Pentacon Six & Biometar 80 focus problem

[Tor-Einar Jarnbjo]

I am not going back to read this whole thing, but what kind of ground glass do you have? If you have a fresnel between the lens and the actual focusing surface that could account for the focus shift. I'd check that. The preferred arrangement would be lens/ground glass/fresnel. The fresnel could be incorporated in the screen as well, so make sure the screen isn't in upside down.

It is just a very simple piece of glass, matted on one side. No Fresnel lens.

 

[Theo Sulphate]

I believe this summarizes the situation:

 

[Marco Gilardetti]

Now that this phenomenon has an official name and also some technical sketches, you should indeed propose it as a unique feature. I could list countless examples of obvious engineering flaws which are commercially proposed as "special features", this may very well be one of those and make you rich.

Seriously, to check if the image moved over the groundglass was another good test that I couldn't think of, and why it also didn't explain what is happening here remains a mistery to me.

 

[Fin]

Right... Just read through this lot. Can I summarise that:

Old lens focuses correctly on both Pentacon Six and DSLR
New lenses focus correctly on DSLR but not on the P6?

If the above is correct, it means that there is a problem between the new lenses and the mount on the camera. On a P6, the bayonet is on the lens but it is clamped down to the camera with a rotating ring. If there is a slight difference (and quite possibly some wear) between the old lens and the new ones, which may not affect the DSLR adaptor but will mean the lens is held slightly out of position on the P6.

 

[Tor-Einar Jarnbjo]

Right... Just read through this lot. Can I summarise that:

Old lens focuses correctly on both Pentacon Six and DSLR
New lenses focus correctly on DSLR but not on the P6?

Yes.

If the above is correct, it means that there is a problem between the new lenses and the mount on the camera. On a P6, the bayonet is on the lens but it is clamped down to the camera with a rotating ring. If there is a slight difference (and quite possibly some wear) between the old lens and the new ones, which may not affect the DSLR adaptor but will mean the lens is held slightly out of position on the P6.

It has already been suggested several times that the new lenses might not sit properly in the camera mount and I have probably each times asked, if that should be the case, how it can explain the problem I am having. IMHO, there is absolutely no need for a lens to sit exactly in a specific position to project the same image both onto the groundglass and onto the film. There are several situations, where a lens is even deliberately moved out of its intended position, e.g. when using extension tubes or tilt/shift adapters. Even if the lens is intentionally moved out if its default position, if everything else is as it should, the optical path from the lens to the grounglass and the path from the lens to the film plane remains the same and you can still manually focus based on the projection onto the groundglass.

I have already asked, but I can ask again: I my reasoning is wrong and the problem could be caused by the lens not sitting properly in the mount, can you please explain what exactly is wrong in my reasoning?

 

[Mozg31337]

Tor: I will be driving through Germany at the end of December and I will take with me my Pentacon Six and both of my lenses, the 80mm f2.8 and 50mm f4, which are of the new build type. I will not have much time to run the tests unfortunately as I will be passing Germany on the way to Poland and will likely only spend a night in Germany, but you are welcome to run a few tests with my camera and your lens and my lenses and your camera. I am not sure where I will be spending the night as I've not booked a hotel, but most likely it will be somewhere around Berlin.

 

[Tor-Einar Jarnbjo]

Tor: I will be driving through Germany at the end of December and I will take with me my Pentacon Six and both of my lenses, the 80mm f2.8 and 50mm f4, which are of the new build type. I will not have much time to run the tests unfortunately as I will be passing Germany on the way to Poland and will likely only spend a night in Germany, but you are welcome to run a few tests with my camera and your lens and my lenses and your camera. I am not sure where I will be spending the night as I've not booked a hotel, but most likely it will be somewhere around Berlin.

Thank you for your offer, but I will not be in Germany myself around the end of December.

 

[shutterfinger]

If it were in my possession I would buy a piece of common size ground glass off ebay, cut it to fit the film plane, secure it in place, attach the problem lens, mount it on a tripod, and film plane focus on an infinity target at least 1 mile/1.6 kilometers away then adjust the view screen to match the ground glass at the film plane using a 4x to 8x loupe to check focus on each.
Next I would mount the older lens of the same maximum aperture and film plane focus it then check the viewfinder for accuracy/error.

Focal length divided by entrance pupil diameter equals f stop. A marked 80mm lens may actually be 78mm to 82mm in focal length. At wide maximum apertures differences in focal length will show up quicker than smaller maximum apertures.
Focal length divided by f stop number equals entrance pupil diameter.
78/2.8=27.857 mm; 80/2.8=28.571 mm; 82/2.8=29.2857 mm. Enough difference that if the view screen is set to an extreme a lens of the same marked values at the opposite extreme may show errors.
78mm f2.8 focused at 5 meters has a DOF of 4.495 meters to 5.632 meters; focused at 1609 meters DOF is 42.69 meters to infinity, circle of confusion .049 mm.
80mm f2.8 focused at 5 meters has a DOF of 4.518 meters to 5.596 meters; focused at 1609 meters DOF is 44.849 meters to infinity; circle of confusion .049mm.
82mm f2.8 focused at 5 meters has a DOF of 4.539 meters to 5.564 meters, focused at 1609 meters DOF is 47.05 meters to infinity; circle of confusion .049mm.

