The mirror's focal distance seems to run between about 1-2" from whatever you are looking at. I just ballparked the 2" from my eye. It will still magnify clearly when held at much greater distances from the eye, just needs to be close to what you're looking at. I'm not an optics guy, can't explain the whys and whatfors, just know that it lets me look at my own eyeball with exceptional clarity. I'd been using this when machining in tight places. I'd asked my dentist about them. He says they're back surface mirrors and distort the image slightly. He prefers using front surface mirrors and those neat (but expensive) glasses with the long standoff lenses. Sorta like a microscope you wear...
Jon