Even to the most seasoned photography veteran, lenses are often still a mystery. Ask any photographer what "focal length" really refers to, and he'll probably shrug his shoulders in ignorance. If there was anything you ever wanted to know about camera lenses, then this article by Haje Jan Kamps will most likely have an answer for you, in plain language that anyone can understand. This article is a must-read and must-bookmark for future reference, full of very useful information.
Haje Jan Kamps writes:
If you've ever wondered what focal length was, you might have looked it up on Wikipedia or similar, and you'll have been met with the extremely unhelpful "When a photographic lens is set to infinity, its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image