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Canon 20mm f/2.8 USM — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published JP H08-110467A optical prescription (11 elements in 9 groups) with a 7.1 mm entrance pupil and 0.25 m minimum focus.
Optical & Portrait Specifications (Upper Body Framing)
| Optical & Portrait Metric | Measured / Patent Value | What It Means in Practice |
|---|---|---|
| Patent Prescription | JP H08-110467A · 11 elements in 9 groups | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 7.1 mm (at f/2.8) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 40% at f/2.8 · -1.9 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 85° horizontal | Width of the background room visible behind the subject |
| Upper-Body Working Distance | 0.84 m | Camera-to-subject distance for head-to-waist framing |
| Effective Focal Length (Breathing) | 19.5 mm (-2.3%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | 46 cm (67 cm – 1.12 m) | Sharp zone at wide-open aperture |
| Minimum Focus Distance | 0.25 m | Closest focusing distance from the sensor |
Questions
How much background blur and corner cat's-eye does the Canon 20mm f/2.8 USM produce?
Wide open at f/2.8, the Canon 20mm f/2.8 USM has a 7.1 mm entrance pupil (20 mm ÷ 2.8) and 40% bokeh roundness in the 90% corner (-1.9 EV corner falloff). In an upper-body portrait at 0.84 m, the sharp depth-of-field zone spans 46 cm (67 cm to 1.12 m) with a 85° horizontal background angle of view.
Does the Canon 20mm f/2.8 USM show focus breathing at portrait distance?
Based on patent JP H08-110467A, tracing the focusing group at 0.84 m gives an effective framing focal length of 19.5 mm (-2.3% vs. the nominal 20 mm). You can toggle "Match subject size" in the simulator to see the framing shift live.
What optical design is inside the Canon 20mm f/2.8 USM?
The patent prescription (JP H08-110467A) specifies 11 lens elements in 9 groups. The interactive Optical X-Ray above traces rays through every surface in real time.
Related comparisons
- Canon 24mm f/2.8 IS
- Canon 15mm f/2.8 Fisheye
- 24mm vs 35mm focal length comparison
- 24mm vs 50mm focal length comparison
- 28mm vs 35mm focal length comparison
- 28mm vs 50mm focal length comparison
Compare with similar portrait lenses
- Nikon 20mm f/2.8 AI-s
- Sony E 20mm F2.8
- Sony Batis 18mm F2.8
- Nikon 18mm f/2.8D FX
- Nikon 20mm f/4 AI
- Nikon 24mm f/3.5 PC-E FX
Also see: depth of field, bokeh, crop factor, lens compression.