... when its a camera obscura !
As part of New Mills Festival a 16ft Yurt was turned into a camera obscura for the day. Sited in High Lea Park the Yurt was made 'light tight' (as dark as possible) with the 'pin hole' pointing towards The Hall to project an amazing image inside the Yurt.
The Hall was about 60 metres away from the Yurt and the image projected although upside down was very clear.
The camera obscura (Lat. dark chamber) was an optical device used in drawing, and one of the ancestral threads leading to the invention of photography. In English, today's photographic devices are still known as "cameras".
The principle of the camera obscura can be demonstrated with a rudimentary type, just a box (which may be room-sized, or even hangar sized) with a hole in one side, (see pinhole camera for construction details). 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 overhead version (illustrated in the Discovery and Origins section below), it is also possible to project a right-side-up image. Another more portable type, is a box with an angled mirror projecting onto tracing paper placed on the glass top, the image upright as viewed from the back.
As a pinhole is made smaller, the image gets sharper, but the light-sensitivity decreases. With too small a pinhole the sharpness again becomes worse due to diffraction. Practical camerae obscurae use a lens rather than a pinhole because it allows a larger aperture, giving a usable brightness while maintaining focus.