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Canon 40mm f/2.8 STM — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US8970966B2 optical prescription (6 elements in 4 groups, 1 aspherical) with a 14.3 mm entrance pupil and 0.3 m minimum focus.
Optical & Portrait Specifications (Upper Body Framing)
| Optical & Portrait Metric | Measured / Patent Value | What It Means in Practice |
|---|---|---|
| Patent Prescription | US8970966B2 · 6 elements in 4 groups (1 aspherical) | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 14.3 mm (at f/2.8) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 98% at f/2.8 · -0.2 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 48° horizontal | Width of the background room visible behind the subject |
| Upper-Body Working Distance | 1.62 m | Camera-to-subject distance for head-to-waist framing |
| Effective Focal Length (Breathing) | 40.3 mm (+0.8%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | 41 cm (1.44 m – 1.85 m) | Sharp zone at wide-open aperture |
| Minimum Focus Distance | 0.3 m | Closest focusing distance from the sensor |
Questions
How much background blur and corner cat's-eye does the Canon 40mm f/2.8 STM produce?
Wide open at f/2.8, the Canon 40mm f/2.8 STM has a 14.3 mm entrance pupil (40 mm ÷ 2.8) and 98% bokeh roundness in the 90% corner (-0.2 EV corner falloff). In an upper-body portrait at 1.62 m, the sharp depth-of-field zone spans 41 cm (1.44 m to 1.85 m) with a 48° horizontal background angle of view.
Does the Canon 40mm f/2.8 STM show focus breathing at portrait distance?
Based on patent US8970966B2, tracing the focusing group at 1.62 m gives an effective framing focal length of 40.3 mm (+0.8% vs. the nominal 40 mm). You can toggle "Match subject size" in the simulator to see the framing shift live.
What optical design is inside the Canon 40mm f/2.8 STM?
The patent prescription (US8970966B2) specifies 6 lens elements in 4 groups with 1 aspherical surfaces. The interactive Optical X-Ray above traces rays through every surface in real time.
Related comparisons
Compare with similar portrait lenses
- Nikon 40mm f/2.8 Macro DX
- Nikon NIKKOR Z 40mm f/2
- Nikon 45mm f/2.8 PC-E
- Nikon 45mm f/2.8P AI-P
- Sony Sonnar FE 35mm F2.8 ZA
- Sony FE 28mm F2
Also see: depth of field, bokeh, crop factor, lens compression.