Retrofocus and Telephoto
When I wrote the Homemade Lens Primer, my goal was to make a wide angle lens with enough BFL to work with my DSLR. As is typical for wide angle lenses, I needed the BFL to be greater than the focal length. (For example: a lens with a focal length of 40 mm and a BFL of 75 mm). The solution was a retrofocus design in which the front element has a negative focal length and the rear element has a positive focal length.
The front (negative) element is double concave, plano-concave or negative meniscus, and the rear (positive) element is double convex, plano-convex or positive meniscus.
When making a lens with a very long focal length, we have the opposite problem. We want the BFL to be less than the focal length so that our lens is not awkwardly long. The solution is the telephoto design (and now I'm using the proper definition) with a positive front element and a negative rear element.
In this case, the front (positve) element is double convex, plano-convex or positive meniscus, and the rear (negative) element is double concave, plano-concave or negative meniscus.