How far away can you see light?

Question

Darkness is dark.

Torches and the Light spell provide bright light for 20 feet, and then dim light for another 20 feet.

A dwarf (with 60 foot Darkvision) standing next to a lit torch could see for 20 feet as if in bright light, 20 feet beyond that as if in dim light, and then, due to Darkvision, a further 20 feet as if in dim light but in grayscale. (Unlike earlier editions, having darkvision nullified by nearby light is not in 5e.)

But how far away can that light itself be seen in the darkness? If there's a huge underground room 200 feet long, and our torch and dwarf are at one end, can a human in darkness at the other end of the room see the light and dwarf? Does the human still count as blinded because he is currently in darkness?

Answer 1

You can see light at any range

I did some googling, and while it's pretty hard to find specific citations from scientific studies, the places that I've been able to find say that the human eye can see a candle from somewhere between 10 and 30 miles away. The curve of the Earth is about 3 miles away. Thus, any significant light is at least barely visible from any range that you're likely to have line of effect. The houserule that I've used for a while now is that you can see a light source at ten times the distance that you can see a non-lit object without penalty.

That said, the vision rules in every edition of D&D that I've seen are actually reversed. They only work if both the looker and the looked-at are in the same lighting conditions. The vision rules on PHB 183 state:

In a lightly obscured area... creatures have disadvantage on Wisdom(Perception) checks that rely on sight.

A creature in a heavily obscured area effectively suffers from the blinded condition.

Nothing in those rules says that you can't see a creature if that creature is inside darkness and you aren't. This is clearly ridiculous. This weirdly reversed rule has existed since at least 3.0, and shows how little the designers thought about what to do about differing light conditions.

What this means is that you should rely on your intuition more than the rules for what will give penalties based on vision. Since, IRL, lights can be seen from the horizon, you can probably see someone with a torch from at least a few hundred feet, probably out to a mile or so.

As far as being blinded is concerned, my intuition was always that that penalty happened because you couldn't see the ground beneath your feet, or the things that are around you. Thus, I would rule that a character who can see a light hundreds of feet off is still blinded, except for the purposes of making checks or attacks against targets who are lit up.

Answer 2

How far away can you see a light on a pitch black night? That's how far you can see a light. Two hundred feet across a cavern, you can easily see a light if you've got eyes in your head.

The game assumes we understand how light and darkness work in a very basic, practical, everyday sense. The rules for vision and light provide mechanics for specific circumstances where mechanics are necessary, but where they are silent they don't eliminate what you know. (Ditto, even though there are no rules for gravity except when falling, the game assumes you know that opening a bag won't make its contents float away!)

For soft rules support of this “light works like it makes sense to work” philosophy, see the Dungeon Master's Guide page 243, where visibility is given in miles, atmospheric effects reduce visibility approximately like we know they do, and it notes that “creatures automatically notice each other once they are within sight or hearing range of one another” if they're not being stealthy. (Carrying a beacon through the darkness is not stealthy.)

The case of a torch 200 feet away across a pitch black cavern is pretty straightforward, but less certain circumstances are going to be at the DM's judgement. Can you see torches moving around at night on a hill three miles distant? I don't know (though I could probably look it up.) What about if the hill is wooded? I still don't know. Those edge cases, where it's not obviously visible and not obviously too far away, are going to be the DM's judgement.

Though you could look it up and I'm sure there's a science-ful answer to be had involving precise equations and atmospheric measurements, during a game I'd recommend the much more pragmatic “whatever suits the unfolding story and moves the game forward.” For most purposes, that should be plenty precise — it's a rare campaign or in-game situation where anything more than hand-wavey precision actually matters or is practically useful. If you do want to get simulation-y, you should consider compiling your own guidelines based on looking up real-world data and equations between games, so you'll have an easy, practical table-use reference to decide this on the fly.

Answer 3

What the Rules Say

See the "Vision and Light" section of the PHB (p. 183), or here in the basic rules:

A given area might be lightly or heavily obscured. [...]

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A).

[...]

Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.

So, the question hinges on what the phrase "blocks vision entirely" means. Does it mean that you cannot see things that are in the area of darkness or does it mean that plus you cannot see through the area of darkness.

