This is a story about a little-known and little-discussed fact in the contact lens industry. Johnson & Johnson makes two types of lenses. One it charges $2 a pair for and tells consumers to throw out after a day. The other, it charges $8 a pair for and tells consumers to wear for up to two weeks. What Johnson & Johnson doesn't tell consumers is that for millions of lens wearers, the two lenses are virtually interchangeable.