CZJ Biotar 58mm f/2 Pre-Set - Misaligned Focus Distance Ring

[BlueOceanWaves]

Hi, I am looking for some help if my CZJ Biotar 58mm f2 pre-set has a misaligned focus ring or not.

The focus ring shows a focus distance below 0.5 meters at minimum focus distance. I took out a tape measure and measured the actual distance between the subject to the (ol) focal plane shutter of my camera. The lens focus ring says "less than 0.5 meters" when the tape measure 0.5 meters.

When I set the Biotar to infinity, the optical focus is actually slightly beyond infinity focus. For infinity to be in total focus, the Biotar's focus ring indicates slightly before infinity.

More context: This is a new purchase, the seller CLA'd the lens. I am using a m42 - EF mount adapter on a Canon t2i (550d) to test the lens. And I wish to use this lens with a film camera in the future (but it is difficult to find a suitable one due to the rear lens hitting the mirror.

How difficult is it to realigned the focus ring.

[stevemark]

It's usually very useful if you can focus slightly beyond infinity (I always calibrate my primes like that if I'm servicing my own lenses).
Since the Biotar 2/58 does not contain any floating elements, you don't have to worry about the lens showing perfectly correct distances near its MFD.

You can read here what I wrote about servicing the Biotar 2/58mm:

https://forum.mflenses.com/repair---carl-zeiss-jena-biotar-2-58mm-quotpre-setquot-t85827.html

Stephan

[BlueOceanWaves]

Hi Stephan, may I ask out of curiosity why setting infinity focus slightly beyond infinity is ideal?

Thanks for sharing your input and your post on the Biotar CLA is amazing as well.

[visualopsins]

Welcome BlueOceanWaves!

[stevemark]

[DConvert]

A small error is common with adapted lenses, down to tiny variations in the adapter length.
But the error here seems VERY minor. If you look at the focusing ring 0.55m is quite a long way round from 0.5m. The extra travel you have is less than 1/6 of this distance & the movement of the focus ring increases as you get closer (as can be seen by the far end of the scale) I'd expect the position of the ring to correlate to ~0.495m not a significant error

[visualopsins]

Adapters thickness error is more noticeable when using wide angle lenses, as the distance the lens must travel within its focus range is much less than with telephoto lens. Adapter thickness is usually most accurate for Normal lenses, 30-35mm for APS-C, 40-58mm for FF. Focus beyond infinity is desirable for accuracy, but can be inconvenient if you are used to turning focus ring to the stop. Manufacturers typically err on the side of too short rather than too long, to accommodate the greatest number of lens variations.

[BlueOceanWaves]

I see, it makes a lot of sense especially for mounting with varying degrees of adapter thickness. I will get to know the lens better and make the adjustment for it when I take it out to shoot some photographs.

I have one more question. The paint for the aperture indicator has been totally wiped off. Would it be a good or bad idea to fill it in with something like Acrylic paint?