If a D&D 5th Edition character with darkvision, such as a half-elf, is standing in light looking into darkness, does darkvision still work, or would the light negatively impact it?
For instance, is a character's ability to see, detect, or perceive negatively impacted by standing outside a dark cave and looking in, or when standing guard by fire and looking out into the night?
The rules don't state that you are unable to use darkvision when also in an area of normal or any other kind of light; therefore, other light sources don't spoil your darkvision. Any area of darkness is treated as dim light within the range of a creature's darkvision.
There's actually an interesting historical note here, since this particular mechanic changed significantly between editions.
In 1e/2e AD&D, creatures did not have darkvision but instead had infravision - the ability to see in infrared, essentially seeing heat. This was explicitly spoiled by the presence of other light sources, as described by the rules and also noted in the example of play section "The First Dungeon Adventure" given in the 1e DMG on page 97:
DM: (nodding agreement) "You remember that the torches will spoil the infravisual capabilities of the gnome and the halfling, don't you?"
LC: "Certainly, but the humans must be able to see! We will go down the stairs now, with weapons drawn and ready."
This changed in 3e, when infravision was replaced by darkvision. Rather than seeing infrared, darkvision seems more like some sort of light amplification effect, and it was explicitly not spoiled by other light sources, as noted in the "Example of Play" section on page 8 of the 3e DMG:
Tordek: Fortunately, the torchlight won't spoil my darkvision - that'll help us navigate in the dark down there.
So, though the precursor to darkvision in earlier editions did behave this way, it was deliberately changed with the introduction of darkvision in 3e and that change seems to have held in subsequent editions - certainly, nothing has explicitly overruled it again.
Aside: the Example of Play section of the 3e DMG is pretty much a direct copy of the story from The First Dungeon Adventure section of the 1e DMG translated to 3e mechanics, which is why "Tordek" feels the need to explicitly mention his darkvision is unaffected by other light sources (in contrast to the previous edition's rules). It's also amusing to see the style differences between editions - both end with a character climbing up to a secret passage and being ambushed by ghouls, but the 1e version of the narrative has the DM decide that the paralysed PC is handily devoured out of sight of the rest of the party, whereas the 3e narrative ends on a cliffhanger at least entertaining the possibility that the party might be able to rescue her...
The 5th edition rules don't impose any penalties for looking against the light. The fact if the character has Darkvision or not is irrelevant. Rules as written, Darkvision only allows you to see in darkness:
Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned.
However, 5th edition empowers the DM in ways that 3rd, 3.5, and 4th did not. While rule zero has always applied, 5th edition chooses not to explicitly codify many things. The rules for stealth and vision are deliberately simple in 5e, all the complicated situations are the subject of the DM's adjudication. If your DM says you can't see against the light, you can't.
