A Pentax 67 lens on the Pentacon Six.  This is the real thing, not a Photoshop simulation! [PX67_01.jpg]

An earlier answer to this question explained the difficulties in doing this.  See here.  This page, new in November 2020, explains the solution!  I am grateful to a visitor to this website, Dominik Samol, for telling me of the solution and for arranging to have the appropriate adapter ring manufactured for me.  Some of his pictures are also featured below.

[PX67_02.jpg]		[PX67_03.jpg]
The lens that we have here mounted on the Pentacon Six is the SMC PENTAX 67 MACRO 1:4 100mm lens.  We have not modified the lens, but the camera.

Lenses for the Pentacon Six are held onto the camera by a locking ring on the front of the camera body: [C442_14A.jpg] The procedure for attaching and removing lenses is explained in detail here.

But it is not difficult to remove this locking ring completely – and to replace it when desired!  This ring rotates through approximately 90º.  In the photograph on the left, it is in its fully clockwise position, when viewed from the front of the camera, which is required to insert and remove lenses.  With the lens in place, this ring is rotated clockwise until it holds the lens firmly in place.  The ring is mounted on a screw thread, so as it is rotated clockwise, it moves down onto the three tabs on the lens, progressively holding them more and more firmly.  This is very useful with some lenses from other manufacturers that have tabs whose thickness does not precisely match the Carl Zeiss Jena and Meyer-Optik Görlitz specifications. To remove this ring, one simply needs to remove the locking screw that limits its travel.  With the ring in the position shown in the picture to the left, this screw is located in approximately the “7 o’clock” position.     [P6ring_03.jpg] Here we can see the locking screw clearly.  It can be helpful to have a magnetic screwdriver when removing it, as it is possible to unscrew it fully but to find that it is inaccessible for removal, as it remains recessed within the screw hole.

[P6ring_01.jpg] Here we can see the camera after removal of the lens locking ring, which is to the left of it in this picture.

[P6ring_02.jpg] [P6_ring_groove.jpg] In this close-up view of the ring, it is just about possible to see the hole for the locking screw at the bottom, and, on the inside at the top, the internal thread of the ring by which it is screwed onto the camera. A tip on re-affixing the ring If one wishes to return the camera to its original specification, the breech-lock ring can easily be re-affixed to the camera.  Before re-affixing it, look at the thread on the outside of the camera ring to which it is to be attached.  Between approximately the “7 o’clock” position and the  “10 o’clock” position you will see the groove in which the locking screw will run. Screw the ring fully on, then look for the hole that contains (or will contain) the locking screw.  This will probably be at about the “3 o’clock” position.  Unscrew the ring to about the “8 o’clock” position, and screw in the locking screw.  Then rotate the ring in both directions to make sure that the locking screw stops it at both ends of the groove.  Then turn the ring fully anti-clockwise, attach a lens with the standard Pentacon Six mount and rotate clockwise to hold the lens firmly. In the photo to the left we can see the groove in which the locking screw runs. Picture courtesy of Dominik Samol

Modify a Pentax 67 to Pentax K (35mm camera) lens adapter It is necessary to cut off the back of this adapter to make its thickness appropriate to allow infinity focus, and then to cut onto the inside surface of the back of the adapter the 68mm thread that is required in order to screw it onto the Pentacon Six camera. The adapter used here has the advantage that it contains both the internal Pentax 67 bayonet mount that is required for most Pentax 67 lenses and also the external Pentax 67 bayonet lugs required for the 400mm, 600mm and 1000mm Pentax 67 lenses.  It seems to be hard to locate the original Pentax versions of these adapters.  However, Chinese manufacturers do advertise adapters that they claim do the same job.  Nevertheless, caution is required, as these manufacturers can change the specification without notifying potential purchasers, which can result in an adapter being received that is difficult or impossible to modify for the Pentacon Six. [P67_on_P6_03.jpg] The top two outer tabs for the longer Pentax 67 lenses can here be clearly seen on this adapter ring.  The lens here is the 45mm Pentax 6×7 lens, which, along with the Pentax 90mm lens, is the smallest in the system. [P67_P6_04.jpg] Front view of the modified P67-PK adapter on a Pentacon six. Both the inner tabs for most Pentax 67 lenses and the outer tabs for the longest lenses can be seen. Above two pictures courtesy of Dominik Samol

