How to adjust Inf. Focus Limit -- Zeiss Biotar 58mm f/2

[ColSebastianMoran]

I have a Zeiss Biotar 58mm f/2 from an Exacta.

Not pretty, but clean inside and focus is smooth. I have it mounted on an adapter for Sony E mount.

Question: Limit on distance scale is exactly infinity, but lens is actually focused at around 15 feet. How can I adjust the focus limit so it reaches infinitey. It's OK for the lens to focus a bit past infinity and I don't care if the focus scale is precise; I never scale-focus, always focus peaking.

 

[Livesteamer]

I am not an expert but I think you need to move the optics closer to the sensor. I think there is a ring in the back to undo and the optics come out.

I like the 58mm Biotar. I have them in Exakta, Praktina and M42 mount. Joe

 

[rick oleson]

If you're focusing at 15 feet when set at infinity, the error calculates out as about 3/4mm (.030"), which is the amount that you need to move the lens to the rear. A quick way to check this is to measure the overall length of the lens when set at infinity, and again when set at 15 feet - the difference is the amount you need to correct. You might be able to get this amount of adjustment by loosening the focus ring and shifting its position. Another possibility is that there may be shims inside the lens, between the lens block and the barrel,that could be removed to shift the optics rearward a bit. Is this lens similarly off target when mounted on an Exakta camera body, or is the error in the adapter?

 

[ColSebastianMoran]

Joe, yes, I have liked this lens, shot it on a Praktina, way back.

Rick, thanks for comment. I only plan to use this lens with focus peaking on a Sony. I'd just like to move the focus limit so it focuses beyond infinity on this adapter. But, I can't see even how to get the mount off the back of the lens or loosen the focus ring.

The only screw visible on the mount is the protruding screw for the mounting lock. No screws visible on the barrel. Rearmost lens group has notches for a spanner. That's all I can see.

 

[Crazy Fedya]

I think, it is the adapter, that makes it nor reach infinity. May be you should look for another one.

 

[ColSebastianMoran]

Thanks, Sam. I might.

In meantime, anyone know how to adjust the focus stop? Or is this fixed forever at the factory?

 

[Crazy Fedya]

You should be able to see a few retaining rings on the back of the lens. One of them holds optical block in the focusing mount. Once you remove it, it should be visible whether it is adjustable. May be just a shim to remove.

 

[ColSebastianMoran]

Sam and Rick -- I decided maybe you are right that it's the adapter. Ordered a more expensive adapter; will see if it solves the problem.

Thanks again for the comments.

 

[mcfingon]

The lenses I have shimmed are pulled apart from the front of the lens. Typically, they separate at the the aperture control ring. Some lenses are held together by a small set screw in the aperure ring area, so it would pay to find a tear-down article on your lens before you get too violent with it. Picture attached of Canon Serenar separated at aperture ring, as do the Leica Summitar and Jupiter-3 and Jupiter-8 lenses.

 

[ColSebastianMoran]

Find a tear-down article

Googling "tear-down biotar" I quickly found this article. OK, that's scary enough for me; I'll try the new adapter first.

 

[ColSebastianMoran]

Followup... I have a new more expensive adapter. Same focus situation, lens set at infinity, focus seems to be around 15'.

The teardown is just too scary for me. I'm going to live with it, my intention for this lens is portraits.

Thanks, everyone, for the comments.

 

[L. M. Tu]

I don't know which adapters you've got, but my Metabones can be adjusted for infinity by loosening a screw and then rotating the rear mount of the adapter back and forth.

 

[Sarcophilus Harrisii]

First:
Forget this adapter nonsense for five minutes, and find a decent Exakta to mount it onto, and determine what the focus is like on that. It's what the lens was made to fit. Even if you need to beg or borrow one from a shop. Are you anywhere near Vince? I believe he has a few trustworthy bodies. Then at least you will know where the problem lies: adapter or lens.

Second:
If it's a fault in the lens my two cents worth is that perhaps the lens has been assembled with the helicals one thread off. That would probably do it. I've been inside a few of these CZJ lenses including three Biotars, in M42, Exakta and Praktina mount, a Biometar in Praktina mount and probably a few more I can't recall. Made a few mistakes, got the re-assembly wrong, learned the hard way and just had to keep going, until it was right again. So I do speak from some experience of their interior specifications.

One thing I can do from afar: if you can give me an accurate height measurement of the lens from bayonet (or rear lens mount, whichever protrudes furthest) to the top of the filter ring threads, I can take a measurement of my own Exakta Biotar (which focuses correctly on a few bodies) with my vernier calipers. If the lens focus is good the height should be the same (allowing for variations in lens version of course). Your version looks like a pre-set one. Mine as I recall is an auto stop down type, but, if you can obtain the dimension of your own, I'll can probably also measure the distance from the rear of the bayonet mount to the rear glass for you if it helps. If yours is in the ballpark (within a decimal place or two) it's probably the adapter at fault.

Lastly:
There is also a link to the lens blueprints for CZJ lenses in the Members sub forum in a thread I started. If you're up to it this may help you pinpoint assembly errors.
Cheers,
Brett

 

[Grytpype]

That looks like the pre-set aperture Biotar. If it is built the same as the M42 version it is extremely difficult to dismantle, because the slotted ring you have to undo from the back is larger than the inside diameter of the mount (and the Exakta mount also has a smaller throat than the M42). You can just see one of the slots in this ring in the picture below of the M42 lens. The semi-automatic is much easier! The infinity focus is set by a spacer between the optics barrel and the focus mount.

​

Two equally wrong adapters does sound a bit like a mis-assembled lens.