Infinity adjustment?

Jiaming · Joined: 09 Mar 2016 Posts: 107

I disassembled and repaired a Carl Zeiss Jena 135/3.5 lens (which is a really good one) some days ago, but after I put all the parts back together, I had trouble getting the infinity correct. Currently, when I turn the focus ring to the end, optically it focuses slightly over infinity (still better than failing to focus to infinity though).
I might probably not bother to actually do the adjustment, but I'm asking out of curiosity - the correct infinity depends on where you start to screw in the focusing barrel, but there has to be a thousand ways to start, while there is only one single correct answer. I can do trial and error until I get the correct position, but how on earth did the Zeiss engineers do it? I don't think they tried thousand times for each lens they produced. So I suppose there has to be an easier method (compared to screwing and unscrewing the barrel again and again) to adjust infinity? Will the barrel guide ring somehow do the trick?
The "anatomy" of the lens can be found here: http://www.aprd31.dsl.pipex.com/articles/czj135svc/

cooltouch · Joined: 15 Jan 2009 Posts: 9109 Location: Houston, Texas

When I disassemble a lens, I will scratch a small mark in each helical so that I can align them back up when I reassemble the lens. The problem with this, though, is the marks have to be made exactly when the two helicals come apart. I'll admit, I haven't always been totally accurate with my alignment marks and I've ended up doing the trial and error reassembly. You do not have a thousand combinations. Only the number of threads on the helicals. But from a practical standpoint, you have only a few. You'll know you're close by seeing how closely the infinity symbol lines up with the index line. And you can usually tell how far off you are, in terms of threads, by how far the infinity symbol is from the index mark. It's a hassle and it can get tedious but it's certainly do-able.

With your lens put back together the way it is, chances are you're probably only one thread off. You've lost some close-focus capability, but you now have the flexibility of being able to focus past infinity too. If it were mine, I'd bite the bullet and pull it apart again. It's not a mystery anymore and, chances are, it won't take you that long.