In uniform, shared light levels, vision penalties due to darkness work fine, intuitively, and exactly as written (no surprises). The deeper the darkness, the more trouble you have seeing, hitting things, and protecting yourself.
This, however, isn't the case you're asking about.
Mixed lighting conditions
Let's stick with RAW to analyse this situation of two people in different lighting conditions. The results are almost entirely the same whether whether Aspartame is in magical darkness or the (possibly very dark) shadow of an oak tree. The results are going to be weird.
These results are pretty counter-intuitive. In real life, the rogue sneaking through the shadows has an advantage on everyone else, and you've probably handled it as such — the rogue themselves isn't blinded by being in the shadows, but they are in Pathfinder!
Pathfinder's vision penalties are counter-intuitive and weird because they don't correctly model vision as we know it: they give you trouble depending on the light level you're standing in, instead of giving you trouble based on the light level of the thing you're looking at. In reality I don't have trouble reading things in the dark because I'm in the dark, I have trouble because those things are.
Like I said though, they work fine when people are sharing the same light levels. They just weren't written with differing lighting conditions in mind and don't handle them well. The sacred RAW should be burned in this case upon the altar of This Could Have Been Written Better For These Cases. Rules are worth the results they produce, and here the results they produce are kinda dumb. Those who wish to apply it exactly because those are the rules can go ahead and suffer the headaches.
Drawing sense out of the ashes.
Pathfinder should probably just be modelling how things really work: darkness affects you based on what you're looking at, not what light level you're in. Accordingly, I'm making a recommendation that in mixed light levels, the RAW be interpreted for what it was probably intended to do — which just requires a small adjustment of the original RAW:
- Dim light: Being in dim light doesn't make you have trouble seeing, but looking at things that are in dim light means you have trouble seeing those things. Past that though the RAW works fine: things in dim light have concealment and you have a 20% miss chance against them.
- Darkness: Being in darkness doesn't make you blind or suffer penalties. However, you are considered blinded for the purposes of observing or interacting with anything that is in the dark. You have a 50% miss chance against those things in combat, you have penalties to AC and perception versus things in darkness, and so on.
- Alternately, stick with the Darkness spell's function, and just grant things in darkness total concealment and forget about blindness.
Wyrmwood brings up in comments that due to firing through concealment, firing past/through dim light introduces a 20% miss chance. This causes sense-making problems if you're on a long archery range, and a tall tree off to the side is casting a shadow across part of the middle of the range: those rules assert your target dummy now has concealment, despite still being in the bright daylight. These rules work fine for actual obstructions like black smoke, but not for light. I suggest that for the purposes of firing through concealment, don't count dim or dark squares — just count what it is you're targeting.
That produces these results, which are probably actually intuitive:
- While Aspartame is in dim light:
- Aspartame is considered as having concealment from Xylitol. Xylitol has trouble seeing him, and has a 20% miss chance.
- Aspartame fires without difficulty, because Xylitol is in bright daylight and clearly visible.
- When Aspartame is in darkness:
- Xylitol is considered as being blinded for the purposes of trying to observe Aspartame. He has a 50% miss chance trying to attack Aspartame.
- Aspartame still fires without difficulty, because Xylitol is in bright daylight and clearly visible.
- Xylitol has all the defensive and sensory penalties for the purposes of any interaction between himself and Aspartame, because he can't see what Aspartame is doing at all.
We get these results, again, whether the dim light or darkness is due to magic or the shade of an oak tree.
You are correct. If you were in total darkness, that would be equivalent to Dim Light/Lightly Obscured to a creature with Dark Vision. However, you would still have to make a stealth check to hide, as the wording is "You can try to hide..." (bold added for emphasis).
Hiding (PHB p. 177)
"When you try to hide, you make a Dexterity (Stealth) check...."
Best Answer
There are no rules for resolving multiple DCs some with and some without disadvantage - use multiple checks or passive checks
Example 3 is the tricky one here, but I will cover the first two anyway. You said not to use passive perception, however both of those two examples should be resolved using passive checks as per the rules.
Refresher on passive checks:
Example 1
When standing guard you are doing something continuously, and should use a passive check. So in this case you don't have to make two rolls, instead you use passive perception and compare that to the stealth check (or DC if the enemy is not stealthy) of the enemy they may notice. When the enemy moves into the light the passive perception increases as the penalty is removed.
Example 2
Normally you would use a passive check for this rather than using an active check. As the trap becomes more well lit the passive check is adjusted and the trap may be revealed.
Example 3
This is the real problematic situation. The goblin is lightly obscured, while the goblin tracks are not. As such, you want to make a roll which covers both searching for the tracks with a normal roll and searching for the goblin with a roll at disadvantage. Unfortunately there are no rules which allow that kind of check to be made.
The problem here is the granularity of the rolls. The player is asking to do two things with one roll: search the grass, and search the bushes. By RAW you make one roll for one thing. So, break it down to two rolls; you can search the grass, then search the bushes at disadvantage.
While making multiple checks works ok for this example, it can be awkward as the scale increases. For example:
It feels strange to ask the player to roll once to search the grass, then a second time to search the barrel and the tree since those are unrelated objects far apart. It also feels a bit excessive to ask them to make 3 rolls - or more if there are more barrels/trees in the area and you don't want to tip them off as to where the items are.
However, 5e has mechanics for resolving "a lot of rolls"; passive checks. If the character needs to search a whole bunch of places, then just use a passive check. There are no guidelines as to when you should switch from active checks to passive checks other than that they are "average result for a task done repeatedly". I would suggest that if your resolution feels unwieldly then you have passed that threshold.
Remember Stealth provides equivalent bonuses
Keep in mind the rules for advantage:
Generally speaking being in darkness or in dense foliage is a circumstance which is advantageous to being stealthy. In example 3 above, the goblin is hiding in bushes and thus is at least lightly obscured. As such, they will be making their stealth check with advantage.
While it is true that the goblin will also be harder to perceive than the tracks due to the bushes, you should keep in mind that the goblin has already gotten half its bonus. A DM who is playing fast and loose with the rules could simply ignore the problem in Example 3 and ask the player to make a normal roll. Since the goblin hid with advantage they will already be somewhat more difficult to find than the tracks.
Personally I do not do this, as I want the goblin to be just as hard to find whether or not there are tracks. But if this situation comes up once in your entire campaign, it may be beneficial simply to make a quick and decisive ruling rather than digging deep.