I dug up a pair of old binoculars to bring on the Dungeness Spit hike, and have since started going to local parks and using them to watch boats. It's generated some good, first-person qualitative data for the wilderness vision distance question.
There's another park across the lake from the park I go to. It's about two miles of water. With 8x magnification, on a clear day, I can count the cars in the parking lot, clearly make out the portable toilet, and can tell one person (or person with dog) from two people from five-ish people, but can't really discern anything about them other than that they're bipeds. With the naked eye, I can often make out the glint of metal or glass from there on a sunny day but otherwise would be hard pressed to tell it apart from a field.
It is also easy to make out the contours of mountains about 40 miles away (I think, that's about where the range begins), and which parts of the mountain have snow on them, again provided clear weather. To the naked eye, the view is still pleasing but I would be hard pressed to make any plans off of it.
With 8x magnification, I can sometimes make out the tail art on airliners coming and going from the airport about 15 miles south of me as they pass overhead, but I have been unable to observe the flight numbers. I was able to identify an F-18 as such, and to see that an H-60 helicopter in the distance had its side doors open (I could see sky through it).
I was able to observe a pair of helium party balloons (lost from a party I presume) ascending about a mile north of my position, though I couldn't see them with naked eye so I stumbled on them accidentally while watching an eagle. They were silhouetted against the sky or I don't think I'd've been able to see them with magnification either.
Sometimes I can read the numbers on the sails of boats; I don't actually know how close they were or how large the numerals were though. Generally reading the names of small boats off their bows is beyond me unless they're quite close, but I have been able to read the names of two large tour boats off of their bows at maybe half a mile of distance, one in twilight. Sometimes in the evening the air over the water shimmers like hot pavement and it gets hard to make out details of anything. The lights from houses across the lake twinkle like stars.
The view of the moon is very good, especially because it is full right now. I could make out some craters that were at an angle to the sun, and to see the shadows inside their rims. I have never seen the moon like that before. I was also able to observe some satellites, as basically dim stars moving on smooth tracks.
I really wonder how fleets coordinated their actions before optics. I feel like making out semaphore at a mile without a spyglass would be really hard. Did they just sail in really close order? Did captains just have a ton of autonomy? I was reading about the audible range of hunting horns and elk bugles a while back and I recall those being about a mile or so depending on terrain, but I would guess that auditory signaling might be hard at sea with the wind carrying it away.
ACKS' description for Eyes of the Eagle has them giving 100x magnification, which is pretty nuts. Low-powered binoculars are 7-8x, high-powered are 10-12x. Spotting scopes for long-range shooting are generally 24-60x, and that lets you observe groupings out to hundreds of yards. 100x is in an awkward spot between high-power spotting scopes and low-power astronomical telescopes. With 100x magnification, I think it would be pretty easy to count individuals a mile or two away (but your field of view would also be obliterated - there's no way you could just wear 100x magnification binoculars like contact lenses in your daily life, never mind dungeoneering).
Amusingly, none of the other rulesets I checked (OSRIC, S&W, 3e, PF, 5e) listed a magnification for their Eyes of the Eagle. 3e, Pathfinder, and 5e did have a nonmagical spyglass though, with 2x magnification for 1000 gp, which is a little funny since they weren't definitely invented until 1608, well into the Gunpowder Age, and since Galileo also developed 8x and 23x telescopes by 1609.
I was surprised how much atmospheric effects mattered even over this fairly-short distance of two miles.
"The glint of metal" is probably a fine way to introduce a distant wilderness encounter with sentients.