Constructing a Functioning Lens
In trying to make a simple focusable monocle with variable aperture for my DSLR, I was faced with the usual problem: lens + focuser + iris = too much distance from the sensor to achieve focus at infinity.
Yes, I could easily put together a 120 mm monocle that would work with my bellows as a focuser. Or I could also use the bicycle inner tube approach to building a focuser, but that's not what I wanted. I wanted a lens in the 35 - 50 mm range that had a real focuser and real aperture control. I wanted something that worked like a real lens! (I don't believe I just said that. Oh well, it's true.)
I decided my best option was to use the focuser from an old junk lens. In this case, that happened to be an old Tamron 28-80 with the optics removed. After cutting off the rear piece as close to the focuser threads as possible and then attaching a nikon mount using super glue, I had a mounted, functioning focuser which could position a lens about 70 mm from the sensor. That was still too long for the 35 - 50 mm focal length I wanted.
I needed to resort to the usual trick of adding a negative lens up front to increase the back focal length. So instead of one element, I would need two: A positive meniscus with a focal length between 40 and 45 as the rear element with its convex side facing the sensor, and a negative lens with a focal length somewhere in the -80 to -300 range in front of the first element and at a distance that provides the required back focal length. The iris is probably best placed between the two elements at about 10 mm in front of the rear element.