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 a spare body-cap for the camera might like to know what the focal length of your pinhole-body-cap will be. If so, you will need to know the distance between the sensor and the inside front of the body cap.
Interchangeable lens cameras are marked somewhere on the body to show where the plane of the sensor lies. Its a small circle with a line through it.
On my APS-C camera that distance was about 23mm. Which is probably the most useful focal length to have as it is the equivalent of a 35mm lens.
If you want to get the best possible resolution out of your pinhole you will need to make some calculations:
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 usually available in sizes 0.1mm, 0.2mm, 0.3mm, 0.4mm and 0.5mm. So for my 23mm focal length, a 0.2mm pinhole would be the closest match.
To calculate the f number for any pinhole-size and focal-length combination divide the focal length by the pinhole size. In the above case dividing 23mm by 0.2mm gives 115, so the pinhole-body-cap will be a equivalent to a 23mm f115 lens.
It is also possible to check the theoretical range of f numbers that will give a good result for a certain pinhole size. For the above result:
First multiply the diameter of the pinhole 0.2 by 505. Note that number (in this case 101). Then multiply that number 101 by 1.7 and note that number too (in this case 172).
This means that for good results the f number should be within the range f101 - f172, and in this case f115 is within that range.
The optimum pinhole-to-sensor distance for a certain pinhole size can also 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. This is not the 23mm that I wanted but because the f number is in the calculated range I should still get good results.
But having gone through all of this trying different combinations of focal lengths and pinhole sizes, I found out that the single most important thing is to have a perfectly round pinhole in very thin metal rather 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.