Pentacon Six & Biometar 80 focus problem

[Tor-Einar Jarnbjo]

I have been using my Pentacon Six for some time now with several different lenses and without any focusing issues, but I recently decided to 'upgrade' my Biometar 80. I previously had one of the earliest version without an automatic diaphragm and with it being rather tedious to use, I decided to get a newer Biometar 80. I found one from the newest series (serial number 44xxx) on eBay, did some test shots with an adapter on my Nikon DSLR, found everything to be ok and went on holiday with the Pentacon and the new Biometar. After getting home and developing the films, I realized that all of the pictures were slightly out of focus and shooting a focus test chart with the new Biometar on the Pentacon confirms this. After having focused on the 0 point, the actual focus is distinctly in front of where it should have been.

I hadn't had any issues with my other lenses, but to be sure, I did some further tests. Here with the old Biometar 80:

And here with my Sonnar 180 (ignore the blur, but I had some issues with my tripod):

I also have a Görlitz Meyer Orestegor 500/f5.6 lens, with which I took some pictures during my holidays without any focusing issues. I have however not done any test shots of the focusing chart with that lens.

Since everything works as expected with my previously owned lenses, my first assumption was that something is wrong with the new Biometar, but after all, I had done some extensive tests with it on my Nikon DSLR before starting to use it on the Pentacon and had not experienced any issues. To be sure, I mounted the new Biometar on the Nikon again and did some test shots of the focus chart. This combination worked perfectly as well, when mounted on the Nikon, the new Biometar focuses just where it should focus:

At the moment, I had the following findings:

- the new Biometar has a focusing issue when mounted on the Pentacon
- when using any of my older lenses on the Pentacon, I do not have a focusing issue
- when using the new Biometar on a Nikon DSLR with adapter, I also do not have a focusing issue

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.

The problem seemed to be related to the exact combination of the new Biometar and the Pentacon. I then managed to get a second new Biometar (serial 45xxx) on eBay of the same version/series, did new test shots of the focus chart and had exactly the same front focus issue with the newest lens as well.

So, the most precise definition I can give of my probem is: If I mount a CZJ Biometar 80 of the newest version on my Pentacon Six, the actual focus point is in front of whatever I tried to focus on. If I use the problematic lenses on another camera, or if I use other lenses on the Pentacon, I have no issues and focusing works as expected.

Is there anybody out there with a suggestion what might be wrong, or what I might be doing wrong?

 

[Marco Gilardetti]

You mean that the image looks perfectly in focus on the groundglass, but then it is not so on the film, correct? If it is so, I cannot think to anything else than "something" displacing or deforming the mirror while the lens is in place. Is there some cam or part of the lens that protrudes enough to touch the mirror, perhaps? Or anything that may hinder the mirror to return correctly in place after shooting?

 

[Tor-Einar Jarnbjo]

You mean that the image looks perfectly in focus on the groundglass, but then it is not so on the film, correct?

Yes. I mean, the Pentacon has no autofocus, so the only possible way to focus is to use the groundglass. When testing with the Biometar on the Nikon DSLR, I have tried both to focus 'by sight' and using the Nikon's autofocus aid (using MF lenses, it shows arrows in the viewfinder indicating in which direction to focus) and both methods work as they should.

If it is so, I cannot think to anything else than "something" displacing or deforming the mirror while the lens is in place. Is there some cam or part of the lens that protrudes enough to touch the mirror, perhaps? Or anything that may hinder the mirror to return correctly in place after shooting?

I hadn't thought about that, but there is nothing in the way. When focusing (shutter cocked, mirror down). there is about 18mm clearance between the back side of the lens and the frontmost edge of the mirror. After exposure (mirror up), there is still about 2mm room between the lens and the mirror. Except for the pin controlling the automatic diaphragm, there is no part of the lens intruding into the camera body. The base of the lens mount is level with the inside of the camera body.

 

[Marco Gilardetti]

And what happens to the mirror if, for one reason or another, the auto diaphragm pin cannot be pushed all the way in (or it's too hard to be pushed)? Will the mirror come in correct place anyhow, or will it be forced to stay in a wrong position / deformed etc.?

Although it looks less likely, it could also be caused by "something" directly acting on the groundglass - that is some part lifting it / displacing it / pushing on it etc.

 

[AgX]

A displacement (swing) of the mirror would result in gradual distribution of unsharpness. Thus checking with a perpendicular, flat plane would show such.

But a lens would rather bear pressure on the mirror than lifting it up. And as a mirror already is at his arrest, how could it be made to swing to the rear? And then it could not swing up to give way for the film plane anyway...

Mirrors typically swing freely once released from the up-position. Any blocking of the release procedure would not affect the travel of the mirror once released. (EDIT: I have automatic-return mirrors in mind.)

