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?