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Lens Fungus Repair

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Tarzankayaker View Drop Down
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Post Options Post Options   Quote Tarzankayaker Quote  Post ReplyReply Direct Link To This Post Topic: Lens Fungus Repair
    Posted: 28 January 2019 at 22:48
Hello everyone,

I purchased a really cheap sigma 400mm f5.6 sight unseen. When the lens arrived it had a pretty terrible fungus issue. I took apart the lens and used hydrogen peroxide to remove the fungus, and now the lens is operational. However, the image quality is significantly degraded. No surprise there.

At Max aperture, the image is pretty soft and flares around bright objects. The fungus did not etch the glass very deeply, but one element has spiral marks all across the lens coating.

My question is this: Am I better off leaving on what coating is left, or polishing the rest of the coating off until I have an even surface?

I appreciate any insight you guys can provide.

Thanks!
 



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sybersitizen View Drop Down
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2019 at 06:23
Originally posted by Tarzankayaker Tarzankayaker wrote:

At Max aperture, the image is pretty soft and flares around bright objects...

My question is this: Am I better off leaving on what coating is left, or polishing the rest of the coating off until I have an even surface?

Since the image is soft, I suspect you might have improperly reassembled the lens elements. If so, that needs to be corrected; otherwise the condition of the coating won't matter much.
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amrep View Drop Down
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Post Options Post Options   Quote amrep Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2019 at 07:41
It really depends on how much damage the fungus have done. If it has just slightly affected the coating only (seen as different surface reflection /color) the optical performance will not be much degraded. If however the fungus has etched trough the coating and also into the surface of the glass itself there may be a huge impact on optical performance, especially if it's one of the smaller rear elements. With a good loupe magnifier such deeper etching can be seen as a deboss relief.

In the latter case you might be able to restore some performance by grinding and polishing if you are able to do it properly. The full performance will not be restored. If the lens isn't usable as it it is you could give it a try.

A lens with "terrible fungus" typically has several elements with just minor coating etching but sometimes one or two elements with deeper etching (maybe because of different glass types).
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Tarzankayaker View Drop Down
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Post Options Post Options   Quote Tarzankayaker Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2019 at 22:23
Just like you said, I disassembled the rear elements and and found a spacer that had flipped backwards. I put the spacer back in the right direction, and there was a significant drop in flaring.

Still fairly soft at max aperture, but sharp enough that I doubt I could get it any better.

Thanks for the advice.
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mambo View Drop Down
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Post Options Post Options   Quote mambo Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2019 at 03:15
[QUOTE=Tarzankayaker] Just like you said, I disassembled the rear elements and and found a spacer that had flipped backwards. I put the spacer back in the right direction, and there was a significant drop in flaring.

Still fairly soft at max aperture, but sharp enough that I doubt I could get it any better.

Thanks for the advice.[/QUOTE

I briefly once had that lens many moons ago. It too was soft wide open and had no fungus, so I sold it.
Charles M

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Tarzankayaker View Drop Down
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Post Options Post Options   Quote Tarzankayaker Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2019 at 15:39
I've been tinkering with this lens since my last post, and I have finally gotten everything reassembled properly. Lens elements were easy to get in the right positions, but it took some time to figure out what direction the spacers between them were supposed to fit. For good measure, two out of six lens elements had black painted rims to reduce reflections, I went ahead and painted the rest black as was suggested on a different forum.

This lens is now pretty sharp overall, and there's no more flaring. Any imperfections I'm seeing can be worked out in a raw processing program. I'm very glad I didn't go with my first bad idea of polishing off lens coatings!

Now that I have everything sorted out, if anyone has need of a diagram of lens and spacer positions for a Sigma 400mm f/5.6 AF, please let me know.

Thanks everyone!
 



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Miranda F View Drop Down
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Post Options Post Options   Quote Miranda F Quote  Post ReplyReply Direct Link To This Post Posted: 05 February 2019 at 13:55
IME cheap old long-focus lenses (I've tried quite a few!) are usually not very sharp, but are okay for making decent snapshots on print film. You will usually find a modern xx-300mm zoom gives better resolution with less CA, and good 200mm will usually beat it as well.
Miranda F & Sensorex, Sony A58, Nex-6, Dynax 4, 5, 60, 500si/600si/700si/800si, various Sony & Minolta lenses, several Tamrons, lots of MF primes and *far* too many old film cameras . . .
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Tarzankayaker View Drop Down
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Post Options Post Options   Quote Tarzankayaker Quote  Post ReplyReply Direct Link To This Post Posted: 07 February 2019 at 22:47
I would agree with this, and I have a 70-210 F4 which I prefer, but there are times I can't get close enough to use it. I photographed an auto race last weekend and 210mm wasn't cutting it.

I made a choice between a pretty clean shot that I needed to crop heavily, and one that I needed to edit heavily.
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