F/Stops and Focal Lengths
Pinhole cameras can be as simple or as technical as you want to make them. I will go into the technical part abit on this page, but not in great detail. I get bogged down if the math starts to get to heavy......I like to keep things fairly simple, chancing my images on guessing and good luck. I do calculate a few things out, and this is what I will go into on this page.
First of all, someone has already done all the mathamatical equations for us, so why should I try and do them over again. I will take their word for it and then use it as a guide line.. That said, I will try and explain a little about pinhole sizes and the best distance of focal length (Focal length meaning the distance between the pinhole and the film or paper surface.)
I read that for every focal length there is an optimal pinhole size. The needles used to make the pinholes are listed here with their diameter sizes. The larger the pinhole you make, the longer your focal length should be.
| Needle No. | Diameter | Best Focal Length | f/Stop |
| 4 | .036" | 20" | f/550 |
| 5 | .036" | 15" | f/490 |
| 6 | .031" | 13" | f/450 |
| 7 | .026" | 10" | f/390 |
| 8 | .023" | 8" | f/350 |
| 9 | .020" | 6 1/2" | f/300 |
| 10 | .018" | 5" | f/280 |
| 11 | .016" | 4" | f/250 |
| 12 | .013 | 2 1/2" | f/190 |
To get your f-stop, you simply divide your focal length by the pinhole diameter. Film speeds you already know, 100, 200, 400 etc. Depending on weather conditions, how much light is reflecting off your subject, and what time of day it is, are all factors to take into consideration when using a pinhole camera. If you make an exposure and your image is too dark or too light, double or half your time and try again. Sometimes it hard to read the light, but with practise you will eventually be able to. Here is a chart with approximate times using RC Paper and ISO 400 film speed for focal lengths of 4" and 8". Be aware that exposure times will vary to a great degree with the reciprocity of different films brands.
| Weather Conditions | f/stop | RC Paper | ISO 400 |
| Snow or light sand | f/250 f/350 |
17 seconds 22 seconds |
1/4 second 1/3 second |
| Sunny day | f/250 f/350 |
49 seconds 2.1 minutes |
1 second 1.5 seconds |
| Hazy sun | f/250 f/350 |
2 mintues 5.1 minutes |
1.3 seconds 3.4 seconds |
| Cloudy but bright | f/250 f/350 |
5.4 minutes 14 minutes |
3.6 seconds 9.2 seconds |
| Heavily overcast | f/250 f/350 |
14 minutes 36 minutes |
9.2 seconds 24 seconds |
| Early morning or evening | f/250 f/350 |
38 minutes 1.6 hours |
25 seconds 1 minute |
The best advice I can give you is to carry a small notebook around with you and record each exposure with the weather conditions, exposure times, ISO's and time of day that you took the picture. After time you will be able to devise your own chart that will suit the brand of film or paper you use.