Pentacon Six & Biometar 80 focus problem

[Tor-Einar Jarnbjo]

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 didn't find any square test chart and didn't have time to make my own, but I used the following chart to take (almost) full frame test shots with the old Biometar and one of the new Biometars:

I have cropped the center and each corner of this chart from the test shots done with the following lens and settings (from left to right):

1. old Biometar at f/2.8
2. new Biometar at f/2.8
3. new Biometar at f/5.6

The rectangular test chart was not perfectly centered on the vertical axis, so the lower corners of the test chart were closer to the edge than the upper corners. That might be the reason for the lower corners to be slightly less sharp than the upper corners. The images were scanned directly from the negatives and I don't have the world's best scanner. I have not done any post-processing except for levels, brightness and contrast. The images are not sharpened.

To me, the old Biometar looks reasonably sharp even at f/2.8. The test shot with the new Biometar at f/2.8 is out of focus, but except for the lower corners beind slightly less sharp (possible explanation see above), there is IMHO no indication of a slanted or uneven focal plane. Stepping down the new Biometar to f/5.6 to get the test chart within the depth of field, gives a similar, but sharper result than the old Biometar at f/2.8.

I am not sure what else I can interpret from these test shots. Feel free to speculate!

 

[Theo Sulphate]

... Feel free to speculate!

I'm tempted to blame Climate Change but I think Demonic Possession is more likely.

Have you used a loupe or magnifier while focusing on the groundglass? When I use the built-in one with Hasselblad or Mamiya RB, I sometimes detect a slight misfocus that I had. Why that would happen with only the new lens... ? Have you had another person try to focus on the groundglass with both lenses and does that person's point of focus agree with yours?

For whatever it's worth, I agree with the idea of using a loupe plus attached groundglass at the film rails to confirm that what appears as perfect focus on the finder groundglass is also perfect focus at the film plane. I understand your point that the film itself proves a discrepancy (only with the new lens but not the old), but there is a subtlety here: the loupe plus groundglass at the film plane is a simultaneous check of what you perceive as perfect focus on the finder groundglass and what you perceive on the film plane groundglass. As stated earlier, it eliminates variables. It can confirm assumptions. In the words of Ronaldus Magnus: "Trust, but verify".

 

[shutterfinger]

Where did you get the new version of the lens, a photo dealer or private individual?
Was there any mention of the lens being recently serviced?
I'm beginning to suspect an element is out of position or the spacing between the main groups is off.

I would follow the instructions in this Zeiss Lens Repair page: https://www.zeiss.com/camera-lenses/us/service/repairs/photo-lenses/non-warranty-repair.html
showing them the test results shown in post 51.

 

[Theo Sulphate]

Where did you get the new version of the lens, a photo dealer or private individual?
...

He has two new Biometars that both exhibit front focus.

He has one old Biometar and at least two other older lenses that have no focus problems.

It would be interesting if a second P6 were available to either replicate or refute these results.

 

[MattKing]

Based on the fact that some lenses focus correctly both at the film, and in the viewing system on the camera, while others focus differently at the film and in the viewing system, the problem isn't in the viewing system.
Do the newer lenses move slightly when the internal levers actuate the aperture diaphragm at the time of exposure? If so, the focus may be changing slightly due to that movement.

 

[shutterfinger]

He has two new Biometars that both exhibit front focus.

He has one old Biometar and at least two other older lenses that have no focus problems.

Do we know if both "new" versions produce the exact same focus error? No, just that they are both off.
The fact that several older versions of lens do focus correctly on the camera says the optical path to the view finder and film are the same optical distance.

We do know that the new version has a different barrel length than the older version.
It is possible to get two copies that are incorrectly set after a CLA or element(s) shifted in shipping for one reason or another.
I do not know if the lens element spacing is mechanical or optically set and this information seems to be limited to the manufacturer and a few repair shops.

 

[Marco Gilardetti]

The test shot with the new Biometar at f/2.8 is out of focus, but except for the lower corners beind slightly less sharp (possible explanation see above), there is IMHO no indication of a slanted or uneven focal plane.

