Simple viewfinder cameras have several drawbacks:
- Although the report viewfinder corresponded to the focal length of the optics, the image seen through the viewfinder does not exactly correspond to that given by this. This "parallax error" was particularly disturbing for close-ups.
- As the viewfinder image is not that of the lens, photographers how disappointed they not have discovered on their pictures placed a finger over the optical or flap of the case or any picture Black saw they had forgotten to remove the lens cap!
It took so develop a type of camera where the viewfinder, by a mirror system, give the image transmitted by the lens.
Going back in 1887, the "Patent Monocular" CR Smith was already a camera "reflex" mirror referred since reversed so the image from the lens.
In the SLR camera lens, the link between optics and the viewfinder has improved and is achieved by a "pentaprism" or mirror game that lets you see the image area in the viewfinder, which was not the case for SLR cameras with two objectives. It is with its Zeiss Ikon Contax S 1948 which inaugurated this advanced technique.
At the outbreak, the mirror is raised to allow light to reach the shutter that opens and allows light to impress the film, and then the mirror back into place to return the image in the viewfinder. At first, the mirror did not go back automatically, and the viewfinder was so dark; it was reset to review the image. Anyway, according to this system, during the shooting, the image disappears from the viewfinder which is not the case for the bi-reflex targets.
A novelty was the introduction of the photoelectric cell (light meter) in the machine, first on the front of the unit and the prism to measure the light reflected by the target or "Through The Lens" (TTL). The first manufacturers to use this technique were Asahi Pentax with its "Spotmatic" in 1964 and Nippon Kogaku with his "Nikon F" equipped with its photomic T in 1965.