Having immersed myself into the internals of a stuck-aperture Vivitar lens, I spontaneously glued a lens element from a broken beamer onto a surplus filter ring and mounted it onto my FD lenses.
It makes those lenses so short-sighted that I used the 24 mm to get enough motive area into the picture. Moving backwards is not an option because the focus ring of the lens from 0.3 to infinity is now equal to maybe 5 mm focus range!
So it allows macro photography but has its restrictions. Since it hasn't cost anything, that is alright :-)
The level of detail is quite surprising for me, but then I am usually not a close-up photographer.
Note: Those pictures are NOT cropped.