The camera obscura was an optical device used in drawing, and one of the ancestral invented objects leading to the invention of photography. In English, today’s photographic devices are still known as “cameras”.
The principle of the camera obscura is just a box (which may be room-size) with a hole without a glass lens in one side. Light from only one part of a scene will pass through the hole and strike a specific part of the back wall. The projection is made on paper on which an artist can then copy the image. The advantage of this technique is that the perspective is accurate, thus greatly increasing the realism of the image.
With this simple do-it-yourself apparatus, the image is always upside-down. By using mirrors, as in the 18th century version, it is also possible to project an up-side-up image. Another more portable type, is a box with an mirror projecting onto tracing paper placed on the glass top, the image upright as viewed from the back.
As the hole is made smaller, the image gets sharper, but the light-sensitivity decreases. With too small a hole the sharpness again becomes worse due to diffraction. Practical camerae obscurae use a lens rather than a hole without a glass lens because it allows a larger aperture, giving a usable brightness while maintaining focus.