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Pinhole Photography
Up to a certain point a small pinhole will produce a sharper image than a
larger one. If the pinhole is too small, the image gets less sharp because of
diffraction. The hole should be perfectly round, without ragged edges. It
may be checked with a magnifier or an enlarger.
Joseph Petzval of Vienna apparently was the first, in 1857, to attempt to find
a mathematical formula of the optimal pinhole diameter for the sharpest
definition in a pinhole image. The British Nobel Prize winner Lord Rayleigh
(John William Strutt, 1842–1919) worked on pinhole diameter formulas for
ten years and published his work in Nature (1891). Lord Rayleigh's formula
is still one of the formulas used to today. A number of others have been
published since the 1880s.
Lord Rayleigh's formula for subject distances above 1 meter may be written
as follows:
d = 1.9 * sqrt (l * f),
where d = pinhole diameter, l = wavelength of light and f = focal length or
distance from pinhole to light-sensitive material.
For the wavelength of light different average values may be substituted.
Often the value of the yellow-green spectrum is used, i.e. 0.00055 mm.
According to Renner (1995: 117) most formulas used today are of the
following general form:
r = sqrt (l * c * f)
r = pinhole radius
l = wavelength of light
c = a constant, usually a decimal fraction between 0.5 and 1
f = focal length
Platt (1989:73) provides the following optimal pinhole formula:
d x d = f/k, where k is a constant of approx. 1300
Dobson (1991) provides this formula:
d = sqrt (f)/25
Lord Rayleigh's formula and those published by Platt and Dobson all give
somewhat different results. Andrew Davidhazy of the Rochester Institute of
Technology lists several other formulas in a posting on the net.
http://www.photo.net/photo/pinhole/pinhole (20 of 28)7/3/2005 2:15:39 AM