where d is the pinhole diameter, c is a constant (usually Lord Rayleigh's constant, which is 1.9), f is the focal length (i.e. the distance between the pinhole and the light sensitive material) and l is the light wavelength. The wavelength used here is yellow/green, which is 0.00055mm.

My box is 65mm, measured from where I will place the pinhole to where the photo paper will go. Using the equation above that gives me the following pinhole size:

So the pinhole diameter should be about 0.36mm for this camera. If you are not all that fond of doing the math youself, here is a table of a number of focal lengths, where you can find the number closest to your camera size and use as a guide when making the pinhole.

Focal length (mm)	Optimal pinhole size (mm)	*	Focal length cont'd. (mm)	Optimal pinhole size cont'd. (mm)
15	0.17		110	0.47
20	0.20		115	0.48
25	0.22		120	0.49
30	0.24		125	0.50
35	0.26		130	0.51
40	0.28		135	0.52
45	0.30		140	0.53
50	0.32		145	0.54
55	0.33		150	0.55
60	0.35		155	0.55
65	0.36		160	0.56
70	0.37		165	0.57
75	0.39		170	0.58
80	0.40		175	0.59
85	0.41		180	0.60
90	0.42		185	0.61
100	0.45		190	0.61
105	0,46		200	0.63

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