What is a telephoto lens?

If asked the question above, what would you say the answer is? Is it:
a) A lens with a focal length greater than 200mm?
b) A lens which can change it's focal length?
c) A lens whose physical length is less than the focal length?

I expect most of you will go for answer "A" but you'd be wrong. In real terms, anything with a focal length over 200mm is termed a telephoto lens in common parlance...but, there is a clue in the fact that you often find lenses shorter than 200mm refered to as telelphotos. So what are they talking about?

Well, the answer lies in techie talk.

According to the Canon Lens Work book, a telephoto lens is defined as follows:
With general photographic lenses, the overall length of a lens (the distance from the apex of the front lens element to the focal plane) is longer than its focal length. This is not usually the case with lenses of particularly long focal length, however, since using a normal lens construction would result in a very large unwieldy lens. To keep the size of such a lens manageable while still providing a long focal length, a concave (negative) lens assembly is placed behind the main convex (positive) lens which is shorter than its focal length. Lenses of this type are called telephoto lenses. In a telephoto lens, the second principal point is located in front of the frontmost lens element.

Translated into simple terms, a telephoto lens is one that has a physical length (from the front lens element to the the focal plane) which is less than its stated focal length.

-blabpictures-