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I have a hero looking at a city under siege from a distance. He has some slight height advantage, say standing on a small mound or hill side, so I can ignore Earth curvature at the distances involved, but not enough to get "look from above" you can achieve in mountains. Assuming he is an average person, and has no sighting help like binoculars (late medieval tech level) at what distance he can see what the seiging army is doing? He should be able to see some detail, not just tell there is a siege going on or notice large groups of troops moving.

Edit: I mean seeing the individual troopers and roughly discern how they are armed and what they are doing, like if they are preparing an assault, or eating a meal.

As far as I can check, at 300m I see quite well and at 400m I can see a person walking, though dark on dark background or light on light become almost invisible.

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    $\begingroup$ (1) An army is not a human-sized object. It is a very very much larger object. (2) It rather depends on what is meant by "in detail", which the question doesn't specify. $\endgroup$
    – AlexP
    Jan 13, 2022 at 16:53
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    $\begingroup$ a quick google search for "how far can a human see" yielded in this answer of: Human-scale objects are resolvable as extended objects from a distance of just under 2 miles (3 km). $\endgroup$ Jan 13, 2022 at 16:54
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    $\begingroup$ Re, "in detail." That's an entirely subjective term. Do you mean, enough detail to be able to tell whether the other person is wearing a red uniform or a blue one? Enough detail to see whether they are carrying a weapon? To see what kind of weapon? What make and model? Enough detail to see whether or not they have spinach stuck in their teeth? $\endgroup$ Jan 13, 2022 at 16:59
  • $\begingroup$ @SolomonSlow - in detail mean getting some useful info about their weapons or activities, but not necessarily count every dent on the shield ;) $\endgroup$
    – Archelaos
    Jan 13, 2022 at 17:06
  • $\begingroup$ This doesn't seem to be a question about building a fictional world but a question about the visual capabilities of humans in the real world. $\endgroup$
    – sphennings
    Jan 13, 2022 at 17:07

1 Answer 1

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The separation power of the average human eye, in good light, is about 1 minute of arc, or about 0.0003 radians, or about 0.29 mils. Which means that an average observer --

  • Can see a person (as a dot) at about 2 or 3 km or 1 or 2 miles, provided that the person wears clothing which makes a good contrast on the background.

  • Can distinguish the upper and lower body of a person at about 500 to 700 meters.

  • Can distinguish the head of a person at about 250 meters.

  • Can distinguish the eyes of a person at about 100 meters.

  • Can distinguish the irises of the eyes of a person at about 25 meters.

Again, this is in good light, and assuming that the target person is not wearing camouflage.

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  • $\begingroup$ Thanks, that's exactly the type of info I was looking for. Prior googling only gave me results on stars and candles in the night... $\endgroup$
    – Archelaos
    Jan 13, 2022 at 17:24
  • $\begingroup$ Way more detail than I expected, great answer. $\endgroup$
    – TKoL
    Jan 13, 2022 at 17:55
  • $\begingroup$ Fun fact: the resolution of the human eye is pretty much determined by the size of the pupil (the term of art is "diffraction-limited".) This makes the concept of super-acute elf vision in Tolkien's work somewhat problematic. $\endgroup$ Jan 13, 2022 at 20:44

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