 

[Marco Gilardetti]

I owned a Pentacon Six circa 20 years ago, and if my memory doesn't fail its mirror is not of the automatic-return spring type. So all you have written that "can't happen", can indeed happen in my opinion.

I agree that the focusing should be tested by framing a plane target, to see if the focus is even or uneven on the framed area.

 

[Tor-Einar Jarnbjo]

And what happens to the mirror if, for one reason or another, the auto diaphragm pin cannot be pushed all the way in (or it's too hard to be pushed)? Will the mirror come in correct place anyhow, or will it be forced to stay in a wrong position / deformed etc.?

If the auto diaphragm pin could not be pushed all the way in, the first thing I would definitely have noticed is that the aperture wouldn't have stayed open when focusing and steppng down the lens. But there is ample clearance between the diaphragm pin lever and the mirror. When cocking the camera, the diaphragm pin is pressed (the camera lever moves forwards) before the front edge of the mirror passes behind the lever. During normal operation, there is a 9mm clearance between the back side of the lever and the front edge of the mirror. The lever moves back and forth only about 5mm, so had the pin been stuck, there would still have been about 4mm clearance between the lever and the mirror.

Although it looks less likely, it could also be caused by "something" directly acting on the groundglass - that is some part lifting it / displacing it / pushing on it etc.

And what exactly could that 'something' be? As I wrote, the problem only shows with a specific lens model (at least three other lenses work perfectly with the camera) and with the groundglass being so far away from the lens mount, I don't see any possibility at all how a specific lens model can displace the groundglass.

 

[Tor-Einar Jarnbjo]

Addition: The Pentacon Six mirror 'rests' in the open/upper (adjacent to the groundglass) position and is not lowered until the film transport is operated. I just checked the camera and the film transport lever is connected mechanically to the mirror in such a way, that the lever is blocked if the mirror is jammed. When in the lower position (after transporting the film), the mirror is locked in position and cannot be moved.

 

[shutterfinger]

I'm not familiar with your camera or lens but know how to find such problems.
Does the "new" Biometar fit as tightly on the Pentacon Six as the lens that have no focus issues?
Does the "new" Biometar fit as tightly on the Nikon adapter as it does on the Pentacon Six?
What is the overall length of the old Biometar and the new Biometar measuring from the front edge of the lens barrel to the edge of the lens mount with the lens set to indicated infinity on their focus scales? Use a caliper to measure.

 

[Tor-Einar Jarnbjo]

I'm not familiar with your camera or lens but know how to find such problems.
Does the "new" Biometar fit as tightly on the Pentacon Six as the lens that have no focus issues?

Just to be precise and repeat myself: I have one specimen of the oldest version of the Biometar and by now two specimens of the newest version. All fit tightly in the lens mount and there are no obvious mechanical damage or clearance between any parts. The focusing problem is consistent with both the specimens of the new version.

Does the "new" Biometar fit as tightly on the Nikon adapter as it does on the Pentacon Six?

Yes, also there, all lenses mount tightly. The mechanics of the Pentacon mount are quite simple and fool-proof. The lens is without mechanical resistance just 'dropped' into place and then a ring on the camera is turned about a quarter rotation, while ithe skewed inner side of this ring grips around three tabs around the outer edge of the lens.

What is the overall length of the old Biometar and the new Biometar measuring from the front edge of the lens barrel to the edge of the lens mount with the lens set to indicated infinity on their focus scales? Use a caliper to measure.

The old BIometar measures 35.2mm and both new Biometars measure 35.0mm from the front edge of the lens to the mount flank, which alignes with the front of the camera body (I hope that was what you meant). In addition, the lenses protrude an additional 12.1mm (old version) or 12.2mm (new version) from this flank into the camera body. My Sonnar 180, with which I have no focusing issues, protrudes 12.5mm into the camera body. I am not sure if any difference is relevant though. There have been several complete redesigns of the housing between the two versions and even the optical calculation is said to be different. It is not obvious from the poor scans, but judging from paper prints I have made of the focus chart test shots, the new Biometars are clearly sharper than the old Biometar.

 

[Theo Sulphate]

At this point we have:

Old lenses on Pentacon = accurate focus
New Biometars on Pentacon = front focus

Old lenses on DSLR with adaptor = unknown
New Biometars on DSLR with adaptor = accurate focus

Also, we know viewfinder focusing is accurate (lens to groundglass path = lens to film plane path) because the prints show the old lenses focus accurately when using the viewfinder.


Although I'm curious where the focus would be for your old lenses on the DSLR and I suspect they would have back focus, what is truly puzzling is why your Pentacon viewfinder shows in-focus with both old and new lenses, but only the new lens has incorrect focus.

 

[AgX]

The only moving part in the optical system (as we can exclude extreme play in the helicoid or the mount) is the mirror.
But so far we have not established whether there is some swing when resp. lenses are mounted, let alone how such swing could be induced by those lenses.

