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    How large should the hole for a pinhole camera be, and why?
    by Dick Koolish 

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    In terms of sharpness, the pinhole should be the same size as the
    diffraction disk it produces.  A larger pinhole has a larger geometrical
    spot while a smaller pinhole has a larger diffraction spot.

    The linear diameter of the diffraction spot (Airy disk) produced by
    a pinhole of a given diameter is:

    spot = (2.44 * wavelength * focal_length) / diameter

    For example, a .5 mm pinhole at 100 mm focal length at 550 nm wavelength
    would produce a diffraction disk of:

    spot = (2.44 * .000550 * 100) / .5  = .268 mm

    So you see that the pinhole is too big in this case because .5 is larger
    than .268.

    The optimal size is where spot = diameter, so:

    diameter * diameter = (2.44 * wavelength * focal_length)

    diameter = sqrt (2.44 * wavelength * focal_length)

    At 100 mm focal distance and 550 nm wavelength, the optimal pinhole size
    is:

    diameter = sqrt (2.44 * .000550 * 100) = sqrt (.01342) = .366 mm

    Sharpness of an image can be improved by using a shorter focal length
    rather than a longer focal length for a given film size.  This allows you
    to use a smaller pinhole.

    Also, if you use orthochromatic (blue sensitive) film, you can use a
    smaller pinhole by eliminating the longer wavelengths.

    If you have a choice, use the largest film size you can, rather than
    a smaller film size and enlarging.  For a given sized final print,
    the optimal size pinhole increases slower than the enlarging factor.

    I have actually done some tests with various size pinholes at 100mm focal
    length and you can see how too large or too small pinholes degrade the
    image.

    By the way, the resolution of pinholes is nowhere near that of a real lens.
    At 100mm focal length, the optimum pinhole might resolve 5 lines/mm while a
    good lens will resolve 80-100 lines/mm.


    Table of pinhole sizes for 550 nm wavelength.

          Distance   Diameter     Diameter     f number
	    mm          mm           in

             10        0.116        0.005        86.3
             20        0.164        0.006        122.1
             30        0.201        0.008        149.5
             40        0.232        0.009        172.6
             50        0.259        0.010        193.0
             60        0.284        0.011        211.4
             70        0.306        0.012        228.4
             80        0.328        0.013        244.2
             90        0.348        0.014        259.0
            100        0.366        0.014        273.0
            200        0.518        0.020        386.0
            300        0.635        0.025        472.8
            400        0.733        0.029        546.0
            500        0.819        0.032        610.4
            600        0.897        0.035        668.7
            700        0.969        0.038        722.2
            800        1.036        0.041        772.1
            900        1.099        0.043        818.9
           1000        1.158        0.046        863.2
           2000        1.638        0.064        1220.8
           3000        2.006        0.079        1495.1
           4000        2.317        0.091        1726.4
           5000        2.590        0.102        1930.2
           6000        2.838        0.112        2114.5
           7000        3.065        0.121        2283.9
           8000        3.277        0.129        2441.6
           9000        3.475        0.137        2589.7
          10000        3.663        0.144        2729.8


    Trial exposures can be determined from the "sunny 16 rule".
    For example, for ISO 100 speed film, the bright sunlight exposure
    is f/16 at 1/100 sec.  If we use common shutter speeds, we get:

	    f/16    1/125 seconds
	    f/22    1/60
	    f/32    1/30
	    f/45    1/15
	    f/64    1/8
	    f/90    1/4
	    f/128   1/2
	    f/180   1     second
	    f/256   2
	    f/360   4
	    f/512   8