Zeiss 10-100mm T* f2.8 - no infinity focus?

Heikki Repo · May 5

I recently bought a Zeiss 10-100mm T* f2.8 (T3.1 Arri-B mount) for use with my Eclair ACL and BMD Ursa Mini 4.6k. The lens seems to be in quite nice condition, but for some reason I'm unable to reach infinity focus. At first I was worried if this was an issue with the Arri-B to EF mount adapter I use with the Ursa, but then I mounted the lens on my ACL and the same issue is present there too. Using Optar Illumina Arri-B lenses I have no such problem.

This is one of those "for use with Arriflex SR only" lenses, but there's nothing protruding from the rear.

Any ideas? Just so that I don't send it to an expensive check-up just to hear that there is something non-standard to begin with with a lens like this and the issue actually is not using an Arriflex SR
😉

Dom Jaeger · May 6

There is an infinity stop in the front focus mechanics that probably needs adjusting, as well as the focus scale itself if it reads infinity but doesn't focus there. If the zoom isn't holding focus as you zoom out from the long end, either the lens or the adapter need back-focus adjustment too. First step is setting back-focus, then front focus, then set the infinity stop.

I'd recommend having a lens tech who knows these zooms look at it if you want it properly set up. Shouldn't be too expensive just to adjust these settings.

Tyler Purcell · May 6

Yea it's gotta be the adaptor, doubtful the lens back focus is randomly wrong.

Dom Jaeger · May 6

10 minutes ago, Tyler Purcell said:

Yea it's gotta be the adaptor, doubtful the lens back focus is randomly wrong.

You obviously don't work on lenses.

As I just wrote, a zoom has back-focus, front focus and infinity stop adjustments, all of which can affect if it reaches infinity.

A lot of people watch a youtube video and think they can fix a lens themselves, then sell it on after they've screwed it up.

It could be a dodgy adapter, but then the more obvious issue would be that the zoom doesn't hold focus. Plus Heikki mentioned that he tried it on his ACL without the EF adapter and had the same problem.

As an experienced tech I would never say "it's gotta be" something, because it can almost always be several things.

Heikki Repo · May 6

Thanks Dom!

I just tested the lens on my ACL again and it holds focus perfectly, it's just that the infinity isn't reachable.

I'll send this lens to Martin Rodensjö, he should have the know-how and necessary facilities for working on it. :)

Dom Jaeger · May 6

Sounds like a good plan Heikki!

Tyler Purcell · May 6

11 hours ago, Heikki Repo said:

I just tested the lens on my ACL again and it holds focus perfectly, it's just that the infinity isn't reachable.

Do the focus indicators match the measured focus range?

Heikki Repo · May 6

55 minutes ago, Tyler Purcell said:

Do the focus indicators match the measured focus range?

Sorry, can't tell - I shipped the lens off already 🙂

Heikki Repo · Tuesday at 01:05 PM

On 5/6/2024 at 6:16 AM, Dom Jaeger said:

A lot of people watch a youtube video and think they can fix a lens themselves, then sell it on after they've screwed it up.

It seems Dom got it right here. Someone had opened the lens and didn't know how to put it back together. The focus scale was completely off and the back focus nowhere near where it should be. Now, after it was correctly put back together it works. I'm also going to have it cleaned relubricated, so that it'll be in much better than before condition when I get it back.

What is interesting is that apparently someone had previously modified this lens so that the focus stop is different, in order to allow for better close focus... 🤔