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Leica Summilux-M 90mm f/1.5 ASPH. vs. Leica APO-Summicron-M 90mm f/2 ASPH.

Pick the 90mm f/1.5 if you want about 33% more background blur, pick the 90mm f/2 if you want less background blur. Compare both side by side.

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

Optical & Portrait Metric Leica Summilux-M 90mm f/1.5 ASPH. Leica APO-Summicron-M 90mm f/2 ASPH. What It Means in Practice
Upper-Body Working Distance 3.61 m 3.61 m Identical working distance—both frame an upper-body portrait from the exact same spot.
Background Field of View (Compression) 22° 22° Identical 22° horizontal background view behind the subject.
Entrance Pupil Diameter 60.0 mm (f/1.5) 45.0 mm (f/2) Distant background blur discs are 33% larger on the Leica Summilux-M 90mm f/1.5 ASPH..
Corner Bokeh Roundness (90% Field) 53% (-1.1 EV) 98% (-0.1 EV) Leica APO-Summicron-M 90mm f/2 ASPH. keeps significantly rounder bokeh balls into the corners wide open (98% vs. 53% roundness).
Effective Focal Length (Breathing) 91.5 mm (+1.7%) 92.3 mm (+2.6%) At 3.6 m, Leica Summilux-M 90mm f/1.5 ASPH. frames like a 91.5 mm lens while Leica APO-Summicron-M 90mm f/2 ASPH. frames like a 92.3 mm lens.
Depth of Field (Sharp Zone) 21 cm (3.50 m – 3.72 m) 28 cm (3.47 m – 3.75 m) The Leica Summilux-M 90mm f/1.5 ASPH. has a 25% thinner focus zone (21 cm vs. 28 cm)—requiring more precise eye focus.
Minimum Focus Distance 1 m 1 m Both lenses share the same 1 m close-focus limit.

Questions

Which blurs the background more, Leica Summilux-M 90mm f/1.5 ASPH. or Leica APO-Summicron-M 90mm f/2 ASPH.?

At the same subject framing, distant background blur scales with the entrance pupil diameter (focal length ÷ f-number). Leica Summilux-M 90mm f/1.5 ASPH. at f/1.5 has a 60.0 mm entrance pupil, while Leica APO-Summicron-M 90mm f/2 ASPH. at f/2 has a 45.0 mm entrance pupil.

How does working distance differ between Leica Summilux-M 90mm f/1.5 ASPH. and Leica APO-Summicron-M 90mm f/2 ASPH.?

For an upper-body portrait, Leica Summilux-M 90mm f/1.5 ASPH. stands 3.61 m from the subject (sharp zone 21 cm, from 3.50 m to 3.72 m), whereas Leica APO-Summicron-M 90mm f/2 ASPH. stands 3.61 m away (sharp zone 28 cm, from 3.47 m to 3.75 m).

How do the optical designs of Leica Summilux-M 90mm f/1.5 ASPH. and Leica APO-Summicron-M 90mm f/2 ASPH. compare?

Leica Summilux-M 90mm f/1.5 ASPH. uses 14 elements in 11 groups (US8508864B2, effective focal length 91.5 mm at portrait distance). Leica APO-Summicron-M 90mm f/2 ASPH. uses 5 elements in 4 groups (US3076383A, effective focal length 92.3 mm).

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