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Devil's Sight (PHB p119) allows someone to "see normally in darkness":

Devil's Sight
You can see normally in darkness, both magical and nonmagical to a distance of 120 feet.

Our ideas of normal vision includes lightness differences: uneven surfaces like crumpled paper catch the light differently, and even a tree in a dark new moon night may cast a subtle shadow away from the moon.

Would someone with darkvision or Devil's Sight be able to discern those lighting differences we consider important to our normal vision, or detect where subtle shadows are when there's a faint light source to cast them? Do the rules have anything to say, or is there guidance from closely connected material or other editions available?

This is a "magical" ability for the most part, but it seems like if you don't need light to see normally, you may not be able to discern different lightness levels at all.

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Breaking your overall question down you ask:

  1. Would someone with darkvision or Devil's Sight be able to discern those lighting differences we consider important to our normal vision?

  2. Would someone with darkvision or Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?

  3. Do the rules have anything to say, or is there guidance from closely connected material or other editions available?

Darkvision

A discussion of the difference between these the rules describing darkvision can be found here what is the correct interpretation of darkvision and the rest of this answer will assume that darkvision is consistently applied as written in the race descriptions (e.g. dwarf PHB p.20) though it actually does not make much difference to the answers.

Darkvision PHB p.20:

You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

There are two specific levels of light covered here:

  • Dim light, where there is a source of light
  • Darkness, where there is no source of light

In both cases darkvision enhances vision such that "you see as if it were [the next higher level of lighting condition]". Obviously you can also see at all light levels in between using the same "as if" rule and there can be no shadows if there is no light source. As always the light level present are adjudged by the DM.

Addressing your questions one by one:

1. Would someone with darkvision be able to discern those lighting differences we consider important to our normal vision?

Using the "as if" rule YES, with the exception that colour is replaced with shades of grey in darkness. If a creature with normal vision would be able to discern the lighting difference in the "as if" lighting condition, then so can a creature with darkvision in the actual lighting condition. Otherwise they can't.

2. Would someone with darkvision be able to detect where subtle shadows are when there's a faint light source to cast them?

The question appears to be asking "would you be able to see subtle shadows if the lighting conditions were less than dim but more than darkness?"

If the DM adjudges that a creature with normal vision in the "as if" light level would be able to see the subtle shadows in question then YES, a creature with darkvision can see them in the actual lighting conditions. There is obviously no sudden jump between darkvision's benefits in dim light to those in darkness, it is continuous.

Devil's Sight

There is a discussion about how the Warlock ability Devil's Sight works here.

Warlock's Devil's Sight (PHB p108):

You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.

This lets you see in total darkness as if it were full light. You gain no bonuses in dim light from Devil's Sight so the answer for darkvision above applies. However for full darkness you get "normal" sight. So it is full colour and any enhancements (goggles of minute seeing) also work. However as there is no light source there will be no shadows. Weird.

Addressing your questions one by one:

1. Would someone with Warlock's Devil's Sight be able to discern those lighting differences we consider important to our normal vision?

In dim light no. In total darkness yes, with the caveat that there would be no shadows. Darkvision is a separate ability.

2. Would someone with Warlock's Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?

No, it has no effect in dim light, only in darkness. Darkvision is a separate ability.

A devil's Devil's Sight works differently, however. For instance the Barbed Devil (MM p.70):

Magical darkness doesn't impede the devil's darkvision

So the answers for darkvision above would apply, with the caveat that magical darkness does not impede the darkvision.

Darkvision and Devil's Sight

3. Do the rules have anything to say, or is there guidance from closely connected material or other editions available?

The RAW from the 5e core rule books actually cover the questions you ask. I can find no other guidance specific to your questions in 5e supplementary material. As stated elsewhere, rules from other editions are not a reliable guide.

To list the sources defining darkvision that I have found in the core rule books:

  • PHB p.183 Darkvision
  • PHB race darkvision descriptions (e.g. p.20 dwarf)
  • Monster Manual p.9 "monster" specific darkvision
  • PHB p.108 Warlock's Devil's Sight
  • Monster Manual p.70 Barbed Devil (and other devils) Devil's Sight
  • (Sage Advice on Devil's Sight)[http://www.sageadvice.eu/2015/11/18/devils-sight-vision/]

And there are also rpg stack exchange questions on this, as referenced above: - what is the correct interpretation of darkvision - how should a warlock's Devil's Sight work

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  • \$\begingroup\$ This is exactly what I was looking for. Thank you. \$\endgroup\$ – Skathix Dec 13 '16 at 15:50
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Darkvision states:

You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

It doesn't rely on light to function, as it grants you vision in total darkness. Expanding on Adam's answer, I would say that darkvision has effects like modern active-infrared night vision:

photo of lions at a watering hole

This image was shot using only infrared light. There are light differences and projected shadows, although the general depth perception is not great.

So yeah, you actually project infrared rays from your eyes. How cool is that xD

On the other hand, Devil's Sight states that you can see normally. Normally includes colors and everything, so I think it's more a magical trait that relies on a reality-twisting effect like most spells or monster traits do (eg: beholder levitation).

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    \$\begingroup\$ I am really wary of using modern science and technology to explain magical effects and fantasy race abilities. It can be misleading as to intent, how the ability works and can break the feel, mystery and mood. Magic ignores our understanding of science anyway it wants, whenever it wants, that's why it is called magic. Darkvision has nothing to do with infrared, photon multipliers, photosensitive chips or light sensitive retinas, if for no other reason than it works even if there is no light whatsoever. Much better to try and explain things in terms of the "alchemical mysteries" of the setting. \$\endgroup\$ – Protonflux Dec 2 '16 at 12:11
  • \$\begingroup\$ Although I see where @Protonflux is coming from, I actually like this answer as I was looking for something along these lines, though it's still nothing canonical. I had hoped I overlooked something in the Lore of D&D but it seems like they have left it ambiguous, probably on purpose because FM Theory rules all. \$\endgroup\$ – Skathix Dec 13 '16 at 15:40
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Devil's Sight

You can see normally in darkness, both magical and nonmagical to a distance of 120 feet.

