The wonderful photographer and writer NK Guy writes,
As photographers and geeks we tend to obsess about the products we buy. Countless hours are spent poring over catalogues, reading reviews, arguing in forums. All to find the perfect camera. There's just one thing missing here: the /lens/ is also a key part in creating high-quality images. But learning about interchangeable camera lenses, and all the strange arcane terminology that goes with them, can be very difficult. Hence "The Lens: a Practical Guide for the Creative Photographer." It explains all the things you actually need to know about lenses, and why.
Why is a lens "fast" or "slow"? Why are seemingly similar lenses so different in price? Why is there alphabet soup printed on the side of the lens barrel? Every type of lens is described in practical terms, and there's even a section on breaking out of manufacturer-dictated choices - how to adapt and modify lenses to fit incompatible cameras!
The Lens [Photonotes.org]