Designed as a compact unit, this 20x Magnification Loupe from Carbatec can fit easily in the pocket, and has a keyring attached for convenience. The light automatically turns on when you slide the loupe open to give that little extra light when you need it. Making sure of a measurement can be the difference between cutting some that little bit too short, and getting it right.
An economical loupe, have one at the drill press, one at the router table, and one inside the house so there is one always at hand, and you're not spending time looking for the one you know you have, somewhere.
Batteries included.