Pinhole photography Technical stuff
This is about making a body-cap-pinhole for a digital camera but if you are making one for a 35mm film camera then you should read 'film' instead of 'sensor'.
If you are making your own pinhole it needs to be made in a piece of very thin metal and the tiny hole needs to be perfectly round. The slightest roughness will cause a loss of resolution. If you intend to mount the pinhole into the body-cap of a camera you need to know the distance between the the sensor and the inside front of the body cap. SLRs usually show where the plane of the sensor lies by marking a little circle with a line through it somewhere on the camera body. For the best possible resolution there are some calculations that can help.
On my APS-C camera that distance was 23mm. Which is probably the most useful focal length to have as it is the equivalent of a full frame 35mm lens.
The optimum size of the pinhole required can be worked out by taking the square root of the pinhole-to-sensor measurement and then multiplying it by 0.0368. So ideally a 23mm pinhole-to-sensor distance requires a pinhole of 0.176mm.
Ready made pinholes are readily available in sizes 0.1mm, 0.2mm, 0.3mm, 0.4mm and 0.5mm. So if a 23mm focal length was required, a 0.2mm pinhole seems to be the closest match.
Working back the other way, the optimum pinhole-to-sensor distance for a certain pinhole size can be calculated by dividing the pinhole diameter by 0.0368 and then squaring it. This means that my chosen 0.2mm pinhole will give an optimum pinhole-to-sensor distance of 29.54mm.
But as I want a focal length of 23mm not 29.54mm I need to use another calculation to see if f number of that pinhole and focal length combination is within a theoretical range that will still give good results:
First divide the focal length by the pinhole diameter to get the actual f number of that pinhole and focal length combination.
Divide 23mm by 0.2mm to get 115. Note that f number (f115).
Then find the range of suitable f numbers that should give good results:
First multiply the diameter of the pinhole 0.2 by 505. Note that number (101).
Then multiply that number 101 by 1.7 and note that number too (172).
So for good results the f number should be within the range 101-172, and in this case f115 is within that range.
But having gone through all of this, it seems that it is more important to have a perfectly round pinhole than a pinhole of exactly the right size.
These simplified calculations were produced from formulae (and constants 0.0368, 505 and 1.7) that I found on Wikipedia and on other pinhole camera related websites.
Contact me here if you have a question or any advice.