Thin lens (Gaussian)
| Thin-lens | 1/f = 1/s_o + 1/s_i |
|---|---|
| Magnification | M = −s_i / s_o = h_i / h_o |
| Lensmaker's | 1/f = (n − 1)(1/R₁ − 1/R₂) |
| Power (diopters) | P = 1/f (meters) |
Camera / photography
| f-number N | = f / D (focal length / aperture diameter) |
|---|---|
| Exposure value | EV = log₂(N² / t) |
| Depth of field | Narrows with larger aperture, longer f, closer subject |
| Angle of view | 2 · arctan(sensor / (2f)) |
| Hyperfocal | H = f² / (N · c) where c = circle of confusion |
f-stop full stops
| f-stop | Relative light |
|---|---|
| f/1.0 | 100% |
| f/1.4 | 50% |
| f/2 | 25% |
| f/2.8 | 12.5% |
| f/4 | 6.25% |
| f/5.6 | 3.13% |
| f/8 | 1.56% |
| f/11 | 0.78% |
| f/16 | 0.39% |
| f/22 | 0.195% |
Focal length feel (35 mm equivalent)
| Focal length | Class | Typical use |
|---|---|---|
| 14 mm | Ultra-wide | Landscape, architecture |
| 24 mm | Wide | Street, environmental |
| 35 mm | Normal-wide | Documentary |
| 50 mm | Normal | Everyday |
| 85 mm | Short tele | Portraits |
| 135 mm | Tele | Portrait / sports |
| 200 mm | Tele | Sports / wildlife |
| 400 mm | Super tele | Distant wildlife |
Notes
- Crop factor (sensor smaller than full-frame): multiply full-frame focal length to get equivalent field of view (APS-C ~1.5×, Micro 4/3 ~2×).
- Diffraction limits sharpness at very small apertures — f/16+ on full-frame softens noticeably.
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