Find a lens for this look
eBay links may earn us a commission, at no extra cost to you.
Matching lenses from each maker's lineup, plus some classic medium and large format lenses. Email me if something is wrong or missing.
Canon 300mm f/4 L IS — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US6618197B2 optical prescription (14 elements in 10 groups) with a 75.0 mm entrance pupil and 1.5 m minimum focus.
Optical & Portrait Specifications (Upper Body Framing)
| Optical & Portrait Metric | Measured / Patent Value | What It Means in Practice |
|---|---|---|
| Patent Prescription | US6618197B2 · 14 elements in 10 groups | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 75.0 mm (at f/4) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 83% at f/4 (≥86% from f/5.6) · -0.2 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 6° horizontal | Width of the background room visible behind the subject |
| Upper-Body Working Distance | 12.00 m | Camera-to-subject distance for head-to-waist framing |
| Effective Focal Length (Breathing) | 319.5 mm (+6.5%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | 56 cm (12 m – 12 m) | Sharp zone at wide-open aperture |
| Minimum Focus Distance | 1.5 m | Closest focusing distance from the sensor |
Questions
How much background blur and corner cat's-eye does the Canon 300mm f/4 L IS produce?
Wide open at f/4, the Canon 300mm f/4 L IS has a 75.0 mm entrance pupil (300 mm ÷ 4) and 83% bokeh roundness in the 90% corner (-0.2 EV corner falloff, reaching ≥86% roundness from f/5.6). In an upper-body portrait at 12.00 m, the sharp depth-of-field zone spans 56 cm (12 m to 12 m) with a 6° horizontal background angle of view.
Does the Canon 300mm f/4 L IS show focus breathing at portrait distance?
Based on patent US6618197B2, tracing the focusing group at 12.00 m gives an effective framing focal length of 319.5 mm (+6.5% vs. the nominal 300 mm). You can toggle "Match subject size" in the simulator to see the framing shift live.
What optical design is inside the Canon 300mm f/4 L IS?
The patent prescription (US6618197B2) specifies 14 lens elements in 10 groups. The interactive Optical X-Ray above traces rays through every surface in real time.
Related comparisons
- Canon 300mm f/2.8 L IS
- 50mm vs 200mm focal length comparison
- 70mm vs 200mm focal length comparison
- 85mm vs 200mm focal length comparison
- 105mm vs 200mm focal length comparison
- 135mm vs 200mm focal length comparison
Compare with similar portrait lenses
- Nikon 300mm f/4.5 ED-IF AI-s
- Sony FE 300mm F2.8 GM OSS
- Nikon 300mm f/2.8 AF-S II FX
- Leica APO-Telyt-R 280mm f/2.8
- Canon 400mm f/5.6 L
Also see: depth of field, bokeh, crop factor, lens compression.