Stain at the back of a lens. Fungus?

What is it?

Could it be cleaned?

Does it affect the image?

It doesn't look like any kind of fungus I've seen. That usually appears as small fuzzy spots and/or a network of fine strands. It could be balsam separation, that is, where two cemented elements start to separate, or perhaps some kind of contamination or reaction in the balsam adhesive itself. Could the lens have got wet at some point>

13 hours ago, boris- said:

What is it?

Could it be cleaned?

Does it affect the image?

Looks like separation from what I can see. If that is correct, it is a non-trivial repair - that is if you can even find someone who would do it.

If it is a lens offered to you for sale, I would pass.

It will affect the image. But if you already own it, there is no harm in trying to see if the degradation is acceptable to you.

Hello. Thank you for the comments. I am sorry for taking so long.

So, it could be about about separation.

Here are other photos:

So basically, repairing this separation is very expensive and it is not worth the money considering the lens, right? Anyway, I could ask a lens repair service in my city.

And does this separation progress gradually or it always stays like this?

Here are some quick photos.

They are taken with a Fujifilm XT3 and a Nikon to Fuji adapter.

This lens was never very sharp. I bought it used like 15 years ago. I think this stain was not present, I guess I would have seen it.

Recently I compared it to an old manual Nikon 200 mm to make a video shot with my Fuji XT3. And the Nikon was much sharper and contrasty and it showed much better colors. The image of the Sigma was also greenish.

But that could be the case even if the Sigma was in perfect condition. So I don’t know how much this “separation” is affecting the image.

On 2/8/2023 at 8:50 AM, Niels - NHSN said:

If it is a lens offered to you for sale, I would pass.

I would like to sell it just because I never use it.

On 2/8/2023 at 5:33 AM, John Seaman said:

Could the lens have got wet at some point>

I don't think so. The lens has been in his box in a cabinet with other photographic equipment. I don't think I bought it like this 15 years ago, but who knows... 

On 2/21/2023 at 6:45 PM, boris- said:

I would like to sell it just because I never use it.

You haven't posted any pics of the front of the lens, or indicated exactly which Sigma lens model you have here, so our replies are sorta "flying blind".

Assuming this is a common consumer-grade Sigma from 15+ years ago, you probably would not get many offers on it even in perfect condition: most buyers today are looking for either camera-branded lenses or newer premium-grade Sigma models. 

The problem visible in your glass is almost certainly separation, which does progress, is not an easy DIY repair, and is rather expensive to get fixed professionally. Professionals usually don't accept such jobs unless the lens is a sought after classic/collectible (vintage Leica or Zeiss, or Nikon/Canon exotic). With more common mass-market old lenses, it is far less expensive to toss the bad lens and just buy another in better condition.

Separation drops the value of most common old Sigma-Tamron-Tokina lenses to near zero. Personally, I would advise not trying to sell it: there are a LOT of clueless, nasty buyers out there who will stupidly buy a defective lens without reading the full description. When they receive it and finally notice the defect, they kick up a fuss claiming the seller (you) tried to deceive them, creating all sorts of headaches for you with the selling site (eBay, etc). This can be worth the risk with expensive items, but is a waste of time for lesser items. 

Some lenses can remain remarkably usable despite separation, esp if shooting a FX-format lens on a DX or Micro 4/3 camera (which crops away the separation and sees only thru the clear center of the lens). Some can be relegated to "soft hazy portrait" work. But if the degraded performance is too much to work around, you'll be happier if you replace it with another lens. Its sounds as if you were never very fond of this lens to begin with: if thats the case, just write it off after 15 years.