Nikon NIKKOR Z 40mm f/2 vs. Nikon NIKKOR Z 50mm f/1.8 S

Pick the 40mm f/2 if you want a wider field of view, pick the 50mm f/1.8 if you want about 39% more background blur. Compare both side by side.

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

Optical & Portrait Metric Nikon NIKKOR Z 40mm f/2 Nikon NIKKOR Z 50mm f/1.8 S What It Means in Practice
Upper-Body Working Distance 1.62 m 2.01 m The Nikon NIKKOR Z 50mm f/1.8 S requires 0.4 m more room behind you for the same subject size.
Background Field of View (Compression) 50° 41° Nikon NIKKOR Z 50mm f/1.8 S shows a 18% narrower slice of the room behind the subject (41° vs. 50°), isolating them against a cleaner background.
Entrance Pupil Diameter 20.0 mm (f/2) 27.8 mm (f/1.8) Distant background blur discs are 39% larger on the Nikon NIKKOR Z 50mm f/1.8 S.
Corner Bokeh Roundness (90% Field) 86% (-0.6 EV) 69% (-0.5 EV) Nikon NIKKOR Z 40mm f/2 keeps significantly rounder bokeh balls into the corners wide open (86% vs. 69% roundness).
Effective Focal Length (Breathing) 39.0 mm (-2.4%) 48.6 mm (-2.9%) Nikon NIKKOR Z 40mm f/2 frames like 39.0 mm (-2.4%) at 1.6 m; Nikon NIKKOR Z 50mm f/1.8 S frames like 48.6 mm (-2.9%) at 2.0 m.
Depth of Field (Sharp Zone) 29 cm (1.49 m – 1.78 m) 26 cm (1.89 m – 2.15 m) The Nikon NIKKOR Z 50mm f/1.8 S has a 11% thinner focus zone (29 cm vs. 26 cm)—requiring more precise eye focus.
Minimum Focus Distance 0.29 m 0.4 m Nikon NIKKOR Z 40mm f/2 focuses 11 cm closer (0.29 m vs. 0.4 m) for tighter close-ups.

Questions

Which blurs the background more, Nikon NIKKOR Z 40mm f/2 or Nikon NIKKOR Z 50mm f/1.8 S?

At the same subject framing, distant background blur scales with the entrance pupil diameter (focal length ÷ f-number). Nikon NIKKOR Z 40mm f/2 at f/2 has a 20.0 mm entrance pupil, while Nikon NIKKOR Z 50mm f/1.8 S at f/1.8 has a 27.8 mm entrance pupil.

How does working distance differ between Nikon NIKKOR Z 40mm f/2 and Nikon NIKKOR Z 50mm f/1.8 S?

For an upper-body portrait, Nikon NIKKOR Z 40mm f/2 stands 1.62 m from the subject (sharp zone 29 cm, from 1.49 m to 1.78 m), whereas Nikon NIKKOR Z 50mm f/1.8 S stands 2.01 m away (sharp zone 26 cm, from 1.89 m to 2.15 m).

How do the optical designs of Nikon NIKKOR Z 40mm f/2 and Nikon NIKKOR Z 50mm f/1.8 S compare?

Nikon NIKKOR Z 40mm f/2 uses 8 elements in 4 groups (JP2021-189351A, effective focal length 39.0 mm at portrait distance). Nikon NIKKOR Z 50mm f/1.8 S uses 13 elements in 9 groups (WO2019220618A1, effective focal length 48.6 mm).

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