I have prescription reading glasses, fine for reading a book, or anything else from about a foot to 6 or 8 feet, but not really strong enough for N scale modeling. My optometrist found a line of small clip-on magnifiers in one of his catalogs, and I have a 2x version. It clips onto the glasses, sits about 2 inches in front of the lenses, and lets me read N scale dimensional data - at 6 inches. Beyond that, it's all a blur.
I also have a retired pair of my prescription glasses, since I can get a new pair every two years, through my insurance. I leave the magnifier on those, as it's easier to change glasses than add and remove the magnifier. They're hinged, so can be flipped up for normal reading, but the hinge wore after a few years, and wouldn't stay up. I glued it solid.