If resetting the view screen to match the newer lens at the film plane resulted in the older f2.8 lens being off then check with the f4 and f5.6 lens at the film plane for accuracy.
If all older lens are off then reset the view screen to the old f2.8 lens.
I use infinity for setting up cameras as it is more accurate than using closer distances although some will use closer distances for convenience and risk inducing error into their setup.
I prefer a celestial object to a building or mountain if it is available.

 

[Tor-Einar Jarnbjo]

If it were in my possession I would buy a piece of common size ground glass off ebay, cut it to fit the film plane, secure it in place, attach the problem lens, mount it on a tripod, and film plane focus on an infinity target at least 1 mile/1.6 kilometers away then adjust the view screen to match the ground glass at the film plane using a 4x to 8x loupe to check focus on each.
Next I would mount the older lens of the same maximum aperture and film plane focus it then check the viewfinder for accuracy/error.

I can try that if you first care to explain how I can circumvent the laws of physics. I have no doubt that I can fix the focussing issue with the new lenses by lifting the groundglass a bit, increasing the distance between the mirror and the groundglass. I have also no doubt, that doing so will introduce exactly the same problem, but in the opposite direction, with my old lenses. If you believe that not to be the case, please explain why.

Focal length divided by entrance pupil diameter equals f stop. A marked 80mm lens may actually be 78mm to 82mm in focal length. At wide maximum apertures differences in focal length will show up quicker than smaller maximum apertures.
Focal length divided by f stop number equals entrance pupil diameter.
78/2.8=27.857 mm; 80/2.8=28.571 mm; 82/2.8=29.2857 mm. Enough difference that if the view screen is set to an extreme a lens of the same marked values at the opposite extreme may show errors.
78mm f2.8 focused at 5 meters has a DOF of 4.495 meters to 5.632 meters; focused at 1609 meters DOF is 42.69 meters to infinity, circle of confusion .049 mm.
80mm f2.8 focused at 5 meters has a DOF of 4.518 meters to 5.596 meters; focused at 1609 meters DOF is 44.849 meters to infinity; circle of confusion .049mm.
82mm f2.8 focused at 5 meters has a DOF of 4.539 meters to 5.564 meters, focused at 1609 meters DOF is 47.05 meters to infinity; circle of confusion .049mm.

That may all be true, but what relevance does it have to my problem? Any SLR with interchangeable lenses is obviously constructed in such a way, that the marked or actual focal length and/or entry pupil diameter are irrelevant for the concept of the groundglass viewfinder to work. Otherwise, you would have been required to adjust the groundglass on any SLR each time you change the lens.

 

[shutterfinger]

The sun when viewed through a filter that protects your optical sensors from its harmful rays makes a very good infinity target. The edge is very sharp and with some lens you can see the spots on it.
Take two lens of exact focal length (optically measured in a lab) , one an f4 the other an f2. Start with the f4 lens and set the ground glass infinity then switch to the f2 lens without changing anything. The f2 lens will be slightly soft until the infinity position is corrected with it then go back to the f4 lens, it will still be sharp.

 

[Theo Sulphate]

I've often used the Moon for an infinity test. While she may be a harsh mistress, she may be viewed directly.

 

[Neil Grant]

If the above is correct, it means that there is a problem between the new lenses and the mount on the camera. On a P6, the bayonet is on the lens but it is clamped down to the camera with a rotating ring. If there is a slight difference (and quite possibly some wear) between the old lens and the new ones, which may not affect the DSLR adaptor but will mean the lens is held slightly out of position on the P6.[/QUOTE]
..it's called a breech-lock bayonet. Slack should non-existent from new, and wear and tear 'taken-up' by turming the locking ring a bit further. They were more common in the past, with maybe the Mamiya RB/RZ series the final adherents. I'm wondering about how a focus screen displays abberations in the lens (and it's influence on how we judge focus), and generally camera body dimensions and collimation of lens.

 

[Theo Sulphate]

... it's called a breech-lock bayonet. Slack should non-existent from new, and wear and tear 'taken-up' by turming the locking ring a bit further.

Remember that when the image is in focus on the groundglass, then with an old lens the image is also in focus at the film plane ...

BUT...

... with a new lens, when an image is also in focus on the groundglass, the image is NOT in focus at the film plane.

