Leonardo da Vinci studied the camera obscura, an optical device that projects an outside scene into a dark chamber.
Light entering through a small opening forms an image of the scene on the opposite wall. Leonardo recorded detailed observations of this effect and used it to investigate vision, light, perspective, and the operation of the human eye. The projected image is upside down because rays from the top and bottom of the scene cross at the opening.
Leonardo did not invent the underlying phenomenon. Descriptions of camera obscura effects existed in earlier optical writing, including work associated with Ibn al-Haytham. Leonardo’s importance lies in his careful Renaissance analysis and his connection of the device to studies of human sight and artistic representation.
A camera obscura is not a camera in the modern photographic sense. It creates a temporary optical image but has no film or electronic sensor unless later technology is added.