Pentacon Six & Biometar 80 focus problem

Discussion in 'Medium Format Cameras and Accessories' started by Tor-Einar Jarnbjo, Sep 11, 2017.

  1. shutterfinger

    shutterfinger Member

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    BULLSHEET.
    The camera has a B setting. The shutter release is threaded for a cable release. If the shutter will not operate with the back open then look in the edges of the camera back for levers/pins that are depressed when the back is closed, lock them depressed with wooden pins such as round toothpicks. With the shutter locked open on B with a locking cable release the film plane to lens element can be measured. With the camera mounted on a tripod, lens critically focused via the viewfinder, shutter locked open on B a temporary ground glass held at the film plane should match the focus at the viewfinder. A temporary ground glass can be made from a piece of glass/or clear acrylic cut to fit the film plane with strips of clear transparent tape placed next to each other without overlapping, the tape side goes toward the lens.
    With the shutter open on B a block, be it your finger or other blunt object can be placed in the shutter curtain track to block the second curtain from closing when the cable lock/shutter is released. The curtain may be movable from full closed with the shutter released, use care not to damage it by poking holes in the curtain or bending the light trap edge.

    The problem is the redesign of the lens, figuring out how to compensate for it will take some careful measurements, investigation, and testing to figure out.
    A modification to the lens or body will likely make the lens usable on that body only, that body only usable with that lens. A modification that cam be added or removed easily to either the lens or body is the idea solution.
    There is no Do This fix.
     
    Last edited: Sep 12, 2017
  2. Theo Sulphate

    Theo Sulphate Subscriber

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    There may be a flaw in one part of the testing method.

    You say all the lenses focus accurately on the DSLR (which would imply both old and new lenses focus identically). However, when you focus with the DSLR, are you relying on the in-focus indicator (e.g. with a Nikon, there's the green LED) or are you focusing using only the groundglass and nothing else to determine the point of best focus?

    If you're using just the indicator, the old and new lenses will certainly be in focus at some point, but the groundglass may show a difference between old and new. Ideally, when you focus either old or new lens, when the in-focus indicator is lit that should also be the point of sharpest focus on the groundglass.
     
  3. shutterfinger

    shutterfinger Member

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    Did you make an exposure with each the new and old the lens wide open on the DSLR using the same target without moving the camera or target? Did the lens focus at the same distance on their scales?
     
    Last edited: Sep 12, 2017
  4. itsdoable

    itsdoable Member

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    ... or just push on the pin actuation plate in the body (with the lens removed) and see if the mirror moves. I don't have a pentacon 6 handy, but if you do this with a Kiev 88, the mirror moves (because the pin actuation plate mechanism is part of the mirror mechanism). Of course the Kiev is Russian and the Pentacon is German, and the mechanisms are different, but are similarly linked, so you can at least rule this out (or not).

    Note: the Kiev is a hasselblad (Swedish) copy, but the auto aperture mechanism is pure Russian.
     
  5. AgX

    AgX Member

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    If it is a mirror issue, then the gradual difference in sharpness should be visible (unless there is some peculiar swing movement.)

    Thus take a target with even spread focusing points, put it perpendicalir to the optical axis. If the pattern is evenly sharp with the "good" lenses, but has a gradual vertical difference with the "bad" ones, the cause of the problem is the mirror.

    I'm ignorant on the Pentacon Six, but as pointed out in this thread the mirror mechanism is different from the free swinging mirrors at my 35mm bodies and here might be the causal connection to the lenses.
     
  6. OP
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    I have tried both, using the autofocus aid and focusing on the DSLR groundglass (Nikon Df), and I get more or less the same results. Since my DSLR only have a plain groundglass (no split-view or microprism aids), I must admit that it is a little bit tricky to focus perfectly with the smaller viewfinder, but even if I on some of the test shots was slightly off with the focus (when focusing without the autofocus aid), there is no consistent front-focus, as I experience with the Pentacon.
     
    Last edited: Sep 13, 2017
  7. OP
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    No, pushing the lever/plate in the body, which controls the automatic diaphragm pin does not move the mirror.
     
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    Yes, I will do so tonight and get back with the results.
     
