Innovations in 35mm SLR Camera Design: 1940 to 1960

The first innovation of this 20-year period came in 1947, in the form of the ambitious but highly unreliable Hungarian Gamma Duflex. This was the earliest SLR camera to use an instant return mirror, and an internal semi-automatic lens diaphragm.

Previously, reflex mirrors had been coupled to the shutter release and were spring actuated so that they rose automatically when the shutter was tripped, but this meant the viewfinder remain blacked-out until the mirror was manually reset to its original position.

Similarly, lens diaphragms also had to be manually closed to the required f-stop before exposure, and opened afterward. The moment before and after exposure was often a period of dim visibility. The Duflex’s semi-automatic diaphragm closed the lens diaphragm to a pre-set f-stop when the shutter was released, but it still needed to be manually re-opened after exposure.

In 1948, the Italian Rectaflex introduced the world to the first Pentaprism SLR, although its eyepiece was angled upward at 45° (no doubt a concession to the way things had been done up until that time?).

Previously, SLRs employed “waist level” viewfinders, in which the photographer looked downward at a focusing screen to observe the reflex mirror’s image. This system was difficult to use since the scene is viewed as a mirror image, resulting in directional movements being reversed. In other words, an object viewed on the left side of the frame was on the right in reality, so bringing that object more to the centre of the frame required the camera to be panned right rather that to the left. The Pentaprism re-reversed the viewfinder image, and was therefore a big step forward, facilitating far more intuitive shot framing. Objects physically on say the right were also on the right of the viewfinder.

A Pentaprism is actually an eight-sided chunk of glass, where only five of those sides are significant. Two sides are silvered and redirect and re-reverse the light from the mirror. Another two sides let light in and out. The fifth side is not used optically but is made flat for the sake of compactness. The three insignificant sides are simply cut off corners.

1949 was the year the Pentaprism started to become adopted by other manufacturers, who used it to offer the eye-level viewing that has become commonplace. The East German Contax S by Zeiss Ikon, was the second Pentaprism equipped camera, followed by the East German Praktica.

In 1950, the East German Ihagee company launched the Exakta Varex, which was the first SLR camera to have an interchangeable viewfinders, focusing screens, and a viewfinder condenser lens.

The first two items – viewfinders, and focusing screens – probably need no further explanation, since customisation and interchangeably of these components became a feature of the best SLRs. The condenser was a lens placed between the viewfinder’s ground glass focusing screen and Pentaprism, which increased light intensity in the viewfinder.

Meanwhile, Germany was not the only Nation making advances. In France, Pierre Angenieux gave his name to the first “retrofocus” wide-angle lens made specifically for 35 mm SLRs.

Regular wide-angle lenses need to be mounted close to the film, but with SLRs, the space required to allow movement of the mirror prevents this, and so 40mm lenses were typically the shortest focal length possible. The retrofocus design (more correctly known as an inverted telephoto) uses a very large negative front element to force back-focus distances long enough to ensure mirror clearance.

1952, was the year the Soviet Union produced their first eye level viewing Pentaprism SLR camera – the Zenit, while Japan merely accomplished their first waist level finder SLR camera – the Asahiflex I.

The majority of technological innovation still arose in Germany, and in the East – in 1953 – Zeiss Ikon made the Contax E; the first SLR with a built-in light meter. This was quite simply mounted on the Pentaprism, above the lens, and had an external selenium photoelectric cell. The meter was uncoupled, so it’s readings had to be manually set as a shutter speed/lens aperture combination.

Most SLRs had Focal Plane shutters, but in 1953 – in West Germany – Zeiss Ikon made the Contaflex I, which was the first leaf shutter 35 mm SLR. Other cameras used leaf shutters, until Focal Plane shutters improved, got faster, and finally dominated SLR design from about the mid-70s.

Still in West Germany, Metz’s Mecaflex became the first and only square format 35 mm SLR, based on the design principles of the 1934 Robot camera.

Now the fact that it proved not to be an influential design innovation might exclude it from this list, but it was quite a sensible idea. 24×36 mm frames are somewhat inefficient, in so far that they require a 43mm lens diameter, and 59% of that lens’ area is used to produce the image. A square 24×24 mm frame requires a smaller 40mm lens diameter, and uses 64% of that lens’ area. Many years later, this idea has resurged in digital camera design.

In 1954, the Praktina FX, had the first spring powered motor drive accessory for an SLR, and first breech-lock lens mount.

1955, and the Miranda T was the first Japanese Pentaprism eye-level viewing 35 mm SLR, while West German manufacturer – Kilfitt – made the first close focusing “macro” lens for 35 mm SLRs.

1957 was the year in which the Asahi Pentax was the first SLR with a right-handed rapid-wind thumb lever, foldout film rewind crank, and micro-prism focusing aid, plus it adopted the M42 screw lens mount. This landmark camera set the standard for future control layout. Elsewhere in the land of the rising sun, the Zunow SLR offered first internal auto-diaphragm – a device coupled to the shutter release and which automatically stopped down the lens diaphragm when the mirror swung up, and reopen it when the mirror swung down to provide almost continuous fully open aperture viewing. Sadly, the Zunow Company sunk into bankrupt before they could benefit from this development.

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