A lens is testing me

[Bob Hart]

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 encountered 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.

[Allan Black]

Hi Bob no replies, maybe it’s because we’re all glad it’s you and not us :(

I guess you’re looking around the web, this is a good site ...

https://www.dpreview.com/forums/thread/4194130

Let’s know how you go, good luck.
Cheers.

[Bob Hart]

Thank you Allan for your reply.


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 alignment 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.

[Bob Hart]

I have a NIkon AIS 35mm f1.4 which was also fungussed. When you do the secondhand rose you take your chances. I pulled that apart last night. Fortunately there was a how-to online which was very helpful as the lens had been previously entered and the re-assembly was slightly off. There are two separate helicoids and travelling groups, not just a simple lens block moving back and forth. The image and focus are better now. I was not impressed with the lens but I now know it had been hobbled slightly which may be why it was sold on. Like most stills lenses, it breathes the image in motion work. Youtube has had its way with the test clip. It is crisper in the camera screen of the "big" URSA which is also a secondhand rose and had to be fixed after it failed to wake up after a winter sleep. - Failed sensor.

NIKON 35MM F1 4 TEST - YouTube

[Bob Hart]

THE SIGMA LENS SAGA CONT'D.


Well there are all the lens groups finally exhumed from within the guts of the beast.

I also stand corrected. As well as fine oil splash, there is fungus in the rear groups after all. Fortunately not deeply embedded in the glass. To think I was nearly going to put up with it and just clean the easy ones.

Apparently some oils feed fungus growth. Except it would probably gas off and fog the glass or corrode the metal, I am tempted to put some tinea lotion in there.

Cleaning the glassware will be easy. Not smudging the pieces refitting them is going to be a vexing challenge. I suspect there must have been a special holding tool to keep them aligned while the screws and pillars were installed in the factory.

Where's Harry Potter when you need him. However wishes are not going to make it spring together all by itself.

[Bob Hart]

After a few travails I got it done. There are several ways one can put one back together and the front will be rotationally out of position. Everything has to be in a correct position for the factory matcharks to work.

More haste, less speed but the deadline for tomorrows ground-to-air shots against a hard sky was getting uncomfortably close.

It will have to be pulled apart again however for now optically it works fine. Had a bit of a "moment" (nearly gibbering and screaming) when two spots turned up when the iris was closed to f16.

However it turned out to be stuff on the IR filter and the focal reducer in the camera itself and that's on me as well because I fixed the big URSA a few days ago.

As luck would have it, one of the zoom cam pins was loosened and has actually deformed the little hole it is threaded into. It could have dropped out and jammed. The thread is ruined so for now two out of the available three and gentle driving has to suffice.