Useless to say, the test target *had* to be at the center of the image field and perfectly orthogonal to the lens axis. However, although the charts are a bit hard to read because it was not so, and although they actually seem to indicate that the focal plane *is* slanted, there is anyway in my opinion not enough slant to justify an horrible target image like the center one, where basically nothing is in focus. Should a slanted focal plane be the cause of your nightmares, at least *something* in the image should have been in focus.

Excepting what MattKing suggested (something inside the barrel may move due to the actuation of the diaphragm - perhaps one of the lenses or groups is loose and is displaced when the diaphragm pin is actuated) there is nothing else compatible with the tests done so far that I can think of. You may perhaps try to lock the diaphragm in stopped-down position (as it was done with the "manual" cursor on M42 thread classic lenses), which would permanently displace the lenses, and see if in this case the lens focuses on the groundglass and on the film in the same way. As a side note, I believe that on your DSLR the Biometar diaprhagm actuating pin is permanently pushed in "stopped down" position by the adapter ring, which, should it be the cause, would explain why the Biometar focuses consistently (although perhaps not correctly) on the DSLR.

Should also this test lead to nothing, and should you have the possibility to borrow another P6 (if you're friend enough with a used camera dealer, he will possibly let you borrow one for a day just to do some tests) I would suggest trying the "new biometar" on a different body and see if the same problem happens; and in that case I would limit my optic harem to the "old" types specifically made for the P6.

I'm sorry I don't think I can be of any other help.

 

[BMbikerider]

A long while ago I had similar problems when using an Exacta clone of the same camera. The lens in question was a 80mm F2.8 Biometar. It was bought on the strength that the camera had just had a full check over and adjustment by a well acclaimed repairer in England. It was returned and the lens and camera was checked. They found that the lens had been dis-assembled and put back together with a very thin shim missing (about .025mm) which affected the focusing at all distances which was more obvious at large apertures. This may be the problem with your equipment

 

[Tor-Einar Jarnbjo]

I'm tempted to blame Climate Change but I think Demonic Possession is more likely.

Have you used a loupe or magnifier while focusing on the groundglass? When I use the built-in one with Hasselblad or Mamiya RB, I sometimes detect a slight misfocus that I had. Why that would happen with only the new lens... ? Have you had another person try to focus on the groundglass with both lenses and does that person's point of focus agree with yours?

Yes and yes. The Pentacon Six has a reasonable built-in loupe in the view-finder and I have also had other persons confirming my opinion of 'correct focus'.

For whatever it's worth, I agree with the idea of using a loupe plus attached groundglass at the film rails to confirm that what appears as perfect focus on the finder groundglass is also perfect focus at the film plane. I understand your point that the film itself proves a discrepancy (only with the new lens but not the old), but there is a subtlety here: the loupe plus groundglass at the film plane is a simultaneous check of what you perceive as perfect focus on the finder groundglass and what you perceive on the film plane groundglass. As stated earlier, it eliminates variables. It can confirm assumptions. In the words of Ronaldus Magnus: "Trust, but verify".

I tried with parchment paper yesterday, but couldn't get any usable readings on it. At least my parchment paper has a far too strong texture or structure to provide a clear enough projection to evaulate focus or sharpness. I tried to get a roll of translucent tape, as somebody else suggested, but I haven't found any. I'll give it another try tonight.

 

[Tor-Einar Jarnbjo]

Where did you get the new version of the lens, a photo dealer or private individual?

One of the new Biometars is from a dealer, the other one from private.

Was there any mention of the lens being recently serviced?

No.

I would follow the instructions in this Zeiss Lens Repair page: https://www.zeiss.com/camera-lenses/us/service/repairs/photo-lenses/non-warranty-repair.html
showing them the test results shown in post 51.

That makes absolutely no sense, since the manufacturer you are linking to has nothing to do with the lenses in question. The lenses are manufactured by Carl Zeiss Jena, which closed their business more than 25 years ago.