 

[shutterfinger]

Each focal length will have its own back focus distance therefore comparing different focal length depth into the camera body tells nothing useful. The same focal length but different maximum aperture may have different back focus also.
Being the lens barrel has changed between the New and Old Biometars the elements may have changed as well as the back focus.
Next test is to measure from the film/image plane to the rear center of the 80mm Biometars.

I suspect the New Biometars are .01 to .1 millimeter farther from the film plane of the Pentacon than they should be.

 

[Tor-Einar Jarnbjo]

Although I'm curious where the focus would be for your old lenses on the DSLR and I suspect they would have back focus, what is truly puzzling is why your Pentacon viewfinder shows in-focus with both old and new lenses, but only the new lens has incorrect focus.

Sorry for not mentioning that right away, but I have tried all five of my Pentacon lenses with Adapter on the DSLR and in that combination, they focus just as they should.

So, your overview should read:

Old lenses (old Biometar 80, Sonnar 180 or Orestegor 500) on Pentacon = accurate focus
New Biometars on Pentacon = front focus

Any of the lenses on DSLR with adaptor = accurate focus

And yes, I realize perfectly that the problem is puzzling, which is why I gave up finding a solution myself and was hoping that any of the readers here might have a suggestion.

I didn't initially want to go into details on anything I believe to have ruled out, but there have been some speculations in the other replies here, so perhaps I should elaborate a bit on that:

As you point out in your reply, I also assume that there is nothing wrong with either the mirror, the groundglass placement or the film plane. Otherwise, I wouldn't have been able to focus properly with the old lenses. There is also nothing on the new Biometars protruding any further into the camera body, than on the other lenses. The mirror is pulled down when operating the film transport lever, and as I found out today and wrote earlier: Had the mirror collided with anything on the rear side of the lens, then the transport lever would have blocked.

On all the test images in my initial post, I used the smaller '0' marking in the provided crop to focus on. AgX and Marco Gillardetti earlier discussed that the issue might be that the focal plane is not flat. And yes, at least when focusing on a very short distance, both the old and the new Biometars have a slightly, but noticably curved focal plane (the focal point is closer to the camera towards the edge of the frame). This can be seen both with the old Biometar and when using the lenses on the DSLR as well. But, since neither a skewed, nor a curved focal plane can IMHO explain why the very exact point I focused on, and which appear in focus on the groundglass, is not in focus on the exposure.

 

[AgX]

Each focal length will have its own back focus distance therefore comparing different focal length depth into the camera body tells nothing useful. The same focal length but different maximum aperture may have different back focus also.
Being the lens barrel has changed between the New and Old Biometars the elements may have changed as well as the back focus.
Next test is to measure from the film/image plane to the rear center of the 80mm Biometars.

I suspect the New Biometars are .01 to .1 millimeter farther from the film plane of the Pentacon than they should be.

If there is some issue with the back focus distance, that would not produce this error, as the focusing nevertheless would be equal on groundglass and film. As long as all lenses were used with similar apertures.
If the "erroneous" lenses were used with wider apertures than the others, then an focusing (wide open) error would show stronger at these lenses. But such focusing error likely would show throughout the whole photography of the OP and not just in these cases.

But checking the eyepiece would not do harm nevertheless.... Maybe it needs a bit of correction.

 

[Berri]

If you set the focusing target precisely at the minimum focusing distance, with the camera on a tripod, do you get to see a sharp image with both your biometar (old and new)? Have you tried to look at the projected image on the focal plane to see if it is sharp when the lens is set to produce a sharp image on the focusing screen? I own a P6 and a Biometar, but mine focuses right.

 

[Tor-Einar Jarnbjo]

I suspect the New Biometars are .01 to .1 millimeter farther from the film plane of the Pentacon than they should be.

That might be, but then they are also .01 to .1mm farther away from the groundglass and any difference would have absolutely no impact on the ability to focus properly. I have extension tubes up to 127.5mm for the Pentacon (for macro photography) and moving the optical elements of the lens farther away from the camera body, does of course not prevent me from focusing properly on the groundglass.

 

[itsdoable]

The Pentacon 6 mount works with the aperture normally closed, and the pin must be depressed fully to open the aperture.

The pin actuation plate in the camera body is linked to the mirror action, that is as the mirror goes up, the plate retracts, allowing the aperture to close.

So, one possibility is that the pin on the new Biometars are a little aft of the older lenses, and the spring system in the actuating plate in the body is not compensating for this, and lifting the mirror.

Its also possible that an element is loose and is moved when the pin is depressed. Less likely as it seems to happen on 2 lenses?

Or something else....

