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Nikon 20mm f/4 AI — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US3549241A optical prescription (10 elements in 8 groups) with a 5.0 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 | US3549241A · 10 elements in 8 groups | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 5.0 mm (at f/4) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 55% at f/4 · -1.4 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) | 19.6 mm (-1.8%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | 70 cm (61 cm – 1.31 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 Nikon 20mm f/4 AI produce?
Wide open at f/4, the Nikon 20mm f/4 AI has a 5.0 mm entrance pupil (20 mm ÷ 4) and 55% bokeh roundness in the 90% corner (-1.4 EV corner falloff). In an upper-body portrait at 0.84 m, the sharp depth-of-field zone spans 70 cm (61 cm to 1.31 m) with a 85° horizontal background angle of view.
Does the Nikon 20mm f/4 AI show focus breathing at portrait distance?
Based on patent US3549241A, tracing the focusing group at 0.84 m gives an effective framing focal length of 19.6 mm (-1.8% 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 Nikon 20mm f/4 AI?
The patent prescription (US3549241A) specifies 10 lens elements in 8 groups. The interactive Optical X-Ray above traces rays through every surface in real time.
Related comparisons
- Nikon AF-S 20mm f/1.8G ED
- Nikon 20mm f/2.8 AI-s
- Nikon 19mm f/4 PC-E FX
- Nikon Nikkor-O 2.1cm f/4
- Nikon 18mm f/2.8D FX
- Nikon 18mm f/4 AI-s
Compare with similar portrait lenses
Also see: depth of field, bokeh, crop factor, lens compression.