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Canon RF 45mm F1.2 STM vs. Canon RF 50mm F1.2 L USM

Pick the 45mm f/1.2 if you want a wider field of view, pick the 50mm f/1.2 if you want about 11% more background blur. Compare both side by side.

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

Optical & Portrait Metric Canon RF 45mm F1.2 STM Canon RF 50mm F1.2 L USM What It Means in Practice
Upper-Body Working Distance 1.82 m 2.01 m The Canon RF 50mm F1.2 L USM requires 0.2 m more room behind you for the same subject size.
Background Field of View (Compression) 44° 39° Canon RF 50mm F1.2 L USM shows a 11% narrower slice of the room behind the subject (39° vs. 44°), isolating them against a cleaner background.
Entrance Pupil Diameter 37.5 mm (f/1.2) 41.7 mm (f/1.2) Distant background blur discs are 11% larger on the Canon RF 50mm F1.2 L USM.
Corner Bokeh Roundness (90% Field) 46% (-1.4 EV) 41% (-1.5 EV) Canon RF 45mm F1.2 STM keeps significantly rounder bokeh balls into the corners wide open (46% vs. 41% roundness).
Effective Focal Length (Breathing) 44.2 mm (-1.8%) 50.8 mm (+1.6%) Canon RF 45mm F1.2 STM frames like 44.2 mm (-1.8%) at 1.8 m; Canon RF 50mm F1.2 L USM frames like 50.8 mm (+1.6%) at 2.0 m.
Depth of Field (Sharp Zone) 17 cm (1.73 m – 1.91 m) 17 cm (1.93 m – 2.10 m) Both lenses deliver the exact same 17 cm sharp focus zone.
Minimum Focus Distance 0.4 m 0.4 m Both lenses share the same 0.4 m close-focus limit.

Questions

Which blurs the background more, Canon RF 45mm F1.2 STM or Canon RF 50mm F1.2 L USM?

At the same subject framing, distant background blur scales with the entrance pupil diameter (focal length ÷ f-number). Canon RF 45mm F1.2 STM at f/1.2 has a 37.5 mm entrance pupil, while Canon RF 50mm F1.2 L USM at f/1.2 has a 41.7 mm entrance pupil.

How does working distance differ between Canon RF 45mm F1.2 STM and Canon RF 50mm F1.2 L USM?

For an upper-body portrait, Canon RF 45mm F1.2 STM stands 1.82 m from the subject (sharp zone 17 cm, from 1.73 m to 1.91 m), whereas Canon RF 50mm F1.2 L USM stands 2.01 m away (sharp zone 17 cm, from 1.93 m to 2.10 m).

How do the optical designs of Canon RF 45mm F1.2 STM and Canon RF 50mm F1.2 L USM compare?

Canon RF 45mm F1.2 STM uses 9 elements in 7 groups (US20260194735A1, effective focal length 44.2 mm at portrait distance). Canon RF 50mm F1.2 L USM uses 15 elements in 9 groups (US20190265441A1, effective focal length 50.8 mm).

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