I had a lot of trouble finding out what J1, J3 etc really meant.
I finally found an online PDF article from "INSIGHT The Journal of the American Society of Ophthalmic Registered Nurses, Inc.", October-December 2004, which says (paraphrased) that: J1means you can read 4 point type, J2 is 5 point, J3 is 6 point, and J5 is 8 point.
You can search the web for "Rosenbaum pocket eye chart" and see one device that measures J values at a distance of 14 inches.