Lens Mounts: Topcon UV

Basic Information on the Topcon UV mount:

Topcon was a famed Japanese camera manufacturer, that in its heyday was seen as competing with Nikon for the spot as top-SLR-dog. Topcon started its SLR manufacture using the Exacta (EXA) mount, but also introduced (1963) their own, rather unusual, SLR lens mount. This later mount is typically referred to as the Topcon UV mount (partially to differentiate between this and the Topcon-proprietary variations of the EXA mount) as the line of lenses was named ‘UV Topcor’.

The Topcon UV cameras are somewhat exceptional in that they are interchangeable lens leaf shutter cameras. Instead of the customary arrangement (echoed in 99% of SLR cameras) of a focal plane shutter (cloth or metal) nearest to the film and a mirror between that shutter and the lens, the line of Topcon UV-mount cameras had a mirror, and a leaf shutter between the mirror and the lens (but no focal plane shutter).

See pictures below for structure of Topcon UV mount:

While there are some inherent advantages to leaf shutter cameras, putting the leaf shutter into the SLR itself (and not into the SLR lens) does lead to some complications, namely that: if the shutter’s maximum opening is large, the fastest shutter speeds cannot be very fast; if the shutter’s maximum opening is small, the maximum theoretical lens aperture is also small. As a result of this, no TopconUV lens has a maximum aperture brighter than f/2. Another problematic aspect with leaf shutter SLR’s is that the arrangement leads to a technically somewhat convoluted image-taking sequence 1.

But there are also undoubtable advantages to lead shutters vis-a-vis focal plane shutters: flash sync at all speeds, and a technically more resilient shutter that (compared to cloth shutters) also suffers less of a risk to develop light leaks.

Topcon UV mount specifications

Mount type: Bayonet (lens release on lens)
Flange focal distance: 55 mm / 52 / 49 mm
Film format: 36mm x 24mm (‘Full frame’)
Mount communication: two levers: one for telling the lens which aperture the user (or automation) has chosen, another to stop down the lens for the shot (camera-to-lens)

The Flange focal distance of the Topcon UV mount is regularly quoted as 55 mm, but this can lead to some confusion, as it is by no means clear what constitutes the flange on a female bayonet mount like the Topcon UV.
• 55 mm is the distance from the film plane to the furthermost protrusions on the camera (effectively the protruding bayonet ring)
• 52 is the distance from the film plane to the external/outer flange (where the rear of the lens meets up with the shutter speed control ring on a Topcon UV camera)
• 49 mm is the distance from the film plane to the inner flange (inside the bayonet mount) of a Topcon UV camera/lens.

Adapting Topcon UV lenses

So and so. On the one hand, the TopconUV mount has a very generous flange focal distance (55 mm), that would allow adaptation to both most 35 mm SLR’s as well as (naturally) all mirrorless cameras.

On the other hand, as the lenses themselves lack an aperture control mechanism, the adapter must furnish that mechanism. Thereby the only way you could use TopconUV lenses on a dumb adapter would be to shoot wide open 24/7. On the other hand, if you can get your hands on a specialised adapter that allows aperture control, there are no limitations.

Identifying the Topcon UV mount

See here.


1 Leaf shutters in SLRs are a bit of a kludge because the mirror needs to double as focal plane shutter (in the sense that the mirror may not let any light through) and the picture-taking process becomes hideously complex:
• Photographer frames, focuses etc. (Mirror is down, leaf shutter is open)
• Photographer pulls trigger (Leaf shutter is closed, mirror is raised, aperture closes to pre-set value)
• The picture is taken (Leaf shutter opens, closes)
• Return to normal (Aperture opens, mirror is lowered, leaf shutter opens)
As one commentator puts it while discussing DKL-mount cameras (another type of leaf shutter SLR’s) “[Camera] mechanics smilingly turn down repair requests [on such cameras], or turn, run and scream” (my translation)

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