Starting Simple
Let's look at some examples, including different ways to address the mechanics.
The first lens uses a positive meniscus from Surplus Shed with an advertised focal length of 65 mm and a diameter of 47 mm (price: $6). As it turned out, Surplus Shed's measurement of the focal length is a bit off. The actual focal length of this lens turns out to be about 45 mm, so it will not quite focus to infinity. (The flange-to-focal-plane distance of a Nikon is 46.5 mm.) Interestingly, at a diameter of 47 mm, this lens exactly fits in a Nikon lens mount without falling into the camera. And that's how I use it: by just holding it against the mount with my fingers as I take the shot. It's not very practical. On the plus side, a 45 mm focal length meant that this lens is theoretically faster than f/ 1.0.
I call this lens the antipinhole.