Disassembly of Lens Groups in Canon FD Fixed Focal Lenses

Herbert · Joined: 07 Sep 2023 Posts: 26

Firstly, I apologise if this basic topic has been covered, but searching this forum didn't turn up anything. I would like to open my Canon FD 24mm / 28mm / 35mm / 50mm lenses, which are up to 50 years old, and thoroughly clean the dust from the lenses. The following service manuals are certainly helpful:
https://www.fou-du-canon-f-1.net/wp-content/uploads/2017/03/repair-FD-part-1.pdf
https://www.fou-du-canon-f-1.net/wp-content/uploads/2017/03/repair-FD-part-2.pdf
I am mechanically skilled, but so far it has been portable hi-fi equipment (e.g. portable CD players and Walkmen) that I have maintained or repaired. What interests me most when disassembling are the lens groups. Dust can also get in there, I assume, unless the lenses are cemented together. In the linked service manuals, Canon only supplies ready-made groups as spare parts, stating that these are individually adjusted by hand at the factory. However, it is also possible that Canon wanted to save labour hours in this way. But I would like to know what is usually adjusted within a lens group. Because when I watch videos of the disassembly of a 50mm nFD, there is no explicit mention that an adjustment within a lens group seems to be necessary. Shim discs are also missing. But I assume, 50mm is a less complex design than 24mm https://www.youtube.com/watch?v=A2cf_UeGfzU
However, I can see that the repairer places marks so that the lenses along the Z-axis are not twisted at an angle.
What do you have to pay attention to when disassembling such a lens group?
Greetings, Herbert

RokkorDoctor · Expire: 2025-05-01

I know that Minolta in their service manuals for the AF lenses had a diagram for each lens where every element was given a priority number. The ones with priority 1 (I seem to recall it was 1) have the greatest impact on image quality when they are rotated.

If the lens failed the IQ test it was advised to take that element and rotate it 120 degrees, or 240 degrees, and mount it in whichever rotated position the lens gave the best IQ.

To be honest, this is all from memory as those Minolta A-lens service manuals that were once downloadable for free are now behind a (modest) paywall, so I can't check now. I would buy them if I worked on Minolta A-lenses, but I never do.

The service manuals for the earlier Minolta ROKKOR lenses that I do have, have no such element identifications. Those lens cells are not listed as available spares either ("NO SUPPLY").

If you take a cell apart I would somehow mark the elements and make note of their relative rotation.

Herbert · Joined: 07 Sep 2023 Posts: 26

Many thanks!
I found those at elektrotanya:
https://elektrotanya.com/minolta_70-210-f4.pdf/download.html
https://elektrotanya.com/minolta_50-f1.4.pdf/download.html#dl
So, for lens groups, the rotation of an element is much more crucial
than the distance between the elements? What is the background?
Found this on wikipedia about astigmatism im lenses:
https://en.wikipedia.org/wiki/Astigmatism_(optical_systems)
All the best, Herbert

RokkorDoctor · Expire: 2025-05-01

Herbert · Joined: 07 Sep 2023 Posts: 26

Again, many thanks! Could a tutorial be found, not only for Canon FD but to give an impression, how those spacers look?
In this video, the repairer dos not seem to care about the angle of the lens elements...

https://www.youtube.com/watch?v=5ChD4ecqzes

RokkorDoctor · Expire: 2025-05-01

Herbert · Joined: 07 Sep 2023 Posts: 26

Many thanks! I will read at least through the pdfs

RokkorDoctor · Expire: 2025-05-01

Herbert · Joined: 07 Sep 2023 Posts: 26

Many thanks for the explanation!

BrianSVP · Joined: 09 Jun 2023 Posts: 354 Location: Philadelphia

Just be aware, as well, that the manuals you have linked in the original post are from 1971 and only cover the very earliest of the FD lenses. The so-called "New" FD lenses have radically different internal mounts - some elements are held in place by plastic clips rather than metal retaining rings, some are swaged-in in plastic housings, some use soft rubber gaskets as spacers, etc.

As far as rotational alignment vs spacing goes, the latter is far more critical, whereas worrying about the rotational position of an element group is like arguing how many angels can dance on the head of a pin. As the late SK Grimes would say, by the time the SLR era came around, the centering on lens elements was so good, it was "like it was done by God himself." That's why lens repairers can get away with using machinists' blocks when recementing doublets instead of using fancy collimating equipment- the elements are so symmetrical, that mere mechanical alignment is enough to ensure proper optical alignment, no matter what the rotational position.

Also if you look at the internal elements of almost any zoom or floating element lens while you zoom and/or/focus, you will see that the internal groups actually rotate as they go in and out. The reason they can do that is that the individual elements and groups are so symmetrical that the there is no optically-significant wobble on the position of their center point as they rotate about the center axis.

Also, also, a bevel and chamfer, while similar, are actually technically terms describing two different things. A bevel is a flat cut across two parallel surfaces, while a chamfer cuts across to perpendicular surfaces.

Image source: https://www.madearia.com/blog/chamfer-vs-bevel/

A rounded chamfer is called a fillet, and a rounded corner that went parallel to parallel would be called a demi-bullnose.

Herbert · Joined: 07 Sep 2023 Posts: 26

Many thanks for the detailed explanation and thanks for the warning!
In fact, only one of my Canon FD's is covered by the documentation linked in the first post:

35mm 1:3.5 Manufactured June 1975

I t would be great to have documentations / service manuals for the following lenses:

FD 28mm 1:2.8 Manufactured July 1978

New FD 24mm 1:2.8 Manufactured January 1979

New FD 50mm 1:1.8 Manufactured June 1984 (But this one is covered in the repair video linked in my first post)

New FD 35-150mm 1:3.5 Manufactured July 1981 and September 1983 (I have two of them)

Many thanks!

ZoneV · Expire: 2011-12-02

Herbert,
Are you sure those lens groups in those Canon manuals are not cemented together?
From my experience I tend to say, on most consumer grade 50 years old photography lenses the rotation is not of big influnece on the image quality.

On older lenses or high end lenses this may likely be different. I think at Leica the rotation was oftem optimized, to get the very best results - but perhaps this was beyond what users would detect outside of astrophotography.

Herbert · Joined: 07 Sep 2023 Posts: 26

At least in the nFD Cleaning tutorials/videos linked in my first post the lens groups are partially cemented together.
And where they were not, dust was able to creep in.
If they were completely cemented I assume this thread would not be needed.
I also assume that alignment within a group is crucial as the assembled groups rotate against each other
which naturally might give some play, even if engineering fit gives fractions of a mm:
https://en.wikipedia.org/wiki/Engineering_fit