The PHB description of the spyglass (on p. 153) simply states that:
Objects viewed through a spyglass are magnified to twice their size.
This isn't restricted to just 5e; it is also the case in 3.5.
I've always been dissatisfied with this answer as it doesn't tell me what to do with that information, which makes it difficult to determine where the limits might be for this item. Given that this is a 1,000 gp item (for some reason), I feel that there should be a notable commensurate benefit to using it.
Due to the lack of rules on this, I would propose the following benefits when a player has access to a spyglass:
- The character has advantage on any active skill check dependent upon observing something that is 30' or further away from them. If someone is hiding where you are looking, they have disadvantage on their Stealth check.
The basis for this is that the 1,000 gp price tag would put the spyglass' value in the realm of a low-end rare magic item. The uncommon magic item Cloak of Elvenkind essentially does the opposite of this by granting advantage to the hider and disadvantage to the observer. Thus, if you are using a spyglass, you are pretty much even with someone using the Cloak of Elvenkind.
Is this proposed house rule unbalanced?
Best Answer
I'd say this creates an imbalance.
A spyglass is expensive not because it has magical powers but because it's very hard to make (using medieval technology) - making the right lenses with the right curvature and smoothness is very hard and only a select few jewelers would be able to make a good spyglass. In effect, a spyglass is equivalent to decent binoculars of the modern age (except it's 1 eye only).
The Cloak of Elvenkind helps the wearer hide better even when someone is right next to them. Looking at someone hiding like that through a spyglass wouldn't give you more - if you're looking from twice as far away you'd see what you'd see from half the distance without the spyglass. However, if you're next to the wearer, looking at them with the spyglass wouldn't make it easier to detect them - they're magically hidden and the spyglass simply magnifies the spot. You could just lean closer for the same effect.
On the other hand, if you're far away (2x), the spyglass wouldn't make the task of finding someone easier - you'd magnify what you're looking at but you still need to realize that someone is hiding at that spot.
You could definitely create your own rule that gives the spyglass an advantage but I think it just degrades the advantages of other magical items.
I hope I descrbed what I mean clearly enough. At the end, it's down to opinion - I can see arguments both ways.