Author Topic: Canon fdn 135mm f3.5 unreachable infinity (Read 1384 times)

Zang · « **on:** July 16, 2022, 17:09:24 »

Hi all,

To start with, Canon fdn 135mm is a weird lens to work with. I did not check the infinity when I bought it (the dirt and oil discouraged me from trying it) so I don't know if the lens reached infinity before I fixed it. I marked the lens's elements very carefully and I am pretty sure I put them back in the right way. Right after the first attempt, it focused only to 15-20m ish. I took it apart again a few times and adjusted it away from the original position hopping to reach infinity. I got to the point where the optical assembly reached the mechanical limit and it cannot be moved any closer. Still, the infinity is not there and I feel I need another millimeter or so. Did I do anything wrong? Did anyone have similar situation?

I was thinking, one of the possibilities was I put the lens element in wrong orientation, but looking at a sample pictures taken after reassembly, I don't think that could be the case.

BTW, adjusting infinity is nightmare as I need to disassemble the lens in order to do it.

Any input is much appreciated.

Cheers,
Zang

Nasos Kosmas · « **Reply #1 on:** July 16, 2022, 18:25:55 »

Are you sure you put helicoid in the right way ?
As far as I can see photos from the other thread every step of helicoid must be 3-4 mm , so changed to the one step further so lens optics go more near to sensor to have infinity focus

Zang · « **Reply #2 on:** July 16, 2022, 18:32:48 »

Quote from: Nasos Kosmas on July 16, 2022, 18:25:55

Are you sure you put helicoid in the right way ?
As far as I can see photos from the other thread every step of helicoid must be 3-4 mm , so changed to the one step further so lens optics go more near to sensor to have infinity focus

Hey Nasos, I am sure I put both the outer and inner helicoid in the right way. The inner one is easy. If you mark it then it is impossible to put it a wrong way. It is single threaded I guess. for the outer one, I know it is very easy to misstep as thread steps are away from each other for les than 1 cm. I marked it very careful and I even try the next thread step and that resulted in the helicoid being impossible to align to the keys because it goes to deep. The most interesting thing here is, the optical assembly cannot be move further as its cage hits the ends of the keys at infinity position. I start thinking about either the mount or the adapter are not right.

Erik Lund · « **Reply #3 on:** July 18, 2022, 13:02:57 »

Most likely you are one thread wrong in the helicoid.

Zang · « **Reply #4 on:** July 18, 2022, 13:07:13 »

Quote from: Erik Lund on July 18, 2022, 13:02:57

Most likely you are one thread wrong in the helicoid.

I don't think so (and I will explain later) but I'll give it another try today and let you know. The lens is too nice to give up

Zang · « **Reply #5 on:** July 19, 2022, 13:13:01 »

The focusing mystery could be resolved soon as I have strong theory on what is happening. This seems to be an optical problem. The lens has 4 elements and I took the last and the second last elements off for cleaning. There was no need to take the further elements off so I left the front group of two untouched. Now, after encountering the issue, I inspected my work and confirmed that the elements I worked on have correct orientation and tightness and I have no doubt about that. However, I have discovered now that the very front element is slightly (very slightly) moving. There are evidences that someone has been to the lens before and most likely tried to unscrew the front element and most likely did that unsuccessfully. Now, the retaining ring is so tight that I did not seem to be able to tighten it, so instead I went for the other direction. It was extremely difficult but I unscrewed it at the end. I now deep it in mineral spirits to resolve the thread lock. I'll put everything back by end of today and let see how that works.

Zang · « **Reply #6 on:** July 19, 2022, 18:52:55 »

That was it! I was able to get perfect infinity still having a decent margin.

All that consumed half of my lunchtime. I cannot wait to finish my work and give it some real life tests.