box or room in which the light of an illuminated scene strikes the opposite rear wall through a narrow hole. On the back wall there is an upside-down and inverted image of this scene. The picture is faint and can only be seen well if it is darkened sufficiently. If the back wall is transparent, you can also view the image from the outside if you provide adequate darkening, for example by using an opaque cloth that covers the back of the back wall and the viewer’s head.

How it works
If light falls through a converging lens or a small hole in an otherwise light-tight hollow body, it creates an inverted and upside-down image , a projection of the outside space. The schematic drawing on the top right shows two beams that enter the hole from two points of an object. The small diameter of the bezellimits the bundles to a small opening angle and prevents the light beams from completely overlapping. Rays from the upper area of an object fall on the lower edge of the projection surface, rays from the lower area are directed upwards. Each point of the object is shown as a disc on the projection surface. The overlay of the slice images creates a distortion-free image. Expressed mathematically, the image is the result of a convolution from an ideal representation of the object with the aperture area.

Imaging geometry of a converging lens
Denotes G the object height (= actual size of the object under consideration), g the object width (= distance of the object from the lens), b the image width (= distance from the perforated disc to the focusing screen) and B the image height (= height of the image created on the Focusing screen), the following applies:

B/G = b/g

Equation is also known from geometric optics as the 1st lens equation. For the mathematical derivation, reference is made to the ray set in geometry. The image size depends only on the distances, but not on the aperture size or hole size.

Use
It was formerly used as an aid for drawing. The image, projected on paper or other support, could serve as a guideline for drawing on it. Subsequently, when the photosensitive materials were discovered, the dark camera became a pinhole camera (the one that uses a simple hole as a target).

These cameras were very limited by the necessary commitment to establish the diameter of the opening: sufficiently small so that the image had an acceptable definition; large enough so that the exposure time was not too long.

The use of the dark camera was a great impulse to devise the way to produce permanent and automatic images. It can be considered as what provided the basis of what we know today as photography.

Dark Chamber and Alchemy
The dark chamber, although it was created in response to the needs of painters and scientists, in ancient times was known as a “magic box” and was closely related to a fantastic animal: the unicorn. Different writings and sketches have been found that describe the dark chamber, but the hole and the effect of producing images that characterize it could only occur if the “box” was pierced with the horn of the unicorn.

Since the fourth century, magicians and alchemists investigate phenomena related to light and images. Fata Morgana , sorceress of the court and sister of Arturo, jealous of Merlin’s prestige managed to steal the magician’s secrets to try to use them, among them, the following writing was found: «(…) The eye of the magic box will have to be pierced with a unicorn horn; otherwise, it will be completely ineffective. (…) » This belief lasted until the eleventh century and it was believed that unicorns were extinguished by the use that was given to their horns in order to use them as described above. With Merlin the first reference to this animal appears and its participation in the “art of learning images”.

Tzung Ching Pung, alchemist of the sixth century, made this other reference: «(…) To get beautiful and delicate reproductions, both of forests and lakes, as well as anything else in general, it is necessary to have the horn of the unicorn of Ycung – * Kuo (…) ».

Abdel-el-Kamir does not describe the dark chamber as his contemporary Merlin. However, it gives a recipe on how to prepare a light sensitive emulsion; This is the photographic film. It is until the eleventh century, with the alchemist Adojuhr, that the dark camera (magic camera according to him) was used for the first time with an extraordinarily sensitive emulsion, which allowed him to print moving images with lens.

The allusions to the unicorn of Merlin and Tzung Ching Pung are vague; in the one of Adojuhr, the opposite happens, since it makes a detailed and detailed description of this animal. In addition, it also points out the usefulness of the horn of the different species for drilling the “target” of the magic boxes. A transcript of Adojuhr is: «(…) A unicorn horn is taken, sharpened by the tip and with it a small hole is made on any glowing surface. Through this hole they will be able to pass, compressing their essence, all kinds of people, objects and places, the same that will have to be carefully stored in a cardboard box where they will remain for eternity, to be taken out when someone needs them (…) ».

Another of the functions that were also given to this magic box is to “capture evil spirits” and find a way to exterminate them, in the alchemist’s representations the previous one is more clearly seen. It was believed that there were different species of unicorns and each one was used in a different way in the magic boxes.

The dark chamber as a show
Some dark cameras were built as tourist attractions, but less and less of these are preserved. Some examples can be found in Grahamstown ( South Africa ), in the Tavira Tower of Cádiz ( Spain ) and Dumfries and Edinburgh ( Scotland ).