 

[Tor-Einar Jarnbjo]

It would be interesting if a second P6 were available to either replicate or refute these results.

That would be at least interesting, but it is unfortunately getting quite difficult to find a well-maintained and working specimen. The original lubrication tends to dry out and unless recently serviced and maintained, most P6s on the used marked have an at least unreliable or stuck shutter.

 

[Tor-Einar Jarnbjo]

Useless to say, the test target *had* to be at the center of the image field and perfectly orthogonal to the lens axis. However, although the charts are a bit hard to read because it was not so, and although they actually seem to indicate that the focal plane *is* slanted, there is anyway in my opinion not enough slant to justify an horrible target image like the center one, where basically nothing is in focus. Should a slanted focal plane be the cause of your nightmares, at least *something* in the image should have been in focus.

I know that I made a mistake when taking the test shots. The reason is that the P6 viewfinder only shows a cropped area of the entire frame and the view is not even properly centered. I tried to compensate for that when composing the test shots, but my brain worked in reverse and I shifted the test chart in the wrong direction, moving it yet farther away from the center of the frame. I am not sure why or how you think that the test shots indicate that the focus plane is slanted. As I already wrote earlier, the Biometars have a slightly curved focal plane, but that is a 'feature' of the old lens design and apply both to the old as well as to the new Biometars. Even with the lens working as it is expected to do, I would have expected that when shooting a flat test target, that the corners are slightly out of focus, when the centre is sharp. Since the lower corners on my test shots are closer to the bottom edge of the frame than the upper corners are to the upper edge, the curved focal plane would explain why the lower corners are less sharp than the upper corners.

Excepting what MattKing suggested (something inside the barrel may move due to the actuation of the diaphragm - perhaps one of the lenses or groups is loose and is displaced when the diaphragm pin is actuated) there is nothing else compatible with the tests done so far that I can think of. You may perhaps try to lock the diaphragm in stopped-down position (as it was done with the "manual" cursor on M42 thread classic lenses), which would permanently displace the lenses, and see if in this case the lens focuses on the groundglass and on the film in the same way. As a side note, I believe that on your DSLR the Biometar diaprhagm actuating pin is permanently pushed in "stopped down" position by the adapter ring, which, should it be the cause, would explain why the Biometar focuses consistently (although perhaps not correctly) on the DSLR.

The diaphragm pin works the other way. If released, the aperture is directly linked and reacting to the aperture control ring. If the pin is pressed, the aperture stays open, even if the aperture ring is set to a stopped-down value. The Nikon adapter leaves the pin in the releasead position. It is worth a try. I am not aware of any possibility to disable the mechanical coupling for the diaphragm control between the lens and the camera, but I can of course fix the pin in the pressed position and do some new tests and see if focus is better if I both focus and expose with a depressed pin.

Should also this test lead to nothing, and should you have the possibility to borrow another P6 (if you're friend enough with a used camera dealer, he will possibly let you borrow one for a day just to do some tests) I would suggest trying the "new biometar" on a different body and see if the same problem happens; and in that case I would limit my optic harem to the "old" types specifically made for the P6.

Without buying another one, I doubt that I can get hold of a second P6 for further tests. Except for a few very small series with barrel design adapted for other cameras, all CZJ medium format lenses were designed and built especially for the P6, both the old and the new Biometars. Pentacon didn't build any own lenses for their cameras, but always sold them bundled with CZJ lenses, with the Biometar 80 being the standard lens.

 

[Tor-Einar Jarnbjo]

Based on the fact that some lenses focus correctly both at the film, and in the viewing system on the camera, while others focus differently at the film and in the viewing system, the problem isn't in the viewing system.
Do the newer lenses move slightly when the internal levers actuate the aperture diaphragm at the time of exposure? If so, the focus may be changing slightly due to that movement.

No, there is no obvious mechanical problems with the new Biometars.

 

[itsdoable]

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.'

