Leica Apo-Summicron-M 50mm f/2 vs Leica Summicron-M 50mm f/2

Portrait At-a-Glance Comparison (Upper Body Framing)

Optical & Portrait Metric	Leica APO-Summicron-M 50mm f/2 ASPH.	Leica Summicron-M 50mm f/2	What It Means in Practice
Upper-Body Working Distance	2.01 m	2.01 m	Identical working distance—both frame an upper-body portrait from the exact same spot.
Background Field of View (Compression)	39°	39°	Identical 39° horizontal background view behind the subject.
Entrance Pupil Diameter	25.0 mm (f/2)	25.0 mm (f/2)	Nearly identical distant background blur size at equal framing.
Corner Bokeh Roundness (90% Field)	91% (-0.3 EV)	89% (-0.3 EV)	Similar corner bokeh shape wide open (91% vs. 89% roundness at 90% field).
Effective Focal Length (Breathing)	50.6 mm (+1.3%)	51.1 mm (+2.3%)	At 2.0 m, Leica APO-Summicron-M 50mm f/2 ASPH. frames like a 50.6 mm lens while Leica Summicron-M 50mm f/2 frames like a 51.1 mm lens.
Depth of Field (Sharp Zone)	29 cm (1.88 m – 2.17 m)	29 cm (1.88 m – 2.17 m)	Both lenses deliver the exact same 29 cm sharp focus zone.
Minimum Focus Distance	0.7 m	0.7 m	Both lenses share the same 0.7 m close-focus limit.

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Questions

Which blurs the background more, Leica APO-Summicron-M 50mm f/2 ASPH. or Leica Summicron-M 50mm f/2?

At the same subject framing, distant background blur scales with the entrance pupil diameter (focal length ÷ f-number). Leica APO-Summicron-M 50mm f/2 ASPH. at f/2 has a 25.0 mm entrance pupil, while Leica Summicron-M 50mm f/2 at f/2 has a 25.0 mm entrance pupil.

How does working distance differ between Leica APO-Summicron-M 50mm f/2 ASPH. and Leica Summicron-M 50mm f/2?

For an upper-body portrait, Leica APO-Summicron-M 50mm f/2 ASPH. stands 2.01 m from the subject (sharp zone 29 cm, from 1.88 m to 2.17 m), whereas Leica Summicron-M 50mm f/2 stands 2.01 m away (sharp zone 29 cm, from 1.88 m to 2.17 m).

How do the optical designs of Leica APO-Summicron-M 50mm f/2 ASPH. and Leica Summicron-M 50mm f/2 compare?

Leica APO-Summicron-M 50mm f/2 ASPH. uses 6 elements in 4 groups (US4123144A, effective focal length 50.6 mm at portrait distance). Leica Summicron-M 50mm f/2 uses 6 elements in 4 groups (US4123144A, effective focal length 51.1 mm).

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Also see: depth of field, bokeh, crop factor, lens compression.