This tool calculates the optimal size of a pinhole that is a given distance away from the sensor / film of your camera. See the article on pinhole photography for further details. To make my own pinhole camera, I simply took a body cap, drilled a hole in it, and taped a pinhole to the front of it. But nothing stops you from making, say, a “tele” pinhole by mounting a cardboard tube on the body cap first…
You provide the focal length (the distance between the pinhole and the sensor / film) and the wavelength of the light. The default wavelength of 550 nm is for green light, which is close to both the maximum sensitivity of the human eye and the middle of its range (about 390 to 750 nm). You probably shouldn’t worry too much about this, although it does have a significant effect on the calculated pinhole size (try it!).
The output is the diameter of the pinhole. The tool also shows is the f-number of this pinhole with your camera, and gives the number of stops that a pinhole with the calculated diameter is slower than an f/10 lens.
I have measured, no doubt in a clumsy and inaccurate way, that the distance between the front (see the self portrait of my camera) of the body cap and the sensor of my Nikon D70 is 49 mm. Given that each Nikkor lens fits each Nikon camera, I assume that it is correct to use 49 mm for all Nikons. If you can confirm or improve this measurement, please let me know in the comments below. I would also be very much interested in a measurement for Canon and the other major brands.
Disclaimer: This tool is made available in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.