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.
A telephoto lens is a camera lens designed to enable people to take long focal length pictures using a lens with an actual length which is shorter than the focal length. For example, a 400mm telephoto lens is not in fact 400mm long, unlike a conventional 400mm lens. There are a number of reasons why people might want to use a telephoto lens, ranging from the need to have a camera which is easier to handle but still capable of long focal length images to a desire to take advantage of specific traits of telephoto lenses.
When people hear the phrase “telephoto lens,” they often think of an extremely long camera lens. Some telephoto lenses are quite long, but this is not necessarily so, and long lenses aren't always telephoto lenses. This term refers to a lens of a very specific construction, not a particular length.
In a telephoto lens, there is a lens element at the front of the camera just as there is with other cameras, but there is a second element in the back of the lens in front of the film which acts to magnify the front lens. This second element effectively increases the focal length of the camera by making it seem as though the first lens is further away than it really is. Historically, a telephoto effect was sometimes accomplished with the use of lenses positioned in front of an optical instrument, but today the structure is built right into the lens.
