The APO Macro-Summarit-S 120 mm f/2.5 plays a true dual role in the Leica S-System: On the one hand, it is a macro lens with a maximum reproduction ratio of 1:2 that renders even the finest details with perfection. On the other hand, its focal length, equivalent to 100 mm in 35 mm format, makes it an ideal moderate telephoto lens for portrait photography, particularly as its unusually large initial aperture allows perfect isolation of portrait subjects from their backgrounds.

Its design and construction is equally elaborate: of the nine lenses arranged in seven groups, three are made of special glass with anomalous partial dispersion. The exceptionally low dispersion (color scattering) of two of these is instrumental in keeping chromatic aberrations to a minimum. Its front-group focusing design features a floating element, i.e. an independently moving lens group that ensures spectacular imaging performance throughout the entire focusing range, even at the closest of focusing distances. Although the lens does not feature a completely sealed barrel due to the extreme extension of its close-up range, it is very effectively protected against the influences of water droplets, dust and moisture.

The optical design of the lens ensures that aberrations such as vignetting or distortion remain imperceptible. The contrast performance of the APO Macro-Summarit-S 120 mm f/2.5 is already so good at maximum aperture that stopping down to smaller apertures brings only an insignificant increase in contrast rendition, and even then, only in the extreme corners of the image. The combination of a moderate telephoto focal length, full macro-capability and an unusually fast initial aperture of f/2.5, and an alternative version with a central shutter, make this a unique lens in every respect.