This diagram shows the Beaulieu reflex viewing system utilized in its most cameras.
100% light thru the lens hits on the film during the exposure phase.
A guillotine-type shutter (reciprocating blade) set at 45° acts as support for a mirror. During the time or exposure, the shutter mirror allows all the light rays to impinge on the film. While the film is moving from one frame to the next, all the light is directed into the viewfinder. This system, which is 100% luminous, gives a viewing image free from any degradation. The luminosity available for viewing is so great that it allows an image magnification of 27X and that a groundglass focusing screen should be put between lens and viewfinder whenever pin-point focusing is necessary. As against this, when the camera is running, the image seen in the viewfinder flickers (alternating motion of the shutter mirror).