Pinhole camera

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

36122550100
200400800160032006400

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

1/40001/20001/10001/5001/2501/1251/601/301/15
1/81/41/212481530
60120240480960    

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 sizediagonalwidth
4:3 width34.6 f/w36 f/w
4:3 diagonal43.3 f/d45 f/d
3:2 width36 f/w36 f/w
3:2 diagonal43.3 f/d43.3 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

6 Sources

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.