What do you see if you cast Darkvision on yourself, but your retinas are damaged and you cannot see normally?

Question

Mike "The Metal Detector" is an almost completely blind sorcerer.

I'm not applying the blind condition, but I impose disadvantage on anything which requires sight, and auto-fail anything which requires seeing detail.

Early in his training, before he was at a level to control it, he ignored his tutor and tried to cast a high level spell which blew up in his face. His eyes are still there but the heat damaged his retinas and he cannot make out any detail, only discern how bright the room is.

What happens if he casts the Darkvision spell (PHB 230) on himself?

Darkvision is always (to my knowledge) used by creatures who also see normal light. So, what would Mike be able to see, for those 8 hours? I can see some possibilities:

1. Mike can see only the additional spectra of light that a darkvision creature can see relative to a non-darkvision creature, although I'm not sure what this would look like.

2. It doesn't work at all — darkvision works by upgrading your existing vision functionality and with such badly damaged retinas, the spell fails.

3. He gains full vision, including the 400–700nm range, and can see as if he was an orc with darkvision.

What happens?

Answer 1

It'll come down to how you rule your player's near-blindness works.

If you rule that they suffer from the Blinded condition, then Darkvision doesn't negate that. Here's what the Blinded condition states:

• A blinded creature can't see and automatically fails any ability check that requires sight.
• Attack rolls against the creature have advantage, and the creature's attack rolls have disadvantage

Since they can't see it would seem that the vision granted by Darkvision is negated by blindness. The darkvision spell doesn't mention anything about negating the blinded condition - that would seem to be reserved for spells like lesser restoration.

So they'd be under the effects of the darkvision spell, but wouldn't be able to see anyways. The spell could hypothetically be dispelled, not that the subject would notice.

I only bring up the blinded condition because it would make sense that you use that for the mechanical effects of his near-blindness, but if you're ruling the effects of his impairment on a case-by-case basis, you'll have to do the same here. The obvious assumption is that the spell doesn't really help, but since you're firmly in homebrew/dm-ruling territory, the rules aren't going to give you a strict answer.

Part of 5e's philosophy is to empower the DM to make the ruling where there are gaps in the rules. With that in mind, you'll have to decide how close you want to hug the rules for the blinded condition.

Answer 2

Scenario number three is correct.

The spell grants darkvision to any willing creature, and does not require any special anatomy. For example, you could cast darkvision on a Grimlock, or a Grell, or any number of monsters that don't actually have eyes. A literal, direct interpretation of the rules states that they would then be able to see. Additionally, note that these creatures, while blind, are immune to the blinded condition.

Remember that this is a magic spell; it doesn't have to hew to real-world physics or logic because it is literally magic. It's ultimately up to the DM to rule on how this will actually work, but I think that there is good RAW support for scenario 3.

Answer 3

PHB p.20, on Dwarves:

Darkvision. Accustomed to life underground, you have superior vision in dark and dim conditions. 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. Vou can't discern color in darkness, only shades of gray.

PHB p.184:

Darkvision: Many creatures in the worlds of D&D, especially those that dwell underground. have darkvision. 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, the creature can't discern color in darkness, only shades of gray.

MM p.9:

Darkvision A monster with darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in darkness as if it were dim light. The monster can't discern color in darkness , only shades of gray. Many creatures that live underground have this special sense.

Even though none of these explicitly refer to the spell Darkvision, the mechanics seem to be consistent in in each incarnation of the ability.

What seems most logical to me is the character would still have the same visual impairment, with the following difference:

• In dim light, the room would appear as if it was lit with bright light.
• In darkness, the room would appear as if it was lit with dim light.

So in other words, the characters vision appears brighter in low light situations. If the character is incapable of seeing color, then in darkness the color would also appear as a shade of gray.

Answer 4

What do you see if you cast Darkvision on yourself, but your retinas are damaged and you cannot see normally?

Exactly the same thing you would if the light level was changed to the level darkvision effectively provides in the circumstances, (with the exception that you lose colour vision when it is completely dark.)

If you are partially sighted normally and are in bright light provided by a lantern, have darkvision cast on you and then shutter the lantern so you are in dim light, you see as if you are partially sighted in bright light again.

Darkvision PHB p.230

You touch a willing creature to grant it the ability to see in the dark. For the duration, that creature has darkvision out to a range of 60 feet.

The spell description states that the effect of the spell is to add the darkvision ability to a creature. It does not change the character's visual sense in any other way.

Any of the PHB races with darkvision (e.g. Teifling PHB p.43) describes Darkvision:

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.

Darkvision just changes the effective lighting level when it is dim or dark. If you can't see very well normally due to damage to your eyes then darkvision does nothing to improve that.

Ps. I believe you should be very careful about applying ideas of modern science to the way magic and abilities work as it will lead you the wrong way as to the intent and playability of the game and it's setting. Talking about wavelengths of light and retinas makes it sound like magic in D&D cares about modern science, which it does not in any way.