Hello there, I am building pinhole cameras from any boxes and tins. Usually I calculate my pinhole diameter as I know the focal length and where to put the light-sensitive material, so I use the formula of Lord Rayleigh: d = 1,9 sqrt(f*0,00055), where d = pinhole diameter, f = focal length in mm, 0,00055 = wavelength of day light. Recently I bought micro drills so I already know the exact diameter of the pinhole and I would like to calculate optimal focal length. To me it sounds simple, I would just mathematically change the formula to get result for "f". Something like that: f = (d)^2/0,0019855). However, I started to search on the internet and I found out that many pinhole photographers use another formula for counting optimal focal length which is probably called Connors formula: f = (d/0,037)^2, therefore formula for optimal pinhole diameter: d = sqrt(f*0,001369). And here is what confuses me! When I compare Rayleigh´s and Connors´formula the results differ. So my questions are: Are there many formulas for counting optimal pinhole diameter or optimal focal length and it just depends on me which one I choose and which result I prefer and are the formulas I wrote correct? Or in the case I need to know pinhole diameter I use Rayleigh´s formula and in the case I need to know optimal focal length I use Connors´one? Or do I calculate anything wrongly from beginning? Thank you for your help and time. I would really like to know all the mathematics and physics about pinhole photography I am just not the best in maths..