To calculate exposure, multiply the Watkins' power number be the distance from the pinhole to the film, and use the result as an f value to calculate exposure. Then, whatever the exposure indicated in seconds, give the same number in minutes.
| Watkins' (metric) power numbers | Pinhole diameter (mm) | Most suitable pinhole-to-film distance (cm) |
|---|---|---|
| 1.2 | 1.25 | 100.0 |
| 1.6 | 1.00 | 50.0 |
| 2.0 | 0.80 | 37.5 |
| 2.4 | 0.70 | 25.0 |
| 2.8 | 0.60 | 20.0 |
| 3.2 | 0.50 | 12.5 |
| 4.0 | 0.40 | 9.0 |
| 4.8 | 0.30 | 6.5 |
Last updated: 28 Mar 1999