How a lens is made?

First they must make the glass. Optical glass is made by melting specially selected sand and various metals to set the "refractive index" - the way the glass bends light when it's made into a lens. This sometimes takes weeks to get the glass exactly the way they want it. Then there are a couple of ways to form the glass into lens elements. One was invented by Minolta, and involves pouring the glass into molds that have the proper curvature already cut into them. This is how most lenses are made. Really good lenses are made by a technique invented by Nikon. Optical glass forms bubbles when you pour it. They don't affect the performance of the lens but they look bad and at the prices Nikon charges that's not a good thing. So...one of Nikon's fine engineers said something like, "why don't we let the glass harden in the pot, then shove the pot off the roof?" They tried it...perfectly flawless pieces of glass, after they got done picking them all up. This, needless to say, is only used on really expensive lenses. (Zeiss pours their lenses into molds with flat tops and bottoms, inspects the finished blanks and throws the ones with bubbles back in the pot.)

After the elements are ground and polished, they are coated with extremely thin layers of metals. This is the antireflection coating and it does two things for you: lets more light through the lens and gives the lens that neat greenish color camera lenses have.

A lens barrel is produced to hold all the elements, plus the diaphragm and sometimes the shutter. All those parts are assembled into a finished lens.

After the lens is tested to ensure it meets the manufacturer's specifications, it is boxed and shipped.