Our current physical model is that vision is a process caused when light that travels through space strikes our eye and causes our visual neurons to fire. This is not what we have always believed; before the scientific era, it was commonly understood to be something that the eye generated - who is to say that this is not the way it works in a magical universe?

Our current scientific concept suggests the first interpretation but the magical interpretation is consistent with the second; it is, in the end, up to the DM.

Using the first interpretation:

1. There is no limit to the distance the light can be seen.
2. The human can see the dwarf, the dwarf cannot see the human.
3. The human never counts as blinded, the rules say effectively blinded. He is not effectively blinded when interacting with the dwarf; he is effectively blinded when interacting with his immediate environment. IMO the use of the phrase effectively blinded is counter-intuitive, it would be better to say that the people and objects in a heavily obscured area are effectively invisible - this creates the same game effect without the confusion.

Using the second interpretation:

1. The light cannot be seen.
2. Neither can see each other.
3. The human is effectively blinded.

What happens in the real world

You have a total visual range of 0.000001 to 100,000,000 candellas per square metre - a range of 10¹⁴.

However, at any given time your effective contrast range is about 100:1 - the brightest thing in your field of view that you can distinguish can be 100 times brighter than the darkest thing that you can distinguish; anything darker than this is too dark to see, anything brighter than this is too bright to see.

Where your current contrast range sits in relation to your absolute range depends on physical (your pupil changing size) and chemical (changes in the retina) responses to the total amount of light hitting your eye.

If you are close enough to the aforementioned dwarf/torch combo that the illuminated region is a significant part of your field of view then you will be adapted to see the things that are illuminated, everything else will be "too dark" to distinguish (200 feet is well within this area). If you are far enough away that it is an insignificant part of your field of view then you will be adapted to the darkness; you will be able to see the illuminated area as a bright spot but not be able to distinguish the things within it as they are "too bright".

For all practical purposes,the light of a torch on a dark night can be seen literally for miles. The transmissivity of the earth's atmosphere at sea level is between 0.2 and 0.6 per nautical mile (about 6,076 feet) for visible light (it is wavelength dependant). The horizon at sea level is about 2 nautical miles away so you will still be getting about 16% of the torchlight at that distance.

What happens in a magical world

Anything ... it's magic.

Answer 4

The Dungeon Master's Guide is actually really clear that one can see a light source from miles away, even if one is in darkness oneself. From "Darkness and Light" in the "Mapping a Dungeon" section of Chapter 5, in the upper-left of p. 105 (my emphasis added):

The light of a torch or lantern helps a character see over a short distance, but other creatures can see that light source from far away. Bright light in an environment of total darkness can be visible for miles, though a clear line of sight over such a distance is rare underground. Even so, adventurers using light sources in a dungeon often attract monsters, just as dungeon features that shed light (from phosphorescent fungi to the glow of magical portals) can draw adventurer's attention.

While there's no range given more specific than "for miles", certainly from 200 ft. away a human in darkness would see your dwarf with a torch. If you somehow had a dark hallway several miles long, or a hallway that wasn't completely dark in the middle, then the DM might need to make a judgement call that there isn't a whole lot of guidance for, but for the most part one can assume that light sources are visible through darkness.

Answer 5

Basically depends...mainly on direct verse indirect lighting.

Since you want a specific answer that would be: "Yes you can see the direct light from a torch 200 feet away; however that light would not be enough to qualify as even dim lighting to the person seeing it."

Much like you can see stars in the night sky but not your hand in front of your face.

I had trouble with a gnome rogue who planned to carry a "closed" hooded lantern for the 5-foot dim light (bright light with darkvision) radius, wondering how that would actually get seen by others in a winding tunnel.

The cell phone in a theater analogy I think is really the answer…how far away could you see a dim light like that being shone straight down and not aimed directly at you (like the closed lantern would) that illuminates a 5-foot circle?

If there was a brighter light source with you it's possible you would not see it at all…and even with darkvision the light from the lantern would be indirect and dim so at best I was thinking you'd see a slightly lighter / brighter shade of grey…that you might not notice unless you were really alert.

In that case darkvision might actually be more of a hindrance because you need discern different shades of grey where as a human in complete darkness would suddenly see "something".

Of course direct light can be seen from virtually any distance even when reflected…if you're paying enough attention to notice it.