d = 2 * sqrt( F*λ )
f-number = (distance to film/paper) / (pinhole diameter)
measure with something lets say that we get 1/60 with f22
let’s calculate with pihole f-number 100
exposure time is then 1/60 * 20.66 = 0.344 seconds
but remember we probably can’t measure with as low ISO as the paper
which probably is between 2 and 10, so we have to count the steps and adjust accordingly
| ISO | speeds | ||||
|---|---|---|---|---|---|
| 3 | 6 | 12 | 25 | 50 | 100 |
| 200 | 400 | 800 | 1600 | 3200 | 6400 |
At least with ILFORD papers, there is minimal need for longer exposure due to reciprocitation
For our papers - provided the exposures are within say a couple of hours, there is no real adjustment you’d need to make to whatever your initial metered reading is telling you to expose to.
https://github.com/ehagan/pinhole-camera-exposure-time
Diagonal of ‘normal’ 35mm film is 43.3mm, 36mm wide and 24mm tall
| image size | diagonal | width |
|---|---|---|
| 4:3 width | 34.6 f/w | 36 f/w |
| 4:3 diagonal | 43.3 f/d | 45 f/d |
| 3:2 width | 36 f/w | 36 f/w |
| 3:2 diagonal | 43.3 f/d | 43.3 f/d |
| (Table from Wikipedia) |
For a 5x7 paper (diagonal 218.7mm) and a true focal length of 100mm we’ll get the following
43.3 100/218.7 = 19.8
or with a 4x6 paper diagonal (127.6mm)
43.3 100/127.6 = 33