Sometimes mold grows inside of a lens -- it's from moisture
inside growing little cultures. I *speculate* that it's due
to a manufacturing flaw -- is it an aftermarket lens? Was it
stored badly over its lifetime (another concept)?

Z

P.S.: * -- Okay, I currently know *most of* a lot. %-}

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Well, it turns out it's not the lens. It took an engineer to suggest to me to try another lens and see what changed. It took the lens off and saw that the crap was still there. Looking inside, he guessed, um, let's see, one thing was that the partially-silvered layer had something something, come off or had a something, so I would see it in the viewfinder but it wouldn't show up on film. Another guess was that some foam rubber part inside had broken and little bits got in there or something. (As you can tell, I really absorbed all of this.)

I hadn't noticed it showing up on my film (although I did notice some spots, and he noticed some little bits of dust or something on the mirror, so I could probably just clean those off, but I'll have to figure out what to clean it off with), but I also hadn't noticed seeing that before.

I think I might just take this roll of film and see if it shows up. That way, before I take it to someone to look at, I'll at least have that information.

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Dust on the mirror will not affect images, just your view
through the viewfinder.

As to what to clean it off with, canned air is good.

Z

P.S.: Something something in the something, but something to
the something! (That can be guaranteed to translate into
either [inclusively] something incredibly smart, as well as
functionally useless.)

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do NOT try to clean anything off your mirror yourself. Only trained professionals should do that.

I tried myself, and now I can never tell whether that blemish or scratch is on my mirror, or on the lens.

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I think it's likely on the reflective mirror that bounces the image into your view finder. This is the mirror that lifts out of the way to expose your shutter. It's likely not affecting your pictures.

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