Adjusting infinity on Zeiss Biotar 75/1.5

[luisalegria]

The CZJ Biotar 75/1.5 is most often found in Exakta mount, which isn't such a convenient mount to adapt to others as the register distance is a bit short vis-a-vis M42, Pentax K, or Nikon. Thats why people don't manufacture Exakta adapters for these. In order to get mine on Pentax K I had to make my own adapter. Fortunately this isn't such a difficult project.

http://forum.mflenses.com/exakta-pentax-k-adapter-t9009,highlight,exakta+adapter.html

I'm sure something similar could be done for a Nikon mount as well.

This still leaves the lens unable to reach infinity. The mount needs to be shortened by about 2mm.

Fortunately, its possible to adjust the Biotar mount by this much at least. These instructions apply to the more common late aluminum version of the Biotar. Earlier versions may also be adjustable, but I haven't had any to experiment on. .



Loosen the three setscrews that attach the focusing ring. Once loose this can be simply lifted off.




The focusing ring comes off this thick band (the bit thats scratched up). This is part of the outside tube of the helical. Make sure the helical is screwed down as far as it will go.




There are two screws on the sides of the lens mount tube. These actually ride in channels - interruptions - of the helical thread. Unscrew these.




Once these screws are removed, the lens mount tube can be screwed on or off. It will be a bit more than 2mm short of the bottom of its thread, so it can be screwed in deeper, shortening the whole mount. Screw it down to the bottom, and then back it off (less than 1/2 turn) until the screw holes (see above) line up with the channels. Thats the maximum this can be shortened. You may find that this turns the lens upside down though, with the aperture ring underneath.




But best is to screw it off entirely, rotate it counter-clockwise half a turn, and screw it back in, which will restore the alignment - the red diamonds on the front and the DOF scale should line up. You may have to do this a few times to get it right.




Put in the lens mount tube screws. Remember, these ride in channels in the helical, make sure the screw holes are aligned with the channels before you try to screw them back in. They should go in without any effort. If they stick, it means they are bearing on the helical threads. If you force the screw it could damage the helical thread. Make sure they are aligned and don't screw them in tightly.




Put the focusing ring back on, with the infinity mark aligned with the triangle in the DOF scale. Tighten the setscrews and you're done.
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[Jesito]

Great explanation, Luis!!

Thanks for sharing,

Jes.
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[piticu]

This part should be screwed in?

I don't know this lens but is looks like it can be pushed in couple of mm. Others (like Super-Takumar 55mm 1.8 ) can't: the focus collar should be shortened too in order to allow full focus, otherwise will stick to the barrel of the lens.
Anyway, thanks for sharing.
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[luisalegria]

Hi Piticu,

Yes thats right, the lens mount tube scews in there. Thats whats used to adjust infinity. The lens originally comes with the lens mount tube not screwed in all the way, in can be screwed in at least 2mm more.
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[Attila]

Many thanks for this great guide!
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[piticu]

[LucisPictor]

Thanks! I think I will try that with my alu Tessar 2.8/50 (EXA).
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[indianadinos]

Hello,

Thanks for sharing the excellent tutorial ...

[Attila]

Welcome Indianadinos!
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[thePiRaTE!!]

Luis, using your instructions I had infinity on my Biotar in half an hour. I think I was lucky to align the mount tube first try.

I did make a note and have one question for you.

Note: The two screws on the mount tube (pic 3) are slightly different from each other in my copy. They had to be put back in the hole they came from or the helical wouldn't spin. Upon close inspection, it looks like one screw had 2 threads and the other had three, changing the depth they sat in the channel.

Question: After the adjustment, my lens over shoots infinity by a bit. This is fine with me and my minimum focus distance is still aproximately correct, but do you see a way it is possible to fine tune this?

Once again, many thanks for posting these instructions, they are a breeze to follow.

Kelly.
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[luisalegria]

Hi Kelly,

I'm glad it worked for you.

I didn't notice that about the screws. It may be the same on mine of course.

Yes, mine overshoots a bit too. I can't think of an easy fix unfortunately, the lens mount barrel aligns only at certain points with the helical slots.

Please post pictures from your Biotar !
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I like Pentax DSLR's, Exaktas, M42 bodies of all kinds, strange and cheap Japanese lenses, and am dabbling in medium format/Speed Graphic work.