A LENS IS TESTING ME

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robert Hart

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A LENS IS TESTING ME

PostFri Oct 16, 2020 4:23 pm

I have a Sigma 50mm-500mm f4-f6.3 zoom. Over the years the image has become a bit spotty. The nature of the design is that in dusty conditions enncountered when chasing aeroplanes in ground-to-air shots, it is inevitable that the lens will hoover up dust as the zoom extension goes in and out. This one has also hoovered up some fungus spores and it is now rampant inside.

The cost of getting it torn down and cleaned is likely more than the cost of buying a used good one off eBay. Lens repairers may also not be too keen on infecting their shops with fungus. I have successfully defungussed Cooke Panchros and Angenieux zooms but this one is going to be a mission. The fungus is on the travelling optical groups deep inside as well as the front element, which is an easy fix.

I cannot find any service literature on the web for this lens. As close as I can get is an exploded view for a 100-400 zoom but there are some differences.

I am putting this query out there in the hope that someone may have a service manual or exploded view of the parts of this lens or even a correct order of dissembly/reassembly. Here's hoping because one can get into a dreadful mess just mining in blindly.
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Uli Plank

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Re: A LENS IS TESTING ME

PostSat Oct 17, 2020 2:45 am

I doubt it makes any sense to tear it apart, these modern AF lenses are very complex with tight tolerances. Thye are not built to be serviced, like a true cine lens would be. And then, get a dry cabinet for your lenses! I've been bitten too.
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robert Hart

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Re: A LENS IS TESTING ME

PostSun Oct 18, 2020 6:01 pm

Thank you Uli for your response.

Alas, I have ventured in like a fool.

What looks to be fine fungus has turned out to be fine oil splashes. The deposit smears across the glass when wiped off. It is on every travelling group in front of and behind the iris so everything has to come out for cleaning. How it got there is anyone's guess.

There are concentric cylinders, the outer barrel and cam tubes. The lube between them is more fluid-like than grease. My theory right or wrong is that air becomes entrained in the lube in use and bubbles eject fine spots of lube into the barrel through the cam slots to mist onto the glass.

Each of the travelling elements is enclosed in a plastic disk with three axial pillars around it. Additional pillar washers are fastened through the cam tube slots by small screws into these pillars. It is very precise plastic castwork. The fit of each lens support is quite loose. It is obviously adequate as the image does not walk when zoom or focus control movements are made.

The pillars being carried in their cam slots are the only support for each travelling element. There is no long sliding cylinder for each travelling element to maintain axial alignment. The alignent relies on the precision of the pillars in their cam slots.

I guess this is to minimise friction which is quite heavy. The plastic pillars seem to be wearing well enough but the lens has not seen a lot of use.

Long life may not be a feature of the system as one of the pillars is deformed. Over time, because the pillar washers are an interference fit in their pillars, the pillar themselves may split or break off. I would like to have seen metal instead of black plastic but then we would be talking Angenieux prices.

When the elements have been cleaned, the challenge is going to be reassembling it. Somewhere in the world a proper lens-smith may read this and have a quiet laugh.
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robert Hart

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Re: A LENS IS TESTING ME

PostSat Oct 24, 2020 3:18 pm

I got it fixed in a bit of haste in time for the ground-to-air shots planned for tomorrow, however if anyone comes across any repair info for this lens it will be appreciated as the lens has to coe apart again for a minor cosmetic alignment.
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ttakala

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Re: A LENS IS TESTING ME

PostSat Oct 24, 2020 10:11 pm

Hats off to you, sir! I have regreased and fixed a couple of old Nikkor primes but opening a modern zoom lens is a job I would not be comfortable attempting.

User 'mikeno62' on Youtube repairs lenses, mostly older ones, but that's one resource you could check.

How was the shoot, does it work properly now?
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robert Hart

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Re: A LENS IS TESTING ME

PostSun Oct 25, 2020 5:57 pm

Thank you for the suggestion. The shoot went fine. When the lens is steeply inclined the zoom extension wants to drop back which is normal for this lens. As things turned out, it very nearly failed before I opened it up. The movement had been a little glitchy. One guide pin screw had worked loose and ovalled the threaded hole it screws into. The screw could not be put back as there was no thread to grip into except the very bottom of the hole. Longer screws bound into the next tube inside. So only two of three guide pins are now working in that piece. This causes the tube to cant slightly and this loads the focus ring which becomes tight. Other than that, the optics are sweet again.
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robert Hart

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Re: A LENS IS TESTING ME

PostMon Oct 26, 2020 8:31 am

For what it may be worth, here is some vision from the event on Sunday I was repairing the lens for. The image is softer in places. I was full of allergy on the day and my reactions were off due to the poison I had to take. The zoom tube wants to drop back from full extension during steep tilts, normal behaviour for this lens but a bit of a hassle for motion vision. There are a couple of mote spots in the image. They are on the rear of the focal reducer which I have installed in the throat of the "big"URSA. I missed them when I was refitting the optic in the camera.

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