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Canon RF-S 7.8mm F4 STM DUAL LENS — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US20250093761A1 optical prescription (9 elements in 7 groups) with a 1.9 mm entrance pupil and 0.15 m minimum focus.
Optical & Portrait Specifications (Upper Body Framing)
| Optical & Portrait Metric | Measured / Patent Value | What It Means in Practice |
|---|---|---|
| Patent Prescription | US20250093761A1 · 9 elements in 7 groups | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 1.9 mm (at f/4) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 62% at f/4 (≥86% from f/5.6) · -1.0 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 114° horizontal | Width of the background room visible behind the subject |
| Upper-Body Working Distance | 0.53 m | Camera-to-subject distance for head-to-waist framing |
| Effective Focal Length (Breathing) | 7.6 mm (-2.2%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | infinity (26 cm – infinity) | Sharp zone at wide-open aperture |
| Minimum Focus Distance | 0.15 m | Closest focusing distance from the sensor |
Questions
How much background blur and corner cat's-eye does the Canon RF-S 7.8mm F4 STM DUAL LENS produce?
Wide open at f/4, the Canon RF-S 7.8mm F4 STM DUAL LENS has a 1.9 mm entrance pupil (7.8 mm ÷ 4) and 62% bokeh roundness in the 90% corner (-1.0 EV corner falloff, reaching ≥86% roundness from f/5.6). In an upper-body portrait at 0.53 m, the sharp depth-of-field zone spans infinity (26 cm to infinity) with a 114° horizontal background angle of view.
Does the Canon RF-S 7.8mm F4 STM DUAL LENS show focus breathing at portrait distance?
Based on patent US20250093761A1, tracing the focusing group at 0.53 m gives an effective framing focal length of 7.6 mm (-2.2% vs. the nominal 7.8 mm). You can toggle "Match subject size" in the simulator to see the framing shift live.
What optical design is inside the Canon RF-S 7.8mm F4 STM DUAL LENS?
The patent prescription (US20250093761A1) specifies 9 lens elements in 7 groups. The interactive Optical X-Ray above traces rays through every surface in real time.
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Compare with similar portrait lenses
- Fujifilm XF 8mm F3.5 R WR
- Leica DG Summilux 9mm f/1.7 ASPH.
- Canon RF 5.2mm F2.8 L DUAL FISHEYE
- Nikon 6mm f/2.8 Fisheye-Nikkor
- Nikon 10.5mm f/2.8G ED DX Fisheye
Also see: depth of field, bokeh, crop factor, lens compression.