Because the images are in focus on the groundglass with either old or new lenses, any difference in the mounting of old vs. new lenses is irrelevant: the question is why an in-focus image on the groundglass produces an out-of-focus image at the film plane ONLY with the new lenses.

My best theory was that the process of mounting a new style lens distorts the P6 body by mechanical force - making it shorter by a fraction of a millimeter. However I've been told that the P6 is as strong as a Panzerkampfwagen.

My second best theory involves Poltergeisten.

 

[Neil Grant]

My best theory was that the process of mounting a new style lens distorts the P6 body by mechanical force - making it shorter by a fraction of a millimeter. However I've been told that the P6 is as strong as a Panzerkampfwagen.

My second best theory involves Poltergeisten.[/QUOTE]

..I did mention body dimensions (and lens collimation), and you've tied this to changes brought about by lens mounting. The screen and the film plane are hopefully par-focal, but there are no guarantees. Maybe something has been 'bounced' at some point in it's life. My recollections of the P6 are bulk, a tiny vf window and very slow travelling FP shutter.

 

[Tor-Einar Jarnbjo]

My recollections of the P6 are bulk, a tiny vf window and very slow travelling FP shutter.

Then you must be remembering a different camera. The Pentacon Six comes with both waist-level and/or prism viewfinders, the screen is of course much larger than on any 135 SLR and the shutter can do 1/1000s if it is a sunny day.

 

[Neil Grant]

...this one only had a prism - with a smaller eyepiece than I was expecting. When I say 'slow-travelling FP', I'm refering to the velocity of the blinds (strikingly slower than my M 645). I believe the sych speed was 1/30 or even longer. It had a 3 lens kit with a Flek 50, an 80 and a very large 180 2.8 -all 'Zebra'. Its breech lock bayonet was only the second one I'd come across on a roll film camera. (the other was an M RB) Can only be a P6. Anyway one day it just disappeared, only its Aluminium case remaining.

 

[John Koehrer]

One of the great advantages to a breech lock mount lens is there's no wear on either the body or lens mount. Any wear
is on the back/inside of either piece.

 

[AgX]

With a SLR mount wear is of lesser importance. It would affect the infinity setting or settings where an exact perpependicular plane of focus is needed.

 

[Theo Sulphate]

My hope is that maybe soon these lenses can be mounted on a different P6 and we can observe whether the same effect occurs.

At least doing so will provide another clue to the puzzle.

 

[Tor-Einar Jarnbjo]

I've been quite busy the last weeks and didn't have time to visit my trusted repair shop until today. The good Mr. Wiener had of course both a spare Pentacon Six body and his own Biometar lens at hand to do some preliminary, quick checks and comparisons in his optical test bench and is convinced that the only issue is a misaligned groundglass in my camera body:

- In his test bench, he was able to focus properly on his body with both my old and my new Biometars, his assesment is that the lenses are in a very good to perfect condition without any faults.
- When he tested my body with his Biometar and my Biometars, he allegedly found the same focusing error and couldn't understand how I have ever been able to focus properly with that body.

I am not really convinced, but I left the camera there for him to adjust the groundglass and it will be ready some time next week. Perhaps he is actually familiar with exorcism and camera poltergeists, but doesn't want to be open about it, so he tries to distract me with the groundglass.

 

[moto-uno]

I have to say that this has been quite the journey ! Here's hoping it works as it should upon return.( An exhausted follower ), Peter

 

[Theo Sulphate]

If a misaligned groundglass is indeed the problem, then this illustrates the value of "an impartial observer".

In engineering, sometimes the designer knows there is a fault in the circuit or the program, but can't find it because the engineer is "too close" (too familiar) with the issue. In such cases, often the problem is found when an engineer who is uninvolved in the implementation of the design starts to look at it.

So, in your case, you could not detect that a misalignment existed.

 

[Tor-Einar Jarnbjo]

If a misaligned groundglass is indeed the problem, then this illustrates the value of "an impartial observer".

So, in your case, you could not detect that a misalignment existed.

But I am unfortunately not convinced. I have throughout this thread posted many test exposures (starting with the first message), showing that at least when I am using them, the old lenses are focusing properly, while the new Biometar is not.

 

[Marco Gilardetti]

I also think that the "misplaced groundglass" theory is inconsistent with the findings. However, let's see what the "exorcist" will come up with. As said, this is of course matter for initiates, he couldn't tell you the whole truth, your brain and heart would not be able to handle it.

 

[shutterfinger]

But I am unfortunately not convinced.

and that's the problem, you've convinced yourself that a screen alignment is not the problem.
Its been explained in posts that no 2 lens are identical even though they have the same markings.
When trouble shooting a problem one makes no assumptions or precludes any possibility. Only when the problem is fixed was the solution definite.
Now there is likely a short between your ears but it could be an open.