Zahn's reflex camera obscura
1685 (Oculus artificialis teledioptricus)
Johann Zahn described small, portable box camera obscuras with a lens, a 45° mirror and a ground-glass screen on top: the design of the reflex camera, 150 years before photography.
How it began
Camera obscuras had been rooms or tents. The German monk Johann Zahn published designs in 1685 for boxes small enough to carry, with a lens at the front, a mirror inside and a screen on top.
How it works
The lens forms an image on the mirror, set at 45°, which reflects it up onto a sheet of ground glass or oiled paper. An artist could look down at the screen, right way up, and trace over it. Zahn also described ways to focus, and lenses of different strengths.
Why it mattered
It was the camera in all but the plate. Box camera obscuras of this kind were used by artists through the eighteenth century, and Niépce and Daguerre adapted similar boxes. The mirror-and-screen layout returned in the reflex cameras of the twentieth century, such as the Kine Exakta and the Nikon F.
Limits and what replaced it
It could only show an image, not keep one. When photography came, cameras briefly dropped the mirror for simplicity, until faster film made reflex viewing worthwhile again.
Today
Every single-lens reflex, from the Exakta to the last digital SLRs, used the same mirror-and-screen idea.