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Nikon 20mm f/2.8 AI-s — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US4690517A optical prescription (12 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 | US4690517A · 12 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) | 49% at f/2.8 (≥86% from f/5.6) · -1.7 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 86° 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) | 19.4 mm (-3.1%) | 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 Nikon 20mm f/2.8 AI-s produce?
Wide open at f/2.8, the Nikon 20mm f/2.8 AI-s has a 7.1 mm entrance pupil (20 mm ÷ 2.8) and 49% bokeh roundness in the 90% corner (-1.7 EV corner falloff, reaching ≥86% roundness from f/5.6). 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 86° horizontal background angle of view.
Does the Nikon 20mm f/2.8 AI-s show focus breathing at portrait distance?
Based on patent US4690517A, tracing the focusing group at 0.84 m gives an effective framing focal length of 19.4 mm (-3.1% 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/2.8 AI-s?
The patent prescription (US4690517A) specifies 12 lens elements in 9 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/4 AI
- 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.