  9. Berri

    Berri Member

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    you can tape some tracing paper on the film plane and, with your camera mounted on a tripod, set the "b" shutter speed, press the release button and rotate the ring unterneath it to lock the shutter in open position. Now focus the lens on your target at the minimum distance with your old biometar (do it by means of a tape mesure being careful of measuring from the film plane to the focusing target and in a perpendicular direction to the film plane, do not use your ground glass to focus). You should be able to se a sharp image projected onto the tracing paper (you may need to use a magnifier to check it accuratelly) repeat this test with the newer biometar. If both lenses produce the same results the problem is in the ground glass that either came loose or moved.
     
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    I honestly doubt that I will be able to mount a piece of tracing paper flat enough or that the image projected on the tracing paper is clear and detailed enough to evaluate focus. I could probably, as shutterfinger suggested, use an additional piece of groundglass, but since it has to be large enough to cover the shutter opening, I would either have to order a custom-made piece of glass or try to make one myself. Both options are either time or money consuming and I am not really convinced that they will bring me closer to a solution. I would also doubt, that the distance scale on the Biometars is accurate enough to make the test you are suggesting.

    And, as pointed out several times: The focus works perfectly with any of my other lenses, so I can at least within any reasonable doubt, rule out that the groundglass in the Pentacon viewfinder is dislocated. If it had been loose or dislocated, I would not have been able to focus with the other lenses either.
     
  11. Berri

    Berri Member

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    the problem you are experiencing is very weird. Perhaps you should look into buyng another pentacon body or get it checked by a professional
     
  12. Berri

    Berri Member

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    Fact A) if both lenses are working fine with other digital cameras, the problem is not the lens.
    Fact B) your camera is working fine with other lenses, including a lens of the same focal lenght 80mm, so your camea is working ok
    Fact C) when you mount new biometar style lenses on your P6 body you experience a front focus issue.

    If you focus to the target so that the image appears in focus on the ground glass, do you get a correct distance on the lense scale? Do you get the same value with the old biometar?

    if that is the case, how many rolls did you shoot with the newer lenses? could it be possible that you miss-loaded the film or that the pressure plate came loose by chance every time you used the new lens?
     
  13. Marco Gilardetti

    Marco Gilardetti Member

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    When you use a mid-format lens on a smaller-format camera, you see only a fraction of the lens coverage, so fact "A" is quite questionable as a "fact". About fact "B", many people (myself included) pointed out that it actually depends on how the camera body interacts with the lens, especially the diaphragm actuator (although Tor is quite persuaded to have ruled that out). You also rely too much on what the distance scales say, Berri. We are talking about a 1cm focus displacement, I don't remember how fine the meter scale of the Biometar was but I would be suprised if it was finer than a-quarter-of-meter.

    Tor, do the test that AgX suggested. If everything else has been ruled out, it may very well be that the lenses are not on-axis inside the barrel. If the key-pin is of the removable type (sometimes it's just a screw) you can also try to mount the lens at different angles and see how it affects focus. If the lens assembly is tilted, focus should change by rotating the lens on its axis.
     
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  15. rockpowered

    rockpowered Member

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    The test that Shuttertfinger suggested is exactly the right approach. I own and shoot with a Pentacon Six and have torn it down and repaired it from top to bottom. It's a tricky camera but this issue is not. It's been standard practice for almost forever to check calibrate focus by placing tape/paper or ground glass in place of a strip of film in position on the gate. The slightly opaque scotch tape happens to work well. Focus on an object that is extremely well lit and compare what you get on the tape with your ground glass. The ground glass may need to be shimmed or may be loose. This method is not difficult. I do it all the time for cameras that don't have through the lens viewing such as TLRs or cameras that offer rely on distance scales to focus.
     
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    As Marco also pointed out: The distance scale on the lens is not accurate enough to make sense out of the readings as you are suggesting.

    I shot some 10-15 rolls of film during my holidays, alternating between the different lenses I have and most, if not all pictures shot with the new Biometar are out of focus. I have now also shot about 5 rolls for different tests with the focus chart. On my last test roll, I alternated each exposure between the old and one of the new Biometars and still it was reliably reproducable that the old Biometar focuses properly, while the new Biometar has the front-focus issue. It is at least very unlikely that the film for some reason is out of position only when using one of the new Biometars.
     
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    If that is the problem, can you please explain why I don't have any focusing problems with any of the other lenses I own and use?
     