Well, the Pentacon 6 is pretty simple, so if there is no focus shift (never heard of a Biometar that had it), and the lenses and focus screen is secure, the only thing left that can change your focus position between different lenses is the mirror (back to the mirror!). I'll bring it up again because, if the mirror lifted a tiny bit when the new Biometar was mounted, that would cause you to focus closer than normal, which is what you experienced.

 

[Tor-Einar Jarnbjo]

Well, the Pentacon 6 is pretty simple, so if there is no focus shift (never heard of a Biometar that had it), and the lenses and focus screen is secure, the only thing left that can change your focus position between different lenses is the mirror (back to the mirror!). I'll bring it up again because, if the mirror lifted a tiny bit when the new Biometar was mounted, that would cause you to focus closer than normal, which is what you experienced.

I must admit that I haven't tried to break the mirror, but using at least some force, I am not able to lift the mirror with my fingertip when it is locked in the lowered position. The mirror can be pushed down a little bit, but a feather will pull it back into the proper position when the applied force is removed, Thinking about it, I also came to the conclusion (and you seem to confirm that) that if something had pushed the mirror down, the focus shift would have been in the other direction. With the actual focus being in front of what I see on the groundglass, the mirror had to be lifted to be the cause of the problem.

I see now however, that I forgot to mention something when I posted the results from the test shots yesterday. To further try to rule out that something is mechanically interfering with the mirror, I did a few shots of the initial focus test chart with an extension tube between the camera and the lens. The diaphragm pin is operated through the extension tube, but at least the exact same object is now inserted into the camera. Even with the test tube, the same problem remains.

 

[rockpowered]

Yes and yes. The Pentacon Six has a reasonable built-in loupe in the view-finder and I have also had other persons confirming my opinion of 'correct focus'.

I tried with parchment paper yesterday, but couldn't get any usable readings on it. At least my parchment paper has a far too strong texture or structure to provide a clear enough projection to evaulate focus or sharpness. I tried to get a roll of translucent tape, as somebody else suggested, but I haven't found any. I'll give it another try tonight.

You don't need anything all that special. If you look at the attached image you can see how I did it. I took a piece of clear 120 leader and roughed it up with some sandpaper to give it texture and taped it flat to the film gate. As you can see it projects a very clear image. It's best to do this during the day when you have bright lighting as it will be very hard to see your DIY ground glass in dim lighting. In all it took me about ten minutes to set this up. If I wanted to make sure it was absolutely flat I would make a carrier out of some foam core and then tape the film to a mask sized to the film transport and stretch it tight. Then I would place the assembled carrier flat against the film gate.

Edit: Revised text for clarity

 

[shutterfinger]

This is the tape you want https://www.amazon.com/s/ref=nb_sb_ss_c_1_13?url=search-alias=aps&field-keywords=scotch+magic+tape&sprefix=scotch+magic+,aps,505&crid=3TAZVBPQIORB5
Scotch Magic tape not transparent tape as I stated earlier.

That makes absolutely no sense, since the manufacturer you are linking to has nothing to do with the lenses in question. The lenses are manufactured by Carl Zeiss Jena, which closed their business more than 25 years ago.

Jena is where the CZ factory was. Before WWII and after the end of the cold war the factory location was of no consequence, Zeiss is Zeiss.
https://en.wikipedia.org/wiki/Carl_Zeiss_AG

 

[Theo Sulphate]

Totally wild idea here: is it possible that mounting the newer Biometars causes the camera body to flex so that the mirror rests in a higher position?

Flexing isn't impossible: I have heard from repair techs that the Hasselblad 501C body had a flex problem when a flash bracket was installed and that interfered with winding.

 

[rockpowered]

Totally wild idea here: is it possible that mounting the newer Biometars causes the camera body to flex so that the mirror rests in a higher position?

Flexing isn't impossible: I have heard from repair techs that the Hasselblad 501C body had a flex problem when a flash bracket was installed and that interfered with winding.

Like most former Soviet bloc gear this thing is built like a tank. It not something that one would ever describe as delicate, troublesome when they need a CLA and loud always.

