The Konica Domirex was a prototype SLR camera which made its debut at Photokina in 1963. It had a very unique mirror mechanism, that effectively eliminated the need for the mirror to flip-up when the shutter release was triggered. The Domirex was a fixed-lens SLR with a 4-speed Seiko leaf shutter, and a Hexanon 57mm f/2.4 lens (but it actually looked like a rangefinder camera). The camera was described in US patent US3274912A, ‘Single-Lens Reflex Camera’, (1966, submitted 1962).
The concept was based on the idea of a beam-splitter reflex (BSR) which appeared in the mid 1960s [1]. Unlike an SLR which uses a mirror to reflect incoming light from the lens up through a pentaprism or waist-level viewfinder, the BSR deflects only a small portion of the light up through the viewfinder, with the remainder continuing on to the image plane. This was by no means a new concept, a similar idea had been used for a number of years in 16mm cine cameras – Bolex-REX and Arriflex 16.

In the case of the Domirex it works in the following manner. Between the lens elements is a small optical block containing two small “asterisk-shaped” semi-reflective silvered surfaces, placed in the path of the light rays. These “mirrors” are fixed and inclined at 45° (these are off-axis, one to the left and one to the right), and carefully positioned so not as to cause issues with the exposed image. They send a portion of light from the lens to the roof prism, while the remaining light continues its trajectory towards the film. The roof prism takes up less space in the top of the housing but protrudes slightly towards the front, covering the upper part of the optical block. The camera also had both horizontal and vertical split-image rangefinders for focusing, and did not use ground-glass for focusing (which might have hindered the minimal amount of light passing through the viewfinder).

Norman Rothschild reviewed the prototype in 1965 in Popular Photography [2]. The first thing he noted was “the one thing you’d expect to hear, is absolutely missing”, of course he is referring to the sound of the mirror. This design has the distinct advantage of not needing a large mirror that has to flip up in order for the light to pass through to the image plane during exposure. A large mirror causes mirror-black out, even if only for an instant, vibration, and of course noise. Get rid of the moving mirror, and there is no black-out, no vibration, and very little noise.
There were of course some inherent downsides to the design. As only a small amount of light is sent to the viewfinder, the viewfinder image would be much darker, than when 100% of light is reflected by means of a mirror. Could this loss of light have been a problem? Rothschild [2] suggested that the light loss was around 20%, meaning 80% of the light passed through to the image plane. But he seems to have had no problem focusing the camera, even in “relatively dim light”. This may be been due to the quality of the pentaprism, the lack of ground-glass, and split-rangefinder.

Perhaps the greatest problem would be the lack of interchangeable lenses. The prototype would have had to be modified to allow for interchangeable lenses. This could be accomplished by creating lenses incorporating the 45° reflective surfaces – but this would ultimately make them more expensive than traditional lenses. The camera also used a leaf-type shutter, which was certainly on the way out in the early 1960s, supplanted by the focal-plane shutter. By 1963 there was likely too much influence from existing SLR technologies to attempt to release a new technology. It is distinctly possible that as Mike Eckman suggests [3], the Canon Pellix, released in 1965, was a better option. It was a more conventional 35mm SLR, but with a semi-reflective mirror that accomplished the same aim as the BSR. But it offered a ground-glass, fully interchangeable lenses and a focal-plane shutter.
Rothschild though that the beam-splitting reflex design would eventually succeed in becoming part of the overall SLR market [2]. Rothschild’s final comment in the article was “the Dominex is worth waiting for, even if this takes a while.”. Sadly he would be wrong. Having likely made only prototypes, there aren’t many of these cameras about, however one of the original prototypes sold in spring 2024 for €12,000.
Further reading:
- Bill Pierce, “SLR’s Without Moving Mirrors: The Split-Beam System”, Popular Photography, 55, pp.64, 126 (June, 1964)
- Norman Rothschild, “Konica Domirex – A prototype”, Popular Photography, 54, pp.65-67 (June, 1965)
- Keppler’s Vault 47: Konica Domirex, Mike Eckman (2019)
- Konica Domirex




















