This is a Sonnar formula lens, first introduced in 1952. It was produced in Nikon S-Mount, Contax RF Mount, Leica Thread Mount, and Nikon F-Mount. In 1973 or so, the Nikkor 105/2.5 changed to a Planar formula, supposedly a little bit sharper. This 1952 lens is known for it's out-of-focus background, was purposely designed under-corrected for spherical aberration. This makes it a great portrait lens. These are all at F2.5, on the M9. Relatively low shutter speeds, shots were close to sunset.