I am looking to do some experiments with my cellphone and a lens, holding the lens in contact with my cellphone lens. I'm looking for a lens that has the shortest back focal length length possible, so a 0mm BFL being the ideal. This is to image/have in focus the surface of the lens itself, like a Stanhope lens.
If I were to pick a lens from surplusshed.com, what type of lens would get me the closest to this possibility?
I know that a ball lens with a refractive index of 2 equals a BFL of 0, but I've found that the higher refractive ball lenses are quite pricey, and surplusshed does not carry them.
It was suggested that I combine a rod lens with a half ball lens, but I'm looking strictly for a single lens to experiment with, as I don't want to order custom lenses or cement my own, as I am a beginner.