This is a wonderful lens that comes in several versions. I’m not sure of the specifics, but the first version in chrome focuses to 1:1 as far as I know–the first lens to do so without external extensions needed. The second version, in black, has either a double or a triple focusing helicoid, allowing focusing way past 1:1 in the latter case, and to greater than 1:1 even in the former.
The second version differs from the first by being redesigned optically to include the world’s first bi-pherical element, but unlike modern aspherics, it was with a step rather than a smooth curve. This creates (wide open at least) an absolutely unique double-ring bokeh. Wide open the lens does suffer from some glow (as do many of these old lenses–due to poorly corrected spherical aberration), but the lens is very sharp, and a true masterpiece of mechanical engineering on its long helicoid. The strength of the double-ring of the bokeh can be adjusted by slightly closing the aperture. It is completely gone by f4, leaving a more usual, but still lovely, single ring. My copy is somewhat yellowed, indicating that one of the elements might be made of thorium-doped glass, but I do not have a radiation counter to test this, and so cannot confirm it.
