The 2.5cm F4

This optically superb and extremely rare 4 element 4 group nearly perfectly symmetrical extremely curved optical formula was first marketed November 1953 about a year before the introduction of the S2. It was first available in a physically robust chrome on brass mount structure. At about sn403600 (after the first 1100 all chrome body version) a much lighter black painted aluminum body version was introduced.

This 2.5cm formulation is a true 2.5cm wide angle formulation and does not suffer the compromises in design that were later necessitated by SLR mirror clearance issues.
Historical Retrofocus Issues

The later black version is normally preferred in other lenses for lightness and comparative ease of use and handling though perhaps not with this one. The Chrome on brass heavier version only weighs in at 4.5oz (128g) while the aluminum "black" is only 2oz (57g) lighter which is almost negligible when mounted on a camera body. The trade off in loss of sturdy mechanical execution for weight savings is not an entire positive aspect.

According to original factory records 2,874 (1924 in Nikon BM and the remaining 950 in Leica SM) embodiments of this lens formula were produced.

The 2.5cm lens Nikon BM version of this lens is unique for two of its physical aspects in the Nikon Rangefinder lens line. It has no focus grip ring so all focusing action must be done by the camera body focus wheel. Because of it relatively enormous depth of field at almost any aperture this is hardly a use crippling issue. The aperture mechanism, unique to this lens, has no external physical coupling ring and the user must reach carefully into the front to adjust a tab.

Unlike either of the Nikon BM physical embodiments the Leica SM version has highly conventional means for both focus and aperture adjustment.