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Canon 400mm f/2.8 L IS — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US6115188A optical prescription (16 elements in 12 groups) with a 142.9 mm entrance pupil and 3 m minimum focus.
Optical & Portrait Specifications (Upper Body Framing)
| Optical & Portrait Metric | Measured / Patent Value | What It Means in Practice |
|---|---|---|
| Patent Prescription | US6115188A · 16 elements in 12 groups | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 142.9 mm (at f/2.8) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 72% at f/2.8 (≥86% from f/5.6) · -0.6 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 5° horizontal | Width of the background room visible behind the subject |
| Upper-Body Working Distance | 15.99 m | Camera-to-subject distance for head-to-waist framing |
| Effective Focal Length (Breathing) | 439.4 mm (+9.8%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | 39 cm (16 m – 16 m) | Sharp zone at wide-open aperture |
| Minimum Focus Distance | 3 m | Closest focusing distance from the sensor |
Questions
How much background blur and corner cat's-eye does the Canon 400mm f/2.8 L IS produce?
Wide open at f/2.8, the Canon 400mm f/2.8 L IS has a 142.9 mm entrance pupil (400 mm ÷ 2.8) and 72% bokeh roundness in the 90% corner (-0.6 EV corner falloff, reaching ≥86% roundness from f/5.6). In an upper-body portrait at 15.99 m, the sharp depth-of-field zone spans 39 cm (16 m to 16 m) with a 5° horizontal background angle of view.
Does the Canon 400mm f/2.8 L IS show focus breathing at portrait distance?
Based on patent US6115188A, tracing the focusing group at 15.99 m gives an effective framing focal length of 439.4 mm (+9.8% vs. the nominal 400 mm). You can toggle "Match subject size" in the simulator to see the framing shift live.
What optical design is inside the Canon 400mm f/2.8 L IS?
The patent prescription (US6115188A) specifies 16 lens elements in 12 groups. The interactive Optical X-Ray above traces rays through every surface in real time.
Related comparisons
Compare with similar portrait lenses
- Canon RF 400mm F2.8 L IS USM
- Nikon NIKKOR Z 400mm f/2.8 TC VR S
- Leica Telyt 400mm f/5
- Nikon 300mm f/2 ED IF
- Sony FE 600mm F4 GM OSS
Also see: depth of field, bokeh, crop factor, lens compression.