There are two difficulties with modifying Pentacon Six lenses to Pentax 6×7.

• Firstly, the image diagonal of the Pentax 6×7 is obviously significantly larger than the image circle of the Pentacon Six:
• The diagonal of Pentacon Six (56m × 56mm) is fractionally under 79.2mm.
• The diagonal of Pentax 6×7 (55mm × 70mm) is 89mm.
• Any lens that one hopes to use on the larger format must have an image circle large enough to cover the diagonal without noticeable vignetting or fall-off in resolution.  (Obviously, the image projected onto the film gate for any Pentacon Six lens has to have a diameter significantly greater than 79mm, as with any lens there is a fall-off in resolution and brightness towards the edge of the image (see the section on shift & tilt lenses).  Likewise, the image circle of any lens used on the Pentax 6×7 must be significantly larger than 89mm.  It might be good to start with lenses that have been adapted for use shifted on the Pentacon Six, as it will be clear that they will have a significantly larger-than-normal image circle with acceptable definition and absence of vignetting.  I have read that the 55mm Arsat shift lens generates image circle of diameter of about 105 mm at f/4.5 (wide open) and about 110 mm at f/22.

• However, the second difficulty is harder to crack.  This is the register distance – the distance between the rear flange of the lens that mates with the front of the camera and the film plane.  The register distance of the Pentacon Six is 74.1mm, and it is clear that if one wishes to mount a Pentacon Six lens on a 35mm camera body that might have a register distance of, say, 45mm, there will be space between the rear flange of the lens and the front of the 35mm body, to insert an adapter.  This is not the case with the Pentax 6×7, for which I am told that the register distance is 84.95mm.  If this is correct, then the Pentacon Six lens would need to be recessed over 10mm (not far short of half an inch!) within the throat of the Pentax 6×7, if one wanted to obtain infinity focus.  If it is possible to cut significantly more than this length off the back of the Pentacon Six lens (to allow space also for the adapter), then using the lens on the Pentax 6×7 may be possible.  But most Pentacon Six lenses have one or more elements within the final 10mm of their length.
• So such lenses may be usable as macro lenses on the Pentax 6×7 – where the bellows extension will also increase the projected image circle – , but could not be used for normal photography with infinity focus.

David Cross in England e-mailed me in March 2006 that he has converted some Pentacon Six lenses for a Pentax 6×7.  Here is some of what he says:

“So far I have adapted the Pentacon 300/4, the CZJ Sonnar 300/4 (which slips nicely into an outer bayonet extension tube with minimal machining) and a zebra finish Sonnar 180/2.8; this particular lens had a rear element assembly/baffle in two parts so required no machining off the length of the baffle-I have another sample of this lens which would require machining here.  These adaptations have no stop-down linkage - not something I feel is too much of a problem in use.
“Feel free to pass on any enquiries on this subject to me.”

I have also recently read a report of a professional photographer working with a Pentax 6×7 with a modified Pentacon 500mm f/5.6 lens.  This is a great manual lens (with pre-set aperture ring, which speeds up operation considerably).

The opposite question has also been asked: “Is it possible to use Pentax 6×7 lenses on the Pentacon Six?”  To see a consideration of this, click here.

To go back to the Frequently-asked Questions front page, click here.

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