>>3654170The original 105mm f/2.5 was a true Sonnar, 5 elements in 3 groups. The asymmetric, 3-group formula is the trademark of the Sonnar, the purpose being flare and keeping contrast high before the advent of multicoating.
Other qualities - bokeh, field curvature, focus shift - stem from this. The original 50mm f/1.5 lens was 7elements 3groups, the 85mm f/2 version the same. Nikon (and Canon and Kiev) cloned these designs, especially in their early RF lenses, as german patents were invalidated after WWII.
The original 105mm f/2.5 seems to be a scaled up version of the 85mm f/2 sonnar, even the aperture seems to be scaled accordingly. Maybe the 5elements instead of 7 was a cost-cutting measure after slight recalculation, as was the fewer aperture blades for instance.
After the '70s, the lens was redesigned as a double Gauss, more common formula.
