Because of my advanced age, I am afflicted with presbyopia. To correct the problem, I have a new pair of glasses with progressive lenses. These are commonly known as no-line bifocals, but really, they are a different beast than the classic bifocal lens, because there is a gradual progression between the two ranges of correction.
This also makes them difficult to get used to. It seems like everything I look at is out of focus until I move my head properly to be able to see it. I'm told I will get used to it. We'll see (no pun intended).
But they got me interested to learn more about lenses. The geometry at work here is extremely complicated. And how exactly are they made? After all, the curves are complex and subtle, and the lenses that result will be used in very sensitive-to-error situations (that is, I'll be looking through them for a few years). How Stuff Works comes through with a good description of how standard lenses are made.