Canon RF 45mm F1.2 STM vs Canon RF 50mm F1.8 STM — Optical Ray-Tracing & Bokeh Comparison

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

Optical & Portrait Metric	Canon RF 45mm F1.2 STM	Canon RF 50mm F1.8 STM	What It Means in Practice
Upper-Body Working Distance	1.82 m	2.01 m	The Canon RF 50mm F1.8 STM requires 0.2 m more room behind you for the same subject size.
Effective Focal Length (Breathing)	44.2 mm (-1.8%)	51.4 mm (+2.8%)	Canon RF 45mm F1.2 STM frames like 44.2 mm (-1.8%) at 1.8 m; Canon RF 50mm F1.8 STM frames like 51.4 mm (+2.8%) at 2.0 m.
Depth of Field (Sharp Zone)	17 cm (1.73 m – 1.91 m)	26 cm (1.89 m – 2.15 m)	The Canon RF 45mm F1.2 STM has a 33% thinner focus zone (17 cm vs. 26 cm)—requiring more precise eye focus.
Entrance Pupil Diameter	37.5 mm (f/1.2)	27.8 mm (f/1.8)	Distant background blur discs are 35% larger on the Canon RF 45mm F1.2 STM.
Minimum Focus Distance	0.4 m	0.3 m	Canon RF 50mm F1.8 STM focuses 10 cm closer (0.4 m vs. 0.3 m) for tighter close-ups.
Patent Prescription	US20260194735A1 (9/7)	US20210263286A1 (6/5)	Published optical prescriptions traced in the 3D simulator above.

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Questions

Which blurs the background more, Canon RF 45mm F1.2 STM or Canon RF 50mm F1.8 STM?

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 F1.8 STM at f/1.8 has a 27.8 mm entrance pupil.

How does working distance differ between Canon RF 45mm F1.2 STM and Canon RF 50mm F1.8 STM?

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 F1.8 STM stands 2.01 m away (sharp zone 26 cm, from 1.89 m to 2.15 m).

How do the optical designs of Canon RF 45mm F1.2 STM and Canon RF 50mm F1.8 STM 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 F1.8 STM uses 6 elements in 5 groups (US20210263286A1, effective focal length 51.4 mm).

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