According to Wikipedia:
In photography and cinematography, a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus lens in a much shorter overall design. The angle of view and other effects of long-focus lenses are the same for telephoto lenses of the same specified focal length. Long-focal-length lenses are often informally referred to as telephoto lenses although this is technically incorrect: a telephoto lens specifically incorporates the telephoto group.
So telephoto lenses are shorter than their focal length. However, when I look at the longer commercially available prime lenses, there's a very clear relationship between the focal length of the lens and its physical length. The lenses get physically longer, to the point of being very, very long indeed, as their focal lengths get longer.
This raises the following questions in my mind:
Are these lenses actually telephoto in the technical sense, or just long-focus?
The telephoto group can be used to make a lens shorter than its focal length, but presumably there is a limit to how much shorter? I.e. you can't make it arbitrarily short, right? What is this limit? (Can you use multiple telephoto groups to compound the effect?)
Looking mainly at "full-frame" and smaller format fixed focal length lenses with an angle of view equivalent to an 85mm or greater focal length lens on a full-frame sensor, how likely are these to actually be telephoto in the technical sense (is there any correlation with focal length and/or format size?), and if both technically-telephoto and merely-long-focus lenses are reasonably popular, what are good examples of each?