 

[shutterfinger]

You may have to put a temporary ground glass at the film plane to find the difference error between the new and old.
The new design may not be compatible with your body. If your body was at fault then all lens would be off.
Always use infinity for alignment testing. The farther the infinity target is from the camera the more accurate the alignment. Always test with the lens wide open. Use a good loupe on the ground glass for the most accurate focus reading.

 

[shutterfinger]

So, one possibility is that the pin on the new Biometars are a little aft of the older lenses, and the spring system in the actuating plate in the body is not compensating for this, and lifting the mirror

Check the mirror position from the film plane/camera back with each the old and new versions of the lens attached.

 

[Tor-Einar Jarnbjo]

If you set the focusing target precisely at the minimum focusing distance, with the camera on a tripod, do you get to see a sharp image with both your biometar (old and new)?

I am not sure, but I can try and get back to you. Just out of curiosity: What knowledge to you expect to gain from such a test, which may explain the problem?

Have you tried to look at the projected image on the focal plane to see if it is sharp when the lens is set to produce a sharp image on the focusing screen? I own a P6 and a Biometar, but mine focuses right.

Sorry, but here I don't understand what you are suggesting. Do you mean looking at the 'projected image on the film plane'? That may make sense, but I have absolutely no idea how to do so, and even if I somehow managed to put something, on which the image could be projected where film is supposed to go, I would expect that projection to show exactly the same front-focus issue as the exposure on 'real' film.

 

[Tor-Einar Jarnbjo]

You may have to put a temporary ground glass at the film plane to find the difference error between the new and old.

I am sorry shutterfinger, but I can't see how your suggestions here contribute in any way to solve the problem.

The new design may not be compatible with your body. If your body was at fault then all lens would be off.

Of course there is some kind of compability issue between the new Biometar and my Pentacon body. That, I already stated clearly in my initial post. What I am trying to find out is what the issue is and how to solve it. The CZJ lenses were both made and are still regularly used on and for the Pentacon Six cameras, so there is no reason to assume that it was not the intention of the manufacturers that this combination should work.

Always use infinity for alignment testing. The farther the infinity target is from the camera the more accurate the alignment. Always test with the lens wide open. Use a good loupe on the ground glass for the most accurate focus reading.

I am not sure what kind of alignment testing you are speaking of here and how it relates to my focus problem. Since I obviously manage to focus properly with the old lenses on the Pentacon and with all the MF lenses on the DSLR, I am not sure why you seem to think that I need to be thought how to manually focus :-/

Check the mirror position from the film plane/camera back with each the old and new versions of the lens attached.

I have no access to the inside of the camera body when a lens is mounted and can not make any measurements there, but I already wrote several posts back, that there is ample clearance between the lens and the mirror, that the film transport will block if the mirror collides with anything when it swings down and that after transporting the film, the mirror is locked in place and can't be moved.

 

[Tor-Einar Jarnbjo]

But so far we have not established whether there is some swing when resp. lenses are mounted, let alone how such swing could be induced by those lenses.

Several of my findings earlier today rule IMHO this option out, but please make a precise suggestion what I can check, if you think the mirror might be relevant.

- Even in the upper position (during exposure), there is about 2mm clearance between the front edge of the mirror and the back side of the lens.
- In the lower position (when focusing), there is about 9mm clearance between the front edge of the mirror and the back side of the lens.

After each exposure, the mirror 'rests' in the upper position, adjacent to the groundglass. The image is still projected on the inside of the shutter curtain, the view finder remains dark. First when I operate the transport lever, several things happen simultaneously. The film is moved to the next frame, the shutter curtain is moved and cocked, a lever inside the camera body depresses the automatic diaphragm pin on the lens and the mirror is moved to its lower position and locked firmly in place. Even if the diaphragm pin, which is the part of the lens protruding farthest into the camera body, can't be pressed, there is still more than enough clearance behind the lever for the mirror to swing freely. The mechanical coupling between the transport lever and the mirror is so, that the transport lever blocks if the mirror collides with anything and can't move freely.

 

[shutterfinger]

I am sorry shutterfinger, but I can't see how your suggestions here contribute in any way to solve the problem.

enjoy your guess work, you might eventually stumble across the cause.
A ground glass at the film plane and the viewfinder should match on all SLR cameras.

 

[Tor-Einar Jarnbjo]

enjoy your guess work, you might eventually stumble across the cause.
A ground glass at the film plane and the viewfinder should match on all SLR cameras.

You obviously don't read what I have been writing here all day. I am out of suggestions what may cause the problem and I am asking here for help, since I have nothing more to guess on. Trying to teach me how to use MF lenses, pointing me in completely wrong directions (as also AgX pointed out to you, a slight misalignment of the lens simply can't be the cause of the problem) or typing down obviousnesses like that the same image should be projected on the film plane and in the viewfinder on any SLR is just wasting my time.