Focal Length and Angle of View
Focal length only has meaning relative to sensor size. Angle of view depends on both dimensions.
For a standard 24 × 36 mm frame with a 43 mm diagonal:
| Focal length | Diagonal angle of view on 24 × 36 mm |
|---|---|
| 20 mm | about 94 degrees |
| 28 mm | about 75 degrees |
| 50 mm | about 47 degrees |
| 135 mm | about 18 degrees |
A normal lens has a focal length close to the frame diagonal. This is why 50 mm is standard on 35 mm film.
Depth of Field
Only one plane is truly in focus. Points outside it blur into discs. They appear sharp while smaller than the circle of confusion (typically 0.03 mm for 35 mm format).
Four factors control depth of field:
- Aperture. Smaller apertures increase depth.
- Focal length. Shorter lenses yield greater depth.
- Subject distance. Greater distance increases depth.
- How big the print is. Larger prints demand tighter tolerances.
Hyperfocal Distance
Focus at the hyperfocal distance to keep everything sharp from half that distance to infinity.
Hyperfocal distances (0.03 mm circle of confusion, 24 × 36 mm frame):
| Focal length | f/5.6 | f/8 | f/11 | f/16 |
|---|---|---|---|---|
| 28 mm | 4.7 m | 3.3 m | 2.4 m | 1.7 m |
| 50 mm | 14.9 m | 10.5 m | 7.6 m | 5.3 m |
| 100 mm | 59.6 m | 41.8 m | 30.4 m | 20.9 m |
For instance, set a 28 mm lens to f/11 and Focus at 2.4 m. Everything from 1.2 m to infinity remains acceptably sharp.
Common pitfall: believing longer lenses inherently possess less depth of field. At matched image scale and aperture, a 200 mm lens matches a 50 mm lens.