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Angenieux 28mm lens R11 from the late 1950s

Toronto.  in the mid 20th century, the Exakta was a rare bird – a 35mm film SLR. The bright view was accomplished by a special waist level viewer, a fast lens and a mirror the size of the film frame.

To view, the mirror rested at 45 degrees bouncing the view through the lens up to the viewfinder. On snapping a photo, the mirror swept up and blocked the finder while fully exposing the shutter mechanism and film frame. For the mirror to work, it had to clear the rear lens element. This made even ‘normal’ lenses a design  challenge. A wide angle lens like the 40mm Zeiss Flektogon was remarkable and even wider lenses like the Angenieux 35mm and 28mm unbelievable!

To work the SLR mirror magic, at infinity the rear element had to be located beyond the edge of the mirror. Angenieux did this by using a ‘retrofocus’ design with a rather difficult trade-off. Their lenses suffered from drastic barrel and pin-cushion geometric distortion. Tilting the camera while shooting straight lines made the distortion even more obvious.

Lens coatings, a wide range of optical glass, computers, etc. led to designs that gave the mirror room to move while over-coming most geometric distortion. And the 35mm SLR became the camera of choice.  In the rangefinder era some companies like Leitz and Zeiss used mirror boxes as a solution. The mirror box purposely used a larger mirror to improve brightness. Lenses beyond, say a 135mm focal length, could fit such a mirror box while shorter focal length lenses that benefitted from a rangefinder could be easily accommodated directly allowing focal lengths down to 28mm to be feasible with little or no geometric distortion.

When the Japanese cameras like Nikon and Canon emerged,  SLRs went from odd film designs to preferred designs and after a century to the common digital SLRs (DSLRs). In time digital technology allowed an instant view of the scene making mirrors unnecessary.

The preferred professional camera design was the DSLR with the less costly  ‘mirrorless’ cameras preferred by advanced amateurs. Smartphones came with a mirrorless camera whose tiny lens usually gave an angle of view like the 35mm film camera lenses –  by far the most used focal length.  Computer technology in the smartphone allowed creation of decent resolution, low noise, photos in spite of a tiny sensor. And mirrors – a point to ponder.