I'm playing D&D 5e, have several characters with Darkvision, one without. The lacking PC (Dragonborn) frequently has Light cast on her shield. When there's a local light on, such as an illumnating shield, I state that the Darkvision-equipped characters can see the immediate area lit by the torch, but not into the dimness beyond.
One player took umbrage when I announced it, and claimed I'd never said anything about it before -- probably true, this is our first full-on "dungeon crawl." But it makes sense to me, as Darkvision in 5e provides 60 feet of "dim light" vision in a monochrome scale. A torch provides 20 feet of bright light, and 20 more feet beyond that of dim light.
Intuitively, if I light a candle in a large, mostly-dark room, I can see stuff within the candle-light radius but beyond it I have difficulty seeing. If I extinguish the candle, my eyes adjust and I can see everything in the room, but nothing as clearly as when the candle was lit.
I found this here: "Beyond that, it's up to the DM to determine what effects, if any, glare has on a character with darkvision looking into a shadow. In some editions of the game darkvision and infravision were explicitly spoiled by brighter light sources, but that doesn't seem to be a rule anymore. The game says nothing either way, so it's up to the DM."
-- so apparently, I'm not utterly crazy, this was a thing at some point.
Can anyone tell me when Darkvision was disrupted by normal light?
How would you rule in this situation?