I was once with a group at a small local observatory, trying to take a look at Venus, low over the horizon. As the rotating roof rolled into the correct position, it turned out that a Yagi-style antenna mounted outside sat right in middle of the view. It seemed like something that'd ruin your day (or night), but it turned out that it really made no noticeable difference.

I guess it's just down to the percentage of the aperture that is blocked?

There's an amusing picture where a large owl has blocked a substantial portion of the view:

https://apod.nasa.gov/apod/ap030401.html

Yes. Given enough distance, the obstructing object will start to come into focus, but if it's close, it just darkens the image. After all reflecting telescopes have a secondary mirror (along with its mount) permanently obstructing part of the primary mirror.

This was not a telescope, it was an all sky camera, the caption explicitly states the owl scratched the optics with its claws.

Guess if you were imaging a bright star you'd might get some interesting diffraction spikes[1] though.

[1]: https://en.wikipedia.org/wiki/Diffraction_spike

You have to block a significant chunk of aperture to get them.

This is useful, however, for focussing using a bahtinov mask.

https://en.m.wikipedia.org/wiki/Bahtinov_mask

Cracks, scratches, dust, fungus, and haze are all invisible until you shoot a photo with strong light in it. It’s mentioned and shown in the article.

It's like driving at night with a dirty windshield. You don't really notice until you meet an oncoming car with bright headlights. Or similar in the daytime, when you're driving into the sun.

Cool. I first learnt about this property when I took my first proper camera with zoom lens to a zoo and was able to take really nice pictures of a squirrel behind some kind of mesh enclosure. the mesh totally disappeared.

Given that, why are specs of dust on glasses so noticeable?

Your eyes are a "wide angle lens." Scratches on wide angle lenses are also very noticeable

Or damn eye floaters.

Like with scratches on lenses, those floaters are only visible when your eyes have a small aperture, like when you’re outside in broad daylight

Floaters are a bit different because they are behind the lens. It's more comparable with dust on the sensor in an SLR (or dust in the sensor cavity)

That is also more noticeable with lower aperture but for a different reason. With a low aperture the light comes more from a single direction so the shadow from the dust particles is more defined. The same happens with the floaters. The reason you only see them when looking at the sky is because they're pretty transparent and you need a bright detailless surface to see the low contrast they provide.

But it's very different from scratches on a lens surface because those are in front of the optics.

I'd love to get rid of mine ... and there is a procedure, but I looked up photos of it and ... nope. Think I'll just stick to using dark mode when using my computer instead.

Can you share a link to that procedure? I have floaters bad enough to make me consider drastic steps.

Talking to an optometrist isn't usually that drastic.

All the ones I talk to say there is no way to fix floaters.

Thus a link to the process would be useful - even if searching for reviews of that process would show it does not work.

During my last eye exam my optometrist advised me to call the office in case of a sudden increase in floaters, which might warn of a condition common in the myopic and potentially progressing to retinal detachment, but which can be easily treated during an office visit by a process I understood to involve lasers.

I didn't name the procedure because, not yet having needed to study it, I don't know what it is called. (I don't recall the proper name of the floater-producing, dangerous but ameliorable condition, either.) But I understood its intent to be preventing any risk of progressive loss of vision, rather than removing the floaters directly. Maybe we're talking about two different things, though.

Yes if floaters increase or change agree go to hospital or optometrist immediately.

As for asking for the name it is that which helps to see that we talk about the same thing without that it is impossible to know what you are talking about.

Vitrectomy would be what you are looking for. It’s fairly drastic as you are pulling vitreous humour from the eye in order to get rid of the floaters.

There’s also laser vitreolysis that’s substantially less invasive.

I’m sure there are more authoritative links but i picked this one because i thought it said ‘burningeye.com’ at first:

https://www.brueningeye.com/laser-floater-removal

And that laser one is the one that there is an issue with.

In the UK my doctors and NHS notes say there is no evidence that this works.

The removal of the vitreous humour does work but the doctors at Moorfields, the top UK eye hospital, do not recommend it as it has what they say is a high risk of making things worse. They will do it but you have to be really really certain that the floaters are impossible to live with.

The one thing that I believe it is successful for is if people have floaters that are suspended directly in their fovea by adhesions to other parts of the eye. They use the laser to cut the ’ligaments’ so it can float away.

I’m not sure ‘breaking up’ floaters would even be desirable, they are still going to be in there making visual noise.

But they aren't, only dust on the sensor is noticeable, dust anywhere else just softens the image somewhat (unless maybe stopped all the way down with some lenses, but it would be unusual).

Edit heh, I was thinking about "lens glasses". I don't find dust on eyeglasses very noticeable unless light catches it in some particular way.

Similarly, you normally don't see any imperfections/dust specks on your cornea - but you will if you look into a very narrow (0.1-0.2 mm) beam of light (e.g., when using a telescope pushed to a very high magnification = very small exit pupil [1]).

[1] https://en.wikipedia.org/wiki/Exit_pupil

Working with a few US, it took me a while to appreciate "eye-glasses", as a thing (noun). TIL "lens glasses", is also a thing. I was rather hoping context would imply what the subject matter is.

Also distinguished from "opera glasses" or "field glasses."

It is in the Fourier domain or just non sharp where the dust is.

Never notice them, only big stains. Maybe it depends on the strength and type?

they also have a blog post about a big house fly that somehow got inside the lens but did not affect image quality

"tis but a scratch".

if only there was a lens that could ignore dust this way...