Modify a Pentax 67 extension tube Again, the same skills are required as for the previous modification, which includes access to a suitable lathe and a thread-cutting tool, plus the necessary required dimensions. The one disadvantage is that on the Pentax 67 extension tube used for the adapter ring that I now have, only the internal Pentax 67 bayonet mount is present; the external bayonet lugs for the 400mm, 600mm and 1000mm Pentax 67 lenses are not there, so one has to be content not to have the option of using these longer lenses. [P67_tube_P6_01.jpg] On this modified Pentax 67 extension tube, the inner bayonet lugs are clearly visible, but the external lugs for the longer lenses are clearly not present. The original back of this extension tube has been cut off and a ring has been added at the back with an internal thread of 68mm, to screw the new ring onto the front of the camera.  The thread and pitch of the mount for the lens locking ring on the front of the Pentacon Six are the same as on the inner sections of the old manual Praktisix/Pentacon Six extension tubes (see here and here), so if necessary one of those rings could be cut down and connected permanently to the cut-down Pentax 67 extension tube. My thanks to Stefano for telling me that this is how he made his Pentax 67 lens mount ring for the Pentacon Six!  Here is what he says: “Having difficulty to find someone who could make the thread m68 I bought an extension tube for p67, disassembled and joined with the [Praktisix/Pentacon Six] bayonet ring, that is already m68 female (plus a step ring 77-82 to keep everything centered.”🙂 [P67_tube_P6_02.jpg] The total depth of the new Pentax 67 lens mount ring for the Pentacon Six, from its front surface to its back surface, needs to be 16mm, as far as I am able to measure (with my caliper tool not currently working!). The ring that I have can here be seen mounted on a Pentacon Six.  Again, the inner bayonet tabs can be seen, as well as the absence of the outer tabs for the longer lenses. Above two pictures courtesy of Dominik Samol

I bought just one lens to test this adaption on, the SMC PENTAX 67 MACRO 1:4 100mm lens illustrated at the top of this page.  The first thing that struck me when it arrived was how big and heavy the lenses for the Pentax 67 are!  I chose this particular lens because reports on it are extremely positive and the focal length means that it could also be used for some general photography – always assuming that infinity focus is possible with this adapter!  It is, after all, relatively easy for a skilled technician to adapt almost any lens for the Pentacon Six, for use on bellows or extension tubes for close-up work.  The challenge is to obtain infinity focus in the small amount of space available for the adapter.

Here are my first two test shots:

[C580_01_Px67_infin.jpg] This is not an exciting composition, but it was the best infinity-focus shot available under Covid-19 restrictions, a view through an open window in my home.  It is early November and about 8.00 a.m., so the shadows are long, but the large area of open sky enables me to check for any light fall-off away from the centre of the frame. Fujifilm PRO400H Pentax 67 100 Macro lens at infinity focus on a Pentacon Six 1/250 f/16 Hand-held As is to be expected when using a lens designed for a “6×7” camera on a “6×6” camera, coverage is faultless.		[C580_03_Px67_5m.jpg] This second test shot of a fairly distant subject turns out not to be on infinity focus but just beyond 5 metres, again taken without leaving the land on which my house stands.  Taken the same day as the previous shot, about half an hour later. Fujifilm PRO400H Pentax 67 100 Macro lens Focus just beyond 5m on a Pentacon Six 1/250 f/14 Hand-held With the sun illuminating more of the scene, contrast has improved.  This is also another excellent test of coverage, and the lens effortlessly passes the test!
For a further example of a picture taken at infinity focus with this lens on a Pentacon Six, see here.
Testing closest-focus with the Pentax 67 100mm Macro lens [C580_4_5_Px67_Cfoc.jpg] The width of this small alarm clock is 9 centimetres (a fraction over 3½"). This shows the closest focus with the “basic” 100mm Pentax Macro lens. Fujifilm PRO400H Pentax 67 100 Macro lens at closest focus on a Pentacon Six 1/8 f/11 Benro tripod & head [Px67_100_cf.jpg] The set-up for the photograph to the left [C580_6_Px67_Cfoc_life_conv.jpg] In addition to what I on the left here call the “basic lens”, the Pentax 67 Macro lens comes with a “LIFE-SIZE CONVERTER” that can be screwed into the front of the lens. Fujifilm PRO400H Pentax 67 100 Macro lens at closest focus on a Pentacon Six with the “LIFE-SIZE CONVERTER” 1/8 f/11 Benro tripod & head We note the blur of the seconds hand as it swings round and a slight blur of the minutes hand as it advances. [P67_100_cfls.jpg] The set-up for the photograph to the left Next we see the camera and lens more clearly as set up for each of these photographs. [PX67_06.jpg] On the left here, the lens is at its closest-focus setting, but the “LIFE-SIZE CONVERTER” is not being used. For this photograph we have placed it beside the lens.  We have also here removed the trumpet-shaped lens hood, which was in place when the above photograph was taken. [PX67_05.jpg] On the left here, the lens is at its closest-focus setting, just as before, but we have removed the lens hood, screwed in the “LIFE-SIZE CONVERTER” and then screwed the lens hood onto the front of the converter. Aperture automation No, that is not something that we can obtain when using Pentax 67 lenses on a Pentacon Six.  However, a lever on this lens and on some other Pentax 67 lenses that I have seen does make it easy to open up the lens aperture temporarily, for instance, to check focus, and then to stop it down again in order to take the photograph.  I also used this method to meter via the working aperture, using my normal Pentacon Six TTL metering pentaprism. Dominik Comments: “I am glad you discovered the auto/manual trick. Except for the Soft, the Shift and the 600mm lens in my posession, all lenses have this option and so they can be used like fast-acting preset lenses.” Conclusion Using this Pentax 67 Macro lens is definitely a viable alternative to taking out other macro lenses, bellows and extension tubes that are described elsewhere on this website.  It can easily go from infinity to life-size, a characteristic often promised by other manufacturers, but not often delivered. It would also be worth exploring if there are other Pentax 67 lenses that fill gaps in our Pentacon Six outfit (doubtful, I think!).  Given the size, weight and cost of Pentax 67 lenses, the Pentacon Six user with a comprehensive outfit may find no need to look to Pentax.  However, for the Pentax 67 user with a range of lenses, it is well worth having the option of also using the Pentax lenses on a Pentacon Six, perhaps as a “second body” to a Pentax 67 outfit. Dominik Samol has written with the following additional information on Pentax 6×7 / Pentax 67 lenses: “ As for "holes" in the Pentacon System. I'll go from short end to long. - 35mm Fisheye (has a different character to the 30mm Arsat/Zodiak and built in filters.) - 45mm F4 (very compact and 5mm wider than the Flektogon)  [TRA, there is of course the 45mm Arsenal Mir 26B and the hard-to-find Mir-69 (see here), plus the shift and shift/tilt lenses derived from the 26B.] - 55mm F4 or F3.5 (3 different versions at Pentax. If one wants to use anything between 45-55mm those would work as well, but except for the last version of the Pentax, any Flektogon is as nice or more nice and none of those is a small lens, especially not the F3.5 version.) [TRA: there is also the Arsat 55mm shift lens in Pentacon Six mount, see here.] - 75mm (no 75 in the Pentacon System, but the 80mm lenses are super small and exceptionally good, the 75s are all bigger, slower but there is one shift lens. If Pentacon is your main system, those are the least desirable.) - 90mm F2.8 (Pentax has a leaf shutter lens here and a not leaf shuttered. Both are nice, but there is the 90mm Vega as well, so no reason to get it, unless you can make the leaf shutter work on the Pentacon.) - 100mm F4 macro - 105mm F2.4 This is the signature lens on the Pentax System. It is roughly twice the size of the Biometar, but will get you more Bokeh wide open and much less busy. Also it is tighter, which might help the look. -120mm Soft (it’s a soft lens. Love it or hate it.) -135mm F4 (a very sharp lens with macro, better close focusing ability. I'd rate it as long general purpose lens. It is about the size of the 100mm macro, maybe a bit longer, but thinner. Also it is the second cheapest lens to get, under 100 pounds.) - 150mm F2.8 / 165mm F2.8 / 165mm F4 These are all portrait lenses, very good performers, but the 180mm Sonnar has more character. The 150 is much smaller (+30% in length compared to the 100 Macro). The 165mm F4 has a leaf shutter, same story as with the 90mm. Size of the 165s is a bit less than the Sonnars in the Pentacon System.)  [TRA: See the 150mm Arsenal Kaleinar and the Hartblei version of it, and the Schneider-Kreuznach Tele-Xenar, here and here.] - 200mm F4 (cheapest 6x7 lens, with luck under 60 pounds. It is sharp and has a modern look to the images. If you can’t afford a Sonnar, this might be an alternative.) - 300mm F4 (size is around the same as the MC Sonnar 300mm, barrel a bit thicker, nice performer.  The ED version is expensive but a stellar performer.) - 400mm F4. (Great lens, compact but thick, no equally fast alternative in the Pentacon System) - 500mm F5.6 (This is the same length as the Pentacon, included, retractable hood, constant lens diameter, so thicker at the base, much smaller at the front.) - 600mm F4 (no lens to walk around with. It’s a little under 6kg. I was not able to use it yet successfully with results that pleased me. It has its place but is a speciality.) - Both Zoom lenses.  Those lenses are one of a kind. They were made very late, the 55-100 was made from 1998, the 90-180 was made 2001. You feel the modern design and quality. The lenses are often referred as "set of primes". You must think of a size close to the 300mm lenses in the Pentacon system. It will make your set huge, but you will not need to change the lens in an outing, because the range wide to portrait, or portrait to near tele is covered. If I would need to narrow down to some lenses, I'd pick: - 45mm F4 (it is the widest available normal lens and the size is good on the P6) - 105mm F2.4 (signature lens) - 135mm F4 (nice general purpose lens with focus down to 75cm) - 200mm F4 (price champion and cheap entry into portraits) - The zoom lenses, if not changing lenses is a benefit to you.” Thank you, Dominik for the information and advice!