 

[Theo Sulphate]

Like most former Soviet bloc gear this thing is built like a tank. It not something that one would ever describe as delicate, troublesome when they need a CLA and loud always.

Well, not the body - if the "mirror box" flexes just a little bit as a consequence of mounting a newer lens, then this may cause the mirror to sit higher.

If the whole thing is built like a Soviet-era T-54, then the quest continues.

If this thread continues much longer, I'll be tempted to get a P6. Note to self: avoid Biometars.

 

[John Koehrer]

On a SWAG is it possible the lens in question and the lens(es) in question are at the opposite extremes of their manufacturing tolerances? There is the slim
possibility that for whatever reason the new lenses were made to a slightly different standard. Not likely I know but we're chasing rainbows anyway.
So you're looking at .0078"=1/64"=.2mm in length.

 

[Theo Sulphate]

...

The fact that several older versions of lens do focus correctly on the camera says the optical path to the view finder and film are the same optical distance.
...

... if ... the lenses and focus screen is secure, the only thing left that can change your focus position between different lenses is the mirror (back to the mirror!). I'll bring it up again because, if the mirror lifted a tiny bit when the new Biometar was mounted, that would cause you to focus closer than normal, which is what you experienced.

The two statements above are the most critical observations. It doesn't matter if the lenses attach by means of a soup can so long as the optical path from lens to viewfinder and from lens to film plane are identical. We know the paths are identical when the old lenses are used. Regardless of barrel or mechanical or optical tolerances of the new lenses, the only thing that will cause an inequality between viewfinder and film plane optical paths is a lifted mirror. This is why I believe the process of mounting the new lenses causes flex in the mirror box.

 

[AgX]

On a SWAG is it possible the lens in question and the lens(es) in question are at the opposite extremes of their manufacturing tolerances? There is the slim
possibility that for whatever reason the new lenses were made to a slightly different standard. Not likely I know but we're chasing rainbows anyway.
So you're looking at .0078"=1/64"=.2mm in length.

Such would be of no effect when focusing on the groundglass, as such eliminates any flange-to-film distance issues caused by the lens.

 

[Tor-Einar Jarnbjo]

To further eliminate some of the suggested causes, I did new tests today. I fixed the diaphragm control pin in the pressed position and mounted the lenses together with an extension tube on the camera.

- Testing with a fixed diaphragm control pin should reveal if the focus shift for some reason is caused by the operation of the pin. With a fixed diaphragm pin, I am not able to stop down the lens. The aperture will always be wide open (f/2.8).

- Using an extension tube will eliminate any possible interaction or incompatibility between the back side of the lens and the mechanics of the camera. The only thin protruding into the camera will be the back side of the extension tube and any chance of mechanical interaction between the lens and the camera should IMHO be completely ruled out.

In my earlier tests, I usually repeated the same shot at different apertures, but since I now couldn't control the aperture, I did a row of test shots, slightly increasing the focus distance for each shot. With the extension tube, the distance readings on the focus ring are of course not correct and I had to move the camera closer to the test target. I started with the focus ring set at 7 meters, moved the camera back and forth to get the test target in focus and then did 12 test shots, while increasing the focus slightly for each shot, reaching a reading of about 15m at the last shot.

Unfortunately, the test shots came out just as I would have expected based on my earlier tests. The first shot (with the target in-focus on the groundglass, the focus ring set to 7m) is clearly out of focus:

I didn't reach optimum focus until shot 10 of 12. Here, the image is clearly out of focus on the groundglass and the focus ring had a reading of about 12m:

Both images are cropped from a 1600 DPI scan of the negative and not post-processed except for adjusting levels, brightness and contrast.

I also managed to get a roll of matte scotch tape yesterday and checked the projection on the scotch tape, when placed in the film plane. As I expceted: When using the old lens, the focus on the groundglass and on the scotch tape matches. When using the new lens, I can see the same front-focus issue on the scotch tape, as I have seen throughout all the other tests.

 

[AgX]

Maybe we should agree on your camera being jinxed. Would make life easier...