1 Pinhole diameter
d = 2 * sqrt( F*λ )
- F = focal length, distance from pinhole to paper/film
- d = diameter
- λ = wavelength of light
- 550nm is good for black and white
- optimally less than 1/100 of focal length
- vignetting when diameter is near the thickness of material
2 f-number
f-number = (distance to film/paper) / (pinhole diameter)
3 Exposure time
measure with something lets say that we get 1/60 with f22
let’s calculate with pihole f-number 100
- light sensitivity change = (target f / measured f)²
- ex (100/22)² = 20.66
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
| 3 | 6 | 12 | 25 | 50 | 100 |
| 200 | 400 | 800 | 1600 | 3200 | 6400 |
ILFORD says about ISO 6 when using their Fibre paper for pinhole
ILFORD MULTIGRADE FB CLASSIC data sheet
MULTIGRADE RC is about 3-6
count from 6 to 100, 4 steps
so lets increase from 1/60 4 steps to 1/4, now we get
1/4 * 20.66 = 5.66 seconds exposure
| 1/4000 | 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 |
| 1/8 | 1/4 | 1/2 | 1 | 2 | 4 | 8 | 15 | 30 |
| 60 | 120 | 240 | 480 | 960 |
4 Reciprocity
At least with ILFORD papers, there is minimal need for longer exposure due to reciprocitation
ILFORD Paper FAQ
“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
5 35mm equivalent focal length
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 5×7 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 4×6 paper diagonal (127.6mm)
43.3 100/127.6 = 33