How does a magnifying glass benefit a player?

Question

The Investigator background grants the player a magnifying glass in their starting equipment. The item's details mention starting a fire in broad daylight, and have the following added:

A magnifying glass grants advantage on any ability check made to appraise or inspect an item that is small or highly detailed.

What I was wondering is, if you could add this benefit to spells like the Identify spell, Find Traps, Detect Poison, skill checks like insight or investigation, or rolls to discern an illusion?

Furthermore, if one had expertise in investigation, would the magnifying glass add any benefits?

Answer 1

It does what it says it does

To your specific questions:

• Spells do what they say they do, no matter if you look through a magnifying glass or not. A magnifiynig glass would not interact with Identify, Find Traps or Detect Poison. Identfiy, for example states "You choose one object that you must touch throughout the casting of the spell. If it is a magic item or some other magic-imbued object, you learn its properties - you do not even have to look at the object or creature for it to work.

• Investigation is "When you look around for clues and make deductions based on those clues" (p. 177 PHB). The magnifying glass description tells you it gives you advantage on "on any ability check made to appraise or inspect an item that is small or highly detailed", so it could help you when you are looking for clues on a small or highly detailed object, but not in general.

• Insight (p. 178 PHB) "involves gleaning clues from body language, speech habits, and changes in mannerisms. None of these sounds as if you could do them better by looking at a creature through a magnifying glass, unless the creature was a diminuitive tiny one that you could not otherwise see clearly, so in nearly all cases it will not help on Insight.

• Illusions "deceive the senses or minds of others. They cause people to see things that are not there, to miss things that are there, to hear phantom noises, or to remember things that never happened. (p. 203 PHB). The magnifying glass would not work against them in general. Surely not against illusion that deceive the mind directly, or that target other senses, but even for illusions that target your visual sense, you either believe the thing is there or not. None of the spells that create illusions state that the illusions have tiny glitches that you could see with a magnifying glass. You might get advantage on illusions that allow for an Investigation check to see through them, if they are so small or intricate that they qualify as discussed under Investigation. Most such illusions are easily discerned by just touching them, so with needing to get close enough to look at them through your magnifying glass, the benefit here is questionable.

• Expertise in a skill means "Your proficiency bonus is doubled for any ability check you make that uses [...] the chosen proficiencies. This is independent of advantage or disadvantage. Having advantage on investigation such as granted by the magnifying glass will not influence the expertise bonus.

Your question reads as if you are looking to get mechanical benefits from a owning a magnifying glass beyond the ones given in the item description. However, anything granted beyond what the item tells it does will depend on the DMs interpretation of what you could reasonably achieve. There are no hidden rules, so there are no hidden mechanical benefits you could demand.

Answer 2

As always, talk to your DM

The specifics of how any particular item can be used is going to be up to the table and your DM.

In these cases, I've taken some cues from 13th Age and Scum & Villainy games. If someone wants to use X (item, mechanic, ability, etc.) and can narratively come up with a good story for why it makes sense, then I'm very likely going to allow it. On the flipside, if you constantly try to shoehorn strong things into weak areas because you want to always optimize, I'm likely going to see that pattern and be a little more strict on what narratives make sense.

But overall, determining if any item is applicable (or any mechanic) is going to be up to the DM to determine. The rules state you simply describe what you want to be doing and the DM will ask for the roll with any bonuses from any mechanics that they think fits based on your description.

Looking for more than is on the tin?

In general, if you are looking to see if something will do something more than what it specifically says, then this is always going to be up to the DM determine.