Camera obscura

A camera obscura (plural: camerae obscurae or camera obscuras) is a natural phenomenon in which light rays entering a dark space through a small hole form an image on a surface. This results in the external view being projected upside down (inverted vertically) and mirrored (inverted horizontally). The term also refers to a dark room, box, or similar structure that projects external images inside or onto a translucent screen visible from the outside. Camera obscuras with lenses have been used since the late 16th century and gained popularity as aids for drawing and painting. This technology evolved into the photographic camera in the early 19th century, with camera obscura boxes used to expose light-sensitive materials to the projected images.

Images produced by camera obscuras without lenses are also known as "pinhole images." Camera obscuras have been used for safely observing solar eclipses without looking directly at the sun. As a drawing tool, they allowed for extremely accurate representations by tracing the projected image, making it a particularly valued method for easily achieving proper graphic perspective. Before the term "camera obscura" was first used in 1604, other terms such as cubiculum obscurum, cubiculum tenebricosum, conclave obscurum, and locus obscurus were used to refer to this device. A camera obscura with a small hole and no lens is often referred to as a pinhole camera, although this term generally refers to simple (homemade) cameras that use photographic film or paper.

Light rays travel in straight lines and change as they are reflected and absorbed, preserving information about the object's color and brightness. Illuminated objects reflect light rays in all directions. If there is a sufficiently small hole in a barrier, the rays entering through that hole allow only the light coming from different points of the opposite scene, forming the image of that scene on the surface opposite the hole. The human eye (and many other animals' eyes) operates similarly to a camera obscura. Light rays enter through the pupil (the hole) and are focused through a convex lens, forming an inverted image on a smooth surface (the retina) as they pass through a dark space. This analogy emerged in the early 16th century and was commonly used in the 17th century to explain the Western theological idea that God created the universe like a machine. It significantly influenced behavioral science, particularly research on perception and cognition. In this context, the projection of the inverted image is noteworthy as a physical optical principle that predates the emergence of living beings. This is not a characteristic of all biological vision.

A camera obscura consists of a box, tent, or room with a small hole on one side or the top. When light from an external scene passes through the hole and strikes the internal surface, the scene is recreated in an inverted (vertically inverted) and mirrored form, but the color and perspective are preserved.

To generate a suitably sharp projected image, the aperture must be smaller than 1/100th of the distance to the screen. As the pinhole gets smaller, the image becomes sharper, but brightness decreases. At very small pinholes, sharpness deteriorates due to diffraction. Optimal sharpness is achieved at an aperture diameter roughly corresponding to the geometric mean of the wavelength of light and the distance to the screen.

In practice, camera obscuras often use lenses instead of pinholes, allowing for larger apertures while maintaining brightness and the ability to focus.

Capturing the projected image on a translucent screen allows it to be viewed from behind, resolving the horizontal inversion, but the vertical inversion remains. Mirrors can also be used to project the image in the correct orientation. The projected image can also be displayed on a horizontal surface (e.g., a table). In an overhead version of a tent in the 18th century, a type of periscope with a mirror was used at the top of the tent.

Box-shaped camera obscuras often use angled mirrors to project the correctly oriented image onto tracing paper. Although the image is viewed from behind, it is mirrored horizontally.