Teleconvertor

These have the useful properties of doubling the focal length while maintaining the same close focusing as the original lens.

I use a Teleplus MC7 seven element one I bought from Jessops in Stoke on Trent back in April 1996 which has works well.
It has a very useful property of having the rear element adjustable to enable life size (1:1) reproduction when used with the 50mm so-called Standard lens for Macro work with the reproduction scale as well as the focus distance marked from when the main lens is set to its minimum focus.