The ability says it all. The intent is that there is no gameplay difference for darkness for the character with devil sight. No bonus, no penalty. Whatever pitiful light sources are available, would cast their pitiful shadows, if that's important. It does not indicate what level of light is available, so assume whatever brightness a character needs to see something they are looking at, they could see it. (except for hide/stealth)

This is not infravision, this is not dark vision. This is an ability that restores normal sight to the character who has it, colors, shadows, subtle patterns, etc. You could read a novel in the darkest cave. I think this answer is just the normal, boring answer, sorry.

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I would think you should be able to see shadows of some sort.

Modern low-light vision devices basically do one of two things, they either amplify ambient light several times over, or they project infrared light as a light source. (More in depth here)

Typical darkvision is likely the former, since it's described much more like a physical adaptation rather than a magical effect. In this case, the object which blocks what little light you are amplifying still casts a shadow, no matter how subtle. If you amplify the light enough, you will also see the shadow. So it is reasonable to assume that darkvision can detect differing lightness levels to some small degree. The shadows will likely be very hard to distinguish though, since darkvision is grey-scale, which is why darkness still acts as dim light with darkvision.

Devil's sight is a magical ability, so it's hard to say how it works. If we assume that it projects light or magic from your eyes, then the objects you see can block those projections, and that will create shadows. In this case, again, you should be able to notice those shadows, especially since Devil's Sight allows you to see as if the darkness was totally illuminated.

However, if Devil's Sight magically tells your brain what to see, then it would be pretty up in the air. Either it would need to assume a certain light level and origin of all light, which would cause you to perceive shadows, or it would give you the image as if there was an equal amount of light coming from all directions, which would elimate all shadows from your sight. Ultimately, because magic should make life easier, I think it's best to just say that you can notice enough variation in light levels to have reasonable depth perception, but nothing more.

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The question as posted, as I understand it, is a non sequitur. Please correct me if I'm wrong and I'll edit this answer accordingly.

Does Darkvision have a concept of detecting lighting differences?

No, it's magic.

Would someone with darkvision or Devil's Sight be able to discern those lighting differences we consider important to our normal vision, or detect where subtle shadows are when there's a faint light source to cast them?

Yes, it's magic.

Do the rules have anything to say, or is there guidance from closely connected material or other editions available?

I won't repeat the relevant citations of dnd-5e rules posted in the other answers, but the concept being used in the 5e game is the same as 3e and 4e. Namely, it's magic. It just works.

As for editions prior to 3e, your question is apt. Just look at the name of the ability; Infravision. Clearly it was meant to mimic what we see in the photo under Manzotin's excellent answer.

The WotC game designers seemingly didn't like that 'old school' approach, as Sean K. Reynolds has said,

Infravision rules have a lot of holes in them, and cleaning up those holes isn't worth the time and effort to do so. Darkvision is an elegant solution (it works just like normal vision and doesn't rely on a science explanation that can be misinterpreted or exploited) and doesn't require all of this extra work.

As you can see from the question by Skathix and the varied answers it's gotten, one has grounds for doubting Mr. Reynold's indictment that Infravision is "full of holes" or especially that Devil's Sight, and similarly Darkvision, is by comparison "elegant".

As just one example, I think the fact that 3e has the Hide in Shadows skill, which can iirc, with a successful opposed role against an opponent's Spot skill, hide against Darkvision... somehow... 'cause... something. Magic maybe?

"Elegance", it would seem, is in the eye of the beholder.

What follows is a deleted comment under the main post where I was trying to ascertain if the question is not asking what it seems to on a plain reading. Since that was disallowed, perhaps repeating it here will answer the question by backing into it (so to speak).

I think a key to this issue is in the PHB quote that starts out this question.

You can see normally in darkness, both magical and nonmagical to a distance of 120 feet.

IMO it should really say,

You can see abnormally in darkness, both magical and nonmagical, to a distance of 120 feet as if you were viewing it under conditions of normal lighting.

Or perhaps, to best get the RAW intent, it could hypothetically be reworded thus,

Regardless of lighting conditions you can see normally to a distance of 120 feet; yes, even in conditions of darkness or magical darkness.

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While your bounty calls for official sources, I think that the "seeing normally" part in your quote sufficiently answers your question: what you can see is perfectly normal, down to the subtle differences in light.

To prove my point, you can take a look at real life darkvision here: https://youtu.be/7HslR6sljco?t=176

This is a video of a Canon camera that can have an ultra-high sensitivity to light, such that it can capture images that look normal in almost total darkness (note that ordinary ISO values range from 100 to several thousand, with higher values corresponding to greater sensitivity, while this camera can have ISO values in the millions). In these images, you can clearly discern subtle variations in lighting, even when the conditions are essentially pitch black.

Additionally, if you're unable to distinguish light levels at all, what does it mean to see?

Finally, I'd argue that determining light levels is a normal part of seeing. Just as someone would recognize a dimming of a light while still being able to see, a darkvision-haver would be able to recognize when the light levels approach darkness.

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    \$\begingroup\$ Keep in mind that that Canon is nowhere near the state of the art in low-light imaging -- the best electron-multiplying CCDs are at the point where they can reasonably be used to count individual photons. \$\endgroup\$ – Shalvenay Dec 1 '16 at 23:13

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