  18. rockpowered

    rockpowered Member

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    You can argue all day long about focus but the proof of focus is what's projected on the film plane. It's either in focus or not and the way to test is a standard method. Your ground glass is not proof of focus. The rest can be dithered on forever. EIther you want to do the actual testing or you dont.
     
  19. OP
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    Where did I write anything else? When I use my old lenses, the image projected on the film plane is in focus, when it appears in focus on the groundglass in the viewfinder. Now once again: How can that be, if the groundglass is displaced or loose?
     
  20. rockpowered

    rockpowered Member

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    I wish you good luck. Testing is a way of eliminating possibilities and narrowing down a diagnostic tree. You're satisfied that you don't need to perform a calibration procedure because you already know the outcome, so be it. I love the P6 it's a great camera.
     
  21. OP
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    I am just asking you exactly what knowledge you are expecting to gain from such a test and how it will help to solve the problem. Wasting time with tests, which do not show anything more than what I already know, will not help me solve the problem either.

    You wrote earlier that I should compare the projection on the groundglass in the viewfinder with the projection on a piece of tape, paper or groundglass placed in the film plane. Now, since I already know, by projecting onto 'real' film, that there is a deviation here, the image projected onto the film is not in-focus when it appears in-focus in the viewfinder: What knowledge to you hope to gain by doing the same test again with a paper, tape or groundglass 'dummy' instead of real film?

    You further write, that the groundglass in the viewfinder may need to be adjusted or may be loose, but I have IMHO ruled that out, both with tests I have already conducted and by successfully using the camera with other lenses. My argument is that if the groundglass in the viewfinder had not been in the correct position, I would not have been able to focus properly with any lens. If you really mean that this conclusion is invalid, please explain why you think so and how it can be, that I can focus properly with a misaligned groundglass.
     
  22. shutterfinger

    shutterfinger Member

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    ScreenHunter_11 Sep. 13 12.02.jpg
    This is the lens diagram for the Biometar, light travels the direction of the arrow. Leaf shutter, if so equipped, and aperture are mounted in the wide space between lens groups.

    A report I read earlier stated that the first 180° turn from was from the closest focus distance of 1 meter to 2 meters, plenty of focus scale on the lens for testing at a target of 1.5 meters from the camera. That report also stated the lens is sharpest from f4 to f16.
    I just love these modern search engines that produce a different set of results for the same search term when the searches are a few hours apart.
     
  23. Theo Sulphate

    Theo Sulphate Subscriber

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    This problem seems to defy logic because:

    (1) on the DSLR, all lenses,both old and new, when in-focus on the groundglass produce a focused result . This implies all of the lenses, old and new, have no focusing anomalies.

    (2) on the P6, only the old lenses when focused on the groundglass produce an in-focus result. The new lenses produce a front-focus result. This contradicts the results of test (1).

    I have a solution to the problem: buy a Hasselblad.

    Ok, seriously... On the P6, is the groundglass plain or does it have a focusing aid such as split-image rangefinder, microprism, or fresnel pattern? If so, what I'm thinking is the angle of the light from the new Biometars is entering the focusing aid at a different angle (the cone of light is wider or narrower) than with the older lenses.

    If it's just a plain groundglass, maybe this simple enhancement to your test would confirm uniform focus: rather than have a single ruler in the center showing distances, add two others at the sides (left and right). This may detect mirror or groundglass issues. Overall, though, I'm totally puzzled.
     
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    If I just wanted something which works, I can always use my Nikon DSLR :wink:

    Pentacon used to produce many different groundglasses for the Pentacon Six, and they are interchangeable with a few other camera models, but I only have a plain groundglass with a grid, but neither split-image nor microprism focusing aids.

    I already did. All images in my initial post are just small crops from the full test chart. I just have a full image taken with the old Biometar at hand right now, but here is how the full setup looks:

    [​IMG]
     
  25. itsdoable

    itsdoable Member

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    ... Focus shift? What aperture were you using on the film shots?
     
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    Tor-Einar Jarnbjo

    Tor-Einar Jarnbjo Member

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    As I wrote in my initial post: 'I did several test shots of the focusing chart with different apertures and the problem is clearly not any kind of aperture dependent focus shift. The focus point stays the same, even if I stop down the lens.'