Any Technicians? Canon Lens Reassembly Help (70-200 F/4)

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JoshuaP1993

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Hello,

I have a 70-200 F4 L Lens which I have removed the front sections to clean and everything is almost perfect however there is a tiny amount of excess pressure when turning the focus ring now, specifically at 1.5m.

I'm quite sure that anyone who repairs these lenses often would easily know what the issue is, I don't believe it to be a major issue, just something simple.

I have checked and I cannot see any debris and I've been rather careful with everything. After a little testing I believe it could be related to the element near the front which moves up and down when focusing because when that is not inserted the focus is nice and smooth. It is when I insert that element with the 3 screws the issues occurs, perhaps it is related to the part which protrudes and inserts into the metal clip to allow movement of the element.

Anyhow, any help would be much appreciated.
 
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Hi Joshua,

This is based on a little interpretation of what you're describing so please forgive me if I've mis-understood.

When you reassemble the front end, and specifically the focusing helicoid mechanism, there are some screws which go through the helical slots and hold the moving front group of lens elements. Each of these screws also passes through a small bushing (cylindrical washer for want of a better description) which acts as a bearing in the slot and is made to very tight tolerances (+/- 0.01mm). The outer diameter of the bushings are chosen when the lens is assembled and they're not all the same.....the helical slots are accurately measured and the required bushing diameter is installed in that particular slot. It's therefore possible that you've mixed up the bushings and have one that tightens as it travels along the length of the helix if the helix narrows slightly at a certain point (a waistline if you prefer).

Hopefully that makes some sense and you're aware of the parts I've detailed.

Bob
 
Many thanks for your response Bob,

I completely understand what you're suggesting. Sadly when dismantling the lens I spent hours trying to find a guide but was unsuccessful therefore everything was done from guess work, which perhaps in retrospect was a terrible idea. I should have known as I have previous experience in repairing mobile phones.

When disassembling I didn't realise that the bushings were specific for each slot therefore I have mixed them up. However for the particular part in question which I certainly believe is causing the issue there are only 2 bushings which may be confused with one another, because the 3rd is different; being the part which protrudes and slots into the metal bracket which enables the up and down movement when focusing.

I guess the angle of the bushings also affect the operation since they are slightly different which I have just noticed. I guess that my only solution would be to try different' combinations with slight alterations and after much trying I may be successful? I don't suppose you have any other advice on rectifying the issue myself?

Kind regards,
Joshua
 
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I'm not sure that the angle (rotation I assume) could be so critical as the assembly technicians would have a nightmare with that. I wasn't sure how many there'd be as there are an increasing number as the front element group gets larger....the 70-200/4 is quite a modest front end in terms of size.

No use now but next time perhaps....have a few different coloured permanent markers to mark the barrel and the insert before dismantling eveything.

I don't have an assembly drawing for that lens but others that I do have (with a similar size front end) seem to show sizes of 3.99mm, 4.00mm and 4.01mm ......tolerances that are certainly to close to eyeball.

Bob
 
Thanks, yes in future I would be wise to do that.

There are only 3 screws w/ bushings holding in the element in question which I believe is causing the issue and as say 2 of them are different, therefore in theory I would only need to try 2 different combinations but I am quite sure that I would have accidentally done so already.

The reason I questioned the rotation is because the unique one is designed different and it looks like it would affect the operation. Please give me an 30 minutes / 1 hour and I will upload some pictures as hopefully this will help rather me trying to explain.

Also someone where suggested that it could be that the focusing helicoid is no longer perfectly round, they said that is an issue with the 50mm 1.4 lens. I am rather hoping that is not the case and I cannot see any logical reason why this would have changed as I have been rather careful but I guess if the measurements are so precise that could be the case?

Kind regards,
Joshua
 
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JoshuaP1993 said:
.....Please give me an 30 minutes / 1 hour and I will upload some pictures as hopefully this will help rather me trying to explain.

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Click to expand...
It may be tomorrow before I see the photos, Joshua, as I'm not in the UK and it's later here than there.

Bob
 
No problem Bob,

I appreciate the help. I will perhaps upload them a little later then so they will be ready for tomorrow.

Kind regards,
Joshua
 
Images:

1. https://i.imgsafe.org/3d3bf3150f.jpg
2. https://i.imgsafe.org/3d3c96145f.jpg
3. https://i.imgsafe.org/3d3c9e3d1f.jpg

I have been doing a little more testing and I have concluded that I believe it could be to do with the part shown in the first picture highlighted in blue.

Hopefully with those images you can make sense of the issue. The parts in the first image show what I believe you're referring to as the "bushings" and the element. If you see the other images you'll see that the element slides into the body and is held in place by 3 screws and 3 bushings. 2 of the bushings are the same (highlighted in green) however the 3rd is shaped differently (highlighted in blue). The 3rd one slots in the middle of the metal clip shown in the second image. This enables control the element to move up and down when you turn the focus ring.

If you look closely you can see that the part is shaped so one side is extended, I believe it needs to be reinserted in the correct orientation. I have tried several positions but I still have had no joy. I rather wish I never started this now.

Kind regards,
Joshua
 
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I can see the part that you mean now, Joshua. It appears on some lenses but not all of them and is an eccentric collar/bushing with tolerances similar to those of the plain collars mentioned earlier.
Looking at the diagram of a different lens then it appears to be used as an anti-backlash fitting and therefore its orientation is quite critical if it's to function as intended.....too tight and there may be some binding and too loose and there'll be excessive backlash (I suspect!). What I can't figure out is how the thing gets adjusted at initial assembly......trial and error would seem to be very time consuming.
The part number is probably YA2-4706-xxx (with xxx being the specific size to be chosen)

Bob
 
Many thanks Bob,

I believe my goal now is to try and find either a diagram or image which will show the correct orientation of the bushings. I am rather reluctant to keep trying without that information because I fear that I could cause damage to them so I am going to wait until I can find this information, although sadly so far I am not having much joy.

Kind regards,
Joshua
 
Perhaps that's a wise approach, Joshua. I have diagrams for about 60 Canon lenses but I've not see one for the 70-200/4.

Bob
 
Hello Bob, just a quick update but today I have tried again and after 2 attempts I've successfully rebuilt the lens and there is no rubbing. I believe the issue was the orientation of the collar/bushing.

Thanks again for your help.
 
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