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.
Pentax SMC Pentax-A* 135mm f/1.8 — Bokeh & Optical Ray-Tracing Simulator
Interactive 3D portrait ray-traced through the published US 4 optical prescription (7 elements in 6 groups) with a 75.0 mm entrance pupil and 1.2 m minimum focus.
Optical & Portrait Specifications (Upper Body Framing)
| Optical & Portrait Metric | Measured / Patent Value | What It Means in Practice |
|---|---|---|
| Patent Prescription | US 4 · 7 elements in 6 groups | Traced surface by surface in WebGL2 |
| Entrance Pupil Diameter | 75.0 mm (at f/1.8) | Controls distant background blur disc size |
| Corner Bokeh Roundness (90% Field) | 94% at f/1.8 · -0.1 EV | Mechanical vignetting (cat’s-eye vs. round corner highlights) |
| Background Field of View (Compression) | 15° horizontal | Width of the background room visible behind the subject |
| Upper-Body Working Distance | 5.40 m | Camera-to-subject distance for head-to-waist framing |
| Effective Focal Length (Breathing) | 139.6 mm (+3.4%) | Actual framing focal length at portrait distance |
| Depth of Field (Sharp Zone) | 25 cm (5.28 m – 5.53 m) | Sharp zone at wide-open aperture |
| Minimum Focus Distance | 1.2 m | Closest focusing distance from the sensor |
Questions
How much background blur and corner cat's-eye does the Pentax SMC Pentax-A* 135mm f/1.8 produce?
Wide open at f/1.8, the Pentax SMC Pentax-A* 135mm f/1.8 has a 75.0 mm entrance pupil (135 mm ÷ 1.8) and 94% bokeh roundness in the 90% corner (-0.1 EV corner falloff). In an upper-body portrait at 5.40 m, the sharp depth-of-field zone spans 25 cm (5.28 m to 5.53 m) with a 15° horizontal background angle of view.
Does the Pentax SMC Pentax-A* 135mm f/1.8 show focus breathing at portrait distance?
Based on patent US 4, tracing the focusing group at 5.40 m gives an effective framing focal length of 139.6 mm (+3.4% vs. the nominal 135 mm). You can toggle "Match subject size" in the simulator to see the framing shift live.
What optical design is inside the Pentax SMC Pentax-A* 135mm f/1.8?
The patent prescription (US 4) specifies 7 lens elements in 6 groups. The interactive Optical X-Ray above traces rays through every surface in real time.
Related comparisons
- 35mm vs 135mm focal length comparison
- 50mm vs 135mm focal length comparison
- 70mm vs 135mm focal length comparison
- 85mm vs 135mm focal length comparison
- 105mm vs 135mm focal length comparison
- 135mm vs 200mm focal length comparison
Compare with similar portrait lenses
- Sony FE 135mm F1.8 GM
- Nikon NIKKOR Z 135mm f/1.8 S Plena
- Canon RF 135mm F1.8 L IS USM
- Nikon NIKKOR 135mm f/2 AI-s
- Nikon AF DC 135mm f/2D
- Nikon 105mm f/1.4 FX
Also see: depth of field, bokeh